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Turbulence analysis and multiscale correlations between synchronized flow velocity and marine turbine power production
Abstract
The correlation between the flow turbulence and the performances of a ma-
rine current turbine is studied. First, the incoming flow encountered in the flume
tank is characterized in the framework of fully developed turbulent cascades in
the inertial range. The Reynolds number, the Kolmogorov dissipation scale and
the integral scale, are estimated from flow measurements. The intermittency
of the turbulence is characterized in the lognormal multifractal framework, and
the influence of the turbulent flow on the turbine power is assessed.
The rotor speed control unit characteristics used for the turbine regulation
induces non-negligible effects on the turbine behavior under fluctuations loads.
Even if the power spectrum does not reveal any scale invariance, a multiscale
analysis allows us to show the correlations between the turbulence time se-
ries and the power produced. The classical Mean Square Coherency function
shows that for scales larger than 10 seconds, the upstream velocity and power
have large correlations. In the framework of the Empirical Mode Decomposition
method, such correlations are studied using the time-dependence intrinsic corre-
lation analysis method. This method allows to zoom into time-frequency scales
where the flow perturbations induced some modifications in power production.
... In France, the Alderney Race (Raz-Blanchard) is one of the most energetic location with a high turbulence rate in the entire water column, originating mainly from large bathymetry variations (figure 1). This turbulence can affect significantly the performance and fatigue of tidal turbines [1,7]. Figure 1b illustrates bathymetry variations profiles in the area of interest for tidal turbine application, with a mean variation of 5m and large elevation exceeding 10m, in profile 2 in particular. ...
... This velocity profile is then spatially averaged and compared to a single point measurement, depicted by the green square on figure 4, at the same distance x of the rotor, but in the very centre of the turbine diameter. This second velocity measurement way is similar to what have been done in previous studies [1,2], but with a closer distance between the velocity measurement point and the turbine; a too far distance being unfavourable to obtain a good coherence between incoming velocity variations and the turbine behaviour. Both these signals are shown on figure 5 with u line and u point and on table 2 for timeaverage and standard-deviation values. ...
... For all cases, no coherence at all is detected past 1Hz, the turbine is acting as a low-pass filter with a cut-off frequency of 1Hz. Same results were found in previous studies but the velocity measurements and the turbine were located further away from each other: at 4D [1] or 2D [2]. Hence, the low-pass filter effect of the turbine persists independently to the spacing. ...
Dans les zones à forts courants telles que celles adaptées à l'exploitation des énergies marines renouvelables, les variations de bathymétrie créent de fortes fluctuations de vitesse. Des expériences ont montré que de larges obstacles cylindriques généraient de fortes fluctuations de vitesses, telles que celles observables en mer. Afin d'étudier les effets de ces fluctuations de chargement sur le comportement d'une hydrolienne, un dis-positif expérimental a été développé dans le bassin à circulation de l'Ifremer à Boulogne-sur-mer, en positionnant un cylindre et une turbine dans son sillage. Des mesures PIV simultanées donnent accèsà la vitesse du fluide en amont de la turbine. La vitesse et les efforts sont comparés en terme de corrélation croisées, de cohérence et leur contenu spectral est étudié. Les résultats montrent que les efforts suivent les fluctuations de vitesses jusqu'à une fréquence de 1H z et les fluctuations de vitesse à basse fréquence ont plus d'impact lorsque la turbine est en fonctionnement. La cohérence entre la vitesse du fluide et la vitesse de rotation diffère de celle des chargements, probablement dû au système de contrôle de rotation de la machine.
... The objective is to generate synthetic turbulence as close as possible to the turbulence of Mycek et al.'s [6] experiment. In addition to the data provided by Mycek et al. [6], an analysis of the upstream and downstream turbulence was made by Medina et al. [38]. The general definition of the turbulence intensity, noted I or I3D(%), is based on the three components of the velocity fluctuations, and is given by Equation (14), where < u 2 i > are the time-averaged velocity fluctuations and < U i > are the time-averaged velocity components. ...
... Experimental spectral data in the wake of the tidal turbine are available in Medina et al. [38]. They used the same turbine as the one simulated here but at TSR = 4.0. ...
... They used the same turbine as the one simulated here but at TSR = 4.0. Figure 26a compares LBM results with the experiment made by [38]. Power spectral density (PSD) computed from the LBM simulation with an upstream turbulence rate of 12.5% is close to the experimental one up to 10 Hz frequency. ...
The lattice Boltzmann method is used to model a horizontal axis tidal turbine. Because tidal turbines generally operate in highly turbulent flows, a synthetic eddy method is implemented to generate realistic turbulent inflow condition. The approach makes use of the open-source code Palabos. Large eddy simulation is employed. A coupling between an immersed boundary method and a wall model is realized to model the turbine. Calculations are performed at two different turbulence rates. The upstream flow condition is first set up to match with experimental results. Numerical simulations of a tidal turbine with realistic turbulent inflow conditions are then realized with the lattice Boltzmann method. The approach is found to be in good agreement with experimental data. Cases with three different inflow turbulence rates are simulated. An almost linear evolution with the turbulence rate is observed for the axial velocity deficit. An analysis of the propagation of tip-vortices in the close wake is carried out. It is found that turbulence has a great impact on the tip-vortices propagation envelope.
... As the onedimensional turbulence intensity, 1 ∞ = ∞ ∕ ∞ , is used in some studies (e.g. McCaffrey et al., 2015;Blackmore et al., 2016;Medina et al., 2017), these results are also given as a matter of information. Independently from the aforementioned campaigns, Table 3 also shows the turbulence statistics obtained from LDV measurements performed at the turbine hub position, without the scaled turbine, for 30 min, for the LTI and HTI conditions as a matter of further comparison. ...
... Contrary to the lower turbulence intensity cases, the flow is almost isotropic for the HTI condition with ∞ ∕ ∞ ≈ 1.1. This is due to the honeycomb (see Fig. 6), placed in the flume tank for the low ∞ configurations, which constrains the flow in both the normal and transverse directions (Medina et al., 2017). ...
... For the MTI, is approximately twice the size. In the same way as Medina et al. (2017), the flow properties are also characterised via a spectral analysis of the upstream velocity. The power spectral densities (PSD) of the streamwise velocity, , presented in Fig. 9, were obtained using the scipy.signal.welch ...
In this paper the turbulence effects are studied for three rotors mounted on the same instrumented hub. Two scaled models of industrial turbines and one open-geometry turbine are considered. The turbulence characteristics are obtained from 2D Laser Doppler Velocimeter measurements and the turbine behaviour is analysed from thrust and torque measurements. Three turbulence intensities and a large range of tip speed ratios and flow velocities are considered. The results are anonymised in order to ensure confidentiality. The rotors have different blade profiles, blade numbers and solidity. The rotor design largely modifies the mean power and thrust coefficients. The turbulence intensity only slightly changes these results but has a larger influence on the fluctuating loads than the different rotor designs. The spectral analysis of the rotor torque and thrust shows that, at low frequencies the load variations are correlated to those of the flow velocity with some differences due to the turbulence intensity levels. The coherences between the loads and the velocity seem to be not affected by the rotor type. At high frequencies, the load variations are correlated to the speed control unit of the scaled model and the rotor design has an impact on the rotational speed and loads coherences.
... Understanding the impact that the extreme marine environment has on the survivability of tidal energy converters is fundamental for the successful development and commercialisation of full scale devices. Recent efforts from the scientific community to drive the development of tidal technology forward include the estimation of turbulence [1] [2] or wave loading [3], [4], [5], [6] effects, on tidal energy devices. These investigations cover a wide range of topics ranging from the influences of diverse waveforms, wave directionality, control strategies or turbulent flow characteristics and intensities. ...
... Damping Withdraw length C 1 8.60 kg 0 5 % C 2 9.35 kg 12 × 3000 kg/s 5 % C 3 10.1 kg 24 × 3000 kg/s 5 % C 4 10.1 kg 24 × 3000 kg/s 10 % ...
... The flow speed spectrum without wave can be superimposed to Fig. 9, except in f = f wave , where it does not present any peak. The spectrum without wave generator has lower levels of power density and also has a -5/3 slope in the inertial range [1]. The wave orbital speed is estimated at the LDV measurement point (x, y, z) = (−2.4L, ...
The dynamic behaviour of an undulating tidal energy converter under wave and current loadings is studied. Therefore, a small scale prototype of a damped pre-stressed undulating membrane is tested in a wave and current recirculatory tank. The membrane’s dynamic profile and hydrodynamic forces are measured synchronously with the surface elevation for both regular and irregular waves in current direction. Spectrum analysis reveals that the response to wave loading depends on the wave frequency and is reduced by damping-type power take-off. Coupling between the wave frequency and the main undulation frequency or its harmonics is also observed. An increase of 30% of the forces must be foreseen in case of wave condition in the tested range.
... The same author has previously shown in Ref. [15] that the ambient turbulence has a huge impact on the thrust coefficient of the machine, using actuator disc. Dur an-Medina et al. studies [16,17]ha v e proven the existence of a strong correlation between the fluctuation of velocity due to the ambient turbulence and the fluctuation of the energy production of a turbine. Frost et al. [18] also compared power performances for tests performed at laboratory scale and in situ. ...
... In a previous study [17], Dur an-Medina et al. give the inertial and integral turbulent lengths scale in the same tank and for the same flow configurations, named C 1 (u ∞ ¼ 0:8 m=s and lowest turbulence rate) and C 2 (u ∞ ¼ 0:8 m=s and highest turbulence rate). These parameters are given in the table B2 of this previous paper. ...
... The present results are computed from the second part of equation (10), similarly to Dur an-Medina et al. [17]. In the same way, f down is introduced as well, corresponding to the downstream turbine rotation frequency, which differs depending on the TSR down cases. ...
The development of marine current turbine arrays depends on the understanding of the interaction effects that exist between turbines in close proximity. Moreover, the ambient turbulence intensity also plays a major role in the behaviour of tidal turbines. Thus it is necessary to take ambient turbulence into account when studying interaction effects between several turbines. In order to highlight these interaction effects, experiments have been carried out in the IFREMER flume tank. These experiments focus on interactions between three horizontal axis turbines. This paper presents the experimental results obtained for three configurations with two ambient turbulence intensity rates.
The results are presented in terms of turbine wakes and performance. The wake characterisation presents complex features for the three configurations and the lowest ambient turbulence rate: upstream turbines wakes are still present at the location of the downstream turbine and their wakes can interact or merge, depending on the tested configurations. On the contrary, for the highest turbulence rate, the downstream turbine wake is not affected in his shape by the two upstream ones which are not visible any more. In fact, as already observed in the previous studies of Mycek et al. [1, 2], the wake shape rapidly spreads out in the stream-wise direction behind the turbines. However, the velocity deficit and the turbulence intensity are higher for the downstream turbine comparing to the upstream ones. In terms of performance, one tested case presents an increase of the downstream turbine power production: when this turbine is exactly in the centre of the two upstream turbines and for the lowest turbulence rate only. A small misalignment of the layout axis with respect to the tidal current may result in a decrease of performance at the end. An analysis of the power spectral density functions of the downstream turbine torque and thrust shows that no signature of the upstream turbines can be found in these answers. Furthermore, the same spectral analysis carried out on the velocity measurements shows no signature of the upstream turbines either, from 3 diameters distance. This result is noticeable for the highest and the lowest tested turbulence cases and whatever the turbines configuration is.
... This method provides a new tool to locally assess the correlations between two time series using different window scales, adapted even for non-stationary and nonlinear data (Chen et al., 2010;Calif et al., 2017). Recently, TDIC method has been used for marine dissolved oxygen (Huang and Schmitt, 2014), finance (Hong and Wang, 2014), marine temperature (Ismail et al., 2015;Derot et al., 2016), environmental field (Sankaran and Reddy, 2016;Reddy, 2016, 2018), marine turbulence (Medina et al., 2017) and renewable resources . To our knowledge, no study has yet used this method in air pollution field. ...
... , the maximum instantaneous period for two IMFs, where T i 1 and T i 2 are the instantaneous periods ensuring that at least one cycle is included when computing the correlation (Medina et al., 2017). Once the size of the sliding window is determined, the TDIC of each pair of IMFs is then defined at each time t k as follows: ...
... Nevertheless, a classical correlation can be misleading since it does not reveal possible localized correlation events through time space (Chen et al., 2010;Medina et al., 2017). In next section, The Time-Dependent Intrinsic Correlation method was used to study the local time correlation. ...
Ground-level ozone (O 3 ) is a major air pollutant which is strongly links to weather conditions. To our knowledge, there are few analysis of O 3 behavior in Caribbean region. The aim of this paper is to study the correlation between O 3 and meteorological parameters (i.e., solar radiation, temperature, relative humidity and wind speed) in a multi-scale way. Firstly, we estimate their Fourier power spectra and consider their scaling properties in the physical space. The power spectra computed follows a power law behavior for all datasets. Thereafter, the multiple scale dynamics and the correlations between simultaneous measurements of O 3 and meteorological parameters was analyzed using the Empirical Mode Decomposition (EMD) based methods. After EMD decomposition of each time series, we investigate their local correlations using a new cross-correlation technique termed Time Dependent Intrinsic Correlation method (TDIC). The time evolution and the scale dependence of their correlation are determined at different time scales and for different intrinsic modes functions. All measured TDICs have highlighted strong correlations between O 3 and meteorological parameters. The impact of weather conditions (e.g., drought, rainfall, windy period) have been locally observed on O 3 formation for all the time scales. This study shows that TDIC method is a powerful tool for perform multi-scale cross-correlation analysis in air pollution field.
... Cette problématique a été abordée à plusieurs reprises au cours de tests de prototypes d'hydroliennes en bassin d'essais (e.g. Maganga et al., 2010, Mycek et al., 2014, Durán Medina et al., 2017. Cependant l'absence de ce type d'étude en milieu naturel manque cruellement. ...
... d'un écoulement turbulent dans un canal de faible profondeur(Fernandez-Rodriguez et al., 2014) ou encore lors d'une étude de corrélation entre vitesse de l'écoulement et puissance générée par une d'hydrolienne installée en bassin d'essai(Durán Medina et al., 2017).Dans le domaine des hautes fréquences (f > 4 Hz), les fluctuations de puissance semblent être insensibles aux mouvements turbulents. Au-dessus de la fréquence critique f c = 4 Hz, les enregistrements de puissance et de vitesse sont clairement affectés par le bruit rendant impossible toute interprétation des fluctuations de ces deux quantités dans ce domaine fréquentiel. ...
... Les travaux présentés dans cette seconde partie de thèse ont démontré que la turbine (W2E) répond à la turbulence sur des échelles semblables au diamètre du rotor (2 m). Des résultats similaires ont notamment été mis en évidence lors d'essais expérimentaux d'un prototype d'hydrolienne tripales réalisés dans le bassin de houle et courant de l'Ifremer de Boulogne/mer (DuránMedina et al., 2017).Les hydroliennes testées opérant dans la couche de surface, l'installation d'appareils de mesures de vitesse de courant dans leur sillage était facilitée. Néanmoins les résultats obtenus doivent être comparés à ceux d'autres campagnes de tests, notamment pour des systèmes de récupération de plus grandes tailles et opérant à mi-hauteur dans la colonne d'eau. ...
Malgré l'intérêt suscité, l'exploitation de l'énergie hydrolienne accuse toujours un sérieux retard par rapport à d'autres ressoures renouvelables. Ce développement tardif s'explique d'une part par l'absence d'une méthodologie pertinente de quantification du potentiel hydrocinétique, d'autre part, par le milieu marin, jugé hostile, au sein duquel des phénomènes turbulents,appliquent des contraintes sur la structure des hydroliennes. Offrant une vision précise de l'écoulement et de son hétérogénéité spatiale et temporelle, la courantographie radar, présentée dans la première partie de cette thèse, constitue un outil puissant d'analyse de la dynamique de la marée. Inédite dans le domaine des énergies renouvelables, elle offre une alternative probante à la modélisation numérique, souvent privilégiée bien que générant des résultats contestables compte tenu de sa conceptualisation simpliste des phénomènes réels. L'étude d'impact d'un écoulement turbulent pleinement développé sur la performance énergétique de prototypes d'hydroliennes de type Darrieus constitue la seconde partie de cette thèse. L'analyse spectrale des signaux temporels de vitesse de courant a permis de révéler les propriétés scalaires de l'écoulement. La caractérisation multi-échelle a montré que l'écoulement est animé par un processus dynamique de brisures successives des structures fluides imbriquées de taille allant de l'échelle d'injection à l'échelle de diffusion moléculaire. Enfin, le partitionnement en échelles dynamiques de la turbulence des spectres de vitesse et de puissance générée par l'hydrolienne a mis en évidence une certaine similarité entre la taille des structures tourbillonnaires qui régissent l'écoulement et les dimensions de la turbine.
... In geosciences, classical correlation is frequently used to study the influence of one parameter on another; however, when applied to nonstationary time series, this approach can modify the true crosscorrelation information between time series and give a misleading information (Hoover, 2003;Rodó and Rodríguez-Arias, 2006). As natural sciences have fluctuations over several scales (Flandrin and Goncalves, 2004), a window based on the local feature scale given by the data itself should be applied to analyze the cross-correlation between time series (Medina et al., 2017). To overcome the sliding window issue, a correlation method originally based on the EMD method and called time-dependent intrinsic correlation (TDIC) was proposed by Chen et al. (2010). ...
... where is adaptive, as it depends on = ( 1 ( ), 2 ( )), i.e., the maximum instantaneous period for two IMFs, where 1 and 2 are the instantaneous periods ensuring that at least one cycle is included in the correlation calculation (Medina et al., 2017). ...
This study investigates the dynamic relationship between particulate matter with less than 10μm aerodynamic diameter (PM10) and meteorological parameters (i.e., solar radiation (SR), air temperature (T), wind speed and direction (U and D), rainfall (R), relative humidity (Rh), and visibility (V)), while using time-dependent intrinsic correlation (TDIC) analysis based on complete ensemble empirical mode decomposition with adaptive noise. The TDIC analysis captured both negative and positive correlations between PM10 and the meteorological parameters at all examined time scales; nevertheless, as high PM10 concentrations were mainly related to synoptic scale sources, the correlations were more significant for a mean time period ranging from 1 to 7 d. In the high dust season (i.e., from May to September), results showed that dust outbreaks have a major impact on climate. Trends differ among meteorological parameters: At daily scale, positive (negative) correlations were found between PM10 and SR, T, U, and V (R and Rh), while correlation strength may change with increasing time scale. In addition, transition periods between the low (i.e., from October to April) and high dust season, but also before and after the passages of rainy events, were identified by the TDIC analysis. The impact of the largest African dust storm in the last 50 years on climate has also been identified locally at a time scale between 1 and 4 d, which corresponds to the duration of its passage.
... Diagonal terms are chosen in order to match with the measured turbulence rate: 3 % and 15 %. [16]. This can induce some differences in the wake. ...
... The size of the structures generated by the SEM, L, is chosen to L = 1.0 m for I = 3 % and to L = 1.2 m for I = 15 %. This last value is twice larger than the one measured byMedina et al. (2017) ...
Tidal turbines are entering an industrial phase and farms will soon be installed. In order to optimize the power output of tidal farms, a good understanding of the interactions between the ambient turbulence and a single turbine is crucial. Computational Fluid Dynamics, and more precisely Large Eddy Simulation, is one way of acquiring such knowledge. This study proposed a comparison between a Lattice Boltzmann Method LES approach and a Navier-Stokes LES approach to model the wake of a tidal turbine. Numerical results are compared with experimental results and a relatively good concordance is observed. Differences inherent to the approaches are then pointed out.
... This PSD appears to converge with respect to the variation σ ( λ) as of σ (λ) = 75% , towards a quasi-linear energy decay. The quality of the PSD curves obtained with sufficient variation on the size of the turbulent structures is further validated by comparison with experimental spectra provided by Medina et al. [41] . While the correspondence between numerical and experimental data is not complete, the similarities between these curves is sufficient to conclude that the SEM is capable of accurately representing the energy behaviour of a real turbulent flow. ...
... Influence of σ ( λ) on the spectral representation for a Gaussian kernel ( λ = 0 . 5 and R f = 2 ) and comparison with experimental data provided by Medina et al.[41] (for I ∞ = 15% and U ∞ = 0 . 8 m/s ). ...
This paper describes a detailed implementation of the Synthetic Eddy Method (SEM) initially presented in Jarrin et al. (2006) applied to the Lagrangian Vortex simulation. While the treatment of turbulent diffusion is already extensively covered in scientific literature, this is one of the first attempts to represent ambient turbulence in a fully Lagrangian framework. This implementation is well suited to the integration of PSE (Particle Strength Exchange) or DVM (Diffusion Velocity Method), often used to account for molecular and turbulent diffusion in Lagrangian simulations. The adaptation and implementation of the SEM into a Lagrangian method using the PSE diffusion model is presented, and the turbulent velocity fields produced by this method are then analysed. In this adaptation, SEM turbulent structures are simply advected, without stretching or diffusion of their own, over the flow domain. This implementation proves its ability to produce turbulent velocity fields in accordance with any desired turbulent flow parameters. As the SEM is a purely mathematical and stochastic model, turbulent spectra and turbulent length scales are also investigated. With the addition of variation in the turbulent structures sizes, a satisfying representation of turbulent spectra is recovered, and a linear relation is obtained between the turbulent structures sizes and the Taylor macroscale. Lastly, the model is applied to the computation of a tidal turbine wake for different ambient turbulence levels, demonstrating the ability of this new implementation to emulate experimentally observed tendencies.
... Blackmore et al. demonstrated in Ref. [12,15] that the increase of turbulence intensity and turbulent length scale slightly increases the power and thrust coefficients. Dur an Medina et al. [13] as well as Payne et al. [14] showed that the flow turbulence intensity significantly affects the temporal variability of the turbine power production. Another consequence is the impact of these velocity fluctuations on the structural loads or on the blade fatigue which has been studied by Davies et al. [16] for instance. ...
... This conclusion is however specific to tidal turbines submitted to that particular flow with turbulence originated from bathymetry variations, like those encountered in the Alderney Race (middle of the English Channel). Dur an Medina et al. [13] show for instance a high level of coherence and correlation for an identical tidal turbine model (placed in the same flume tank) submitted to fully developed and homogeneous turbulence with flow measurements carried out 4 diameters upstream from the turbine. In the wind industry, upstream velocity is measured to continuously readjust the turbine's yaw alignment and therefore enhance the turbine's harvested power [41] or to design collective pitch controls to reduce the loads that wind turbines suffer from incoming turbulent gusts [42]. ...
In high flow velocity areas like those suitable for tidal applications, turbulence intensity is high and flow variations may have a major impact on tidal turbine behaviour. A three-bladed horizontal axis turbine model (scale 1:20) is positioned in the wake of a square wall-mounted cylinder, representative of specific in situ bathymetric variation, to experimentally study these effects in a current flume tank. Local and global loads are acquired in synchronization with velocity measurements to study the turbine response to flow fluctuations. Velocity measurements need to be obtained close to the turbine, contrary to what is commonly considered, to properly correlate velocity and loads fluctuations. Results show that the loads phase average and their dispersion evolve according to the sheared velocity profile. We conclude that the turbine load fluctuations directly respond to the low frequency velocity fluctuations and are dominated by the turbulent structures shed from the cylinder. It is then possible to compare the effects of large coherent turbulent structures on the turbine behaviour to cases with more classical free stream turbulence commonly studied. These results provide a substantive database in high Reynolds number flows for further fatigue analysis or recommendations for turbine positioning in such flows.
... La turbulence générée en amont de la turbine dans la simulation doitêtre la plus proche possible de la turbulence présente dans le canal lors des essais. L'étude de Medina et al. (2017) [69] donne des informations sur la nature de cette turbulence. Ils montrent que pour les deux taux de turbulenceétudiés, la turbulence est pleinement développée.À partir de la densité spectrale de puissance obtenue avec la vitesse axiale mesurée en amont [87]. ...
... La turbulence générée en amont de la turbine dans la simulation doitêtre la plus proche possible de la turbulence présente dans le canal lors des essais. L'étude de Medina et al. (2017) [69] donne des informations sur la nature de cette turbulence. Ils montrent que pour les deux taux de turbulenceétudiés, la turbulence est pleinement développée.À partir de la densité spectrale de puissance obtenue avec la vitesse axiale mesurée en amont [87]. ...
Dans un contexte mondial où l’accès à l’énergie est un problème de premier plan, l’exploitation des courants de marée avec des hydroliennes revête un intérêt certain. Les écoulements dans les zones à fort potentiel énergétique propices à l’installation d’hydroliennes sont souvent fortement turbulents. Or la turbulence ambiante impacte fortement l’hydrodynamique avoisinante et le fonctionnement de la turbine. Une prédiction fine de la turbulence et du sillage est fondamentale pour l'optimisation d'une ferme d'hydroliennes. Un modèle de simulation de l'écoulement autour de la turbine doit donc être précis et tenir compte de la turbulence ambiante. Un outil basé sur la méthode de Boltzmann sur réseau (LBM) est utilisé à ces fins, en association avec une approche de simulation des grandes échelles (LES). La LBM est une méthode instationnaire de modélisation d’écoulement fluide. Une méthode de génération de turbulence synthétique est implémentée afin de prendre en compte la turbulence ambiante des sites hydroliens. Les géométries complexes, potentiellement en mouvement, sont modélisées avec la méthode des frontières immergées (IBM). La mise en place d’un modèle de paroi est réalisée afin de réduire le cout en calcul du modèle. Ces outils sont ensuite utilisés pour modéliser en LBM-LES une hydrolienne dans un environnement turbulent. Les calculs, réalisés à deux taux de turbulence différents, sont comparés avec des résultats expérimentaux et des résultats NS-LES. Les modélisations LBM-LES sont ensuite utilisées pour analyser le sillage de l'hydrolienne. Il est notamment observé qu'un faible taux de turbulence impacte de manière significative la propagation des tourbillons de bout de pale.
... The impact of turbulences on turbine performance and loading has also been explored (Chamorro et al., 2013;Mycek et al., 2014;Fernandez-Rodriguez et al., 2014;Milne et al., 2016;Blackmore et al., 2016). Durán Medina et al. (2017) carried out an in-depth analysis of the correlations between onset flow velocity and turbine power production based on spectral methods, empirical mode decomposition and time-dependent intrinsic correlation. However, there has, to-date, been limited in-depth analysis of the frequency variation of loading of horizontal axis tidal turbine blades and the turbine supporting structure. ...
... The re-sampling is applied using linear interpolation and alternative sampling frequencies were tested to ensure that the chosen frequency was sufficient to resolve the frequency range shown. Alternative methods to compute periodigrams without re-sampling, such as the Empirical Mode Decomposition-Hilbert Spectral Analysis lead to comparable results, as shown by Durán Medina et al. (2017) for the Empirical Mode Decomposition-Hilbert Spectral Analysis method. The spectral density of the velocity fluctuations with respect to the mean is shown in Fig. 3 for streamwise (u ′ ) and transverse (v ′ ) fluctuations for the two TI u levels. ...
Sustainable and cost effective design for tidal current turbines requires knowledge of the complex nature of unsteady loads on turbine components including blades, rotor and support structure. This study investigates experimentally the variation with frequency of rotor thrust and torque loads, of streamwise root bending moment on individual blades and of loads on foundation at the bed. Comparisons between these different load spectra are also established. The impact of absolute rotor angular position on blade and rotor thrust loads is also examined. The study is based on measurements from a 1/15 scale, three-bladed, horizontal axis machine tested in a recirculating flume, in onset flows of 3% and 12% turbulence intensity. It is found that for frequencies below the rotational frequency, load spectra are correlated to spectral density of the onset flow velocity. Above the rotational frequency, loads are mainly affected by turbine operation phenomena. The tower shadowing effect is clearly identified through frequency and angular analysis. Finally, thrust loads as experienced by the rotor alone are for the first time compared with streamwise and transverse foundation loads. Higher frequency loads experienced by the tower are shown to be affected by different vortex shedding regimes associated with different regions of the wake. All the experimental measurements presented in this article can be accessed from http://dx.doi.org/10.7488/ds/2423.
... Such variations are causing a high turbulence rate in the water column, where marine current turbines are meant to be installed. Turbulence can have a major impact on the tidal turbines, on their production [9] and on the structural fatigue [18]. Before trying to reproduce complex structures, we chose to introduce the topic by studying elementary obstacles representatives of real life condition: i.e. with an aspect ratio of the magnitude of the mean bathymetry variations encountered between France and Alderney. ...
... Many studies on dunes underline the importance of turbulent events that can rise high in the water column [2,20,3]. The rise of turbulent structures can have a major impact on turbines used to extract tidal energy [9]. So, the knowledge of these high energetic flow structures is of great importance. ...
The present study aims at investigating turbulence characteristics in high flow velocity areas like those suitable for marine energy application. The Reynolds number, based on the rugosity height and mean flow velocity, is rather high: Re=2.5×10⁷. For that purpose, experiments are carried out in a flume tank with Re as high as achievable in Froude similitude (in the tank: Re=2.5×10⁵ and Fr=0.23). Obstacles are canonical wall-mounted elements chosen to be representative of averaged bathymetric variations: a cube and a cube followed by an inclined floor. First, the wake topology past a canonical wall-mounted cube is illustrated from PIV measurements. Results show a flow behaviour already observed in the literature but for different upstream conditions (Re and turbulence intensity). Second, the impact of the addition of an inclined floor is studied. It is shown that the inclination causes a squeezing of the cube wake that strongly impacts the shape and intensity of the shear layer (up to 10% more intense with the inclined floor). To fully grasp the turbulence organization in the wake for both test cases, an analysis using both complementary Proper Orthogonal Decomposition and quadrant method is performed. POD acts as a turbulent noise filter and quadrant method decomposes turbulent events. Results show the predominance of ejection (Q2) and sweep (Q4) events in the flow Reynolds shear stress. Q2 events are more energetic although Q4 events prevail. It is observed that the inclined floor causes a persistence of Q2 and Q4 events higher into the water column, more than the impulsion given by the floor altitude variations. The rise of the cube wake due to the inclined floor is thus illustrated using Q4 predominance area.
... The authors also documented a reduction in power coefficient by roughly 10% caused by the ambient turbulence intensity increase from 3% to 15%. This range of turbulence level variation is considered in experimental studies [7], [8]. ...
... In the range from 0.1-0.2 Hz to 2 Hz (inertial range) the correlation is stable thus indicating that the output power fluctuations are caused to major extent by the turbulence in tidal flow [8]. ...
... A statistically determined coherent peak indicates the significance of a turbulent flux [9]. The SCS is the Fourier transform of the covariance function, and it is defined by [9,31] where A uw is the cross-spectrum of x and y direction of velocity signals; p fu and p fw are the related PSD. It is relevant to all f, 0 ≤ 2 uw ≤ 1 . ...
Detailed investigation of turbulence flow characteristics due to mechanically generated regular water waves over a submerged cylinder is carried out. Regular waves are generated by a wave maker driven at three different frequencies. Prolonged measurements of the temporal variation in the instantaneous velocity in all three directions were carried out at multiple locations downstream of the cylinder. Particular attention is given to the change in flow characteristics downstream of the cylinder as a result of interaction of the surface wave and bottom mounted cylinder (radius, R = 2.5, 3.75, and 5.5 cm). The resulting flow field is studied in detail using quadrant and velocity spectra analysis. The results show that contributions to the total shear stress from ejection and sweep are prominent for the entire region. Furthermore, the probability density function is determined to describe the instantaneous turbulent velocity signal, which demonstrates a bimodal nature for bottom-normal velocity fluctuations and a sharp distribution for stream-wise velocity fluctuations. Power spectral density shows that the power spectral peak of surface wave is effectively reduced for both stream-wise and vertical velocities. In coastal zones, pipelines of cylindrical cross section are the most dependable and secure, economical, and efficient means of continually transporting natural gas and oil and this study will help in the improved design and implementation of such submarine pipelines.
... Meanwhile, TDIC offers a major advance in evaluating the spatial correlation variation between a pair of turbulence shear signals since the spatial information is highly self-similar in a coherent structure. It was first proposed in the study by Chen [48] and has been applied in various nonlinear research [49][50][51]. Furthermore, the adaptive sliding window size makes it applicable to detect correlation information from nonlinear turbulence shear signals. ...
Multi-scale coherent structures have been observed in ocean currents, which are induced by the interaction of shear flows with different velocities. Understanding the spatial configuration and scale characteristics of coherent structures will promote the explanation of physical ocean phenomena. Considering the self-similarity, we propose a spatial correlation identification model for coherent structure extraction and three-dimensional visualization based on the wavelet transform and time-dependent intrinsic correlation method. The spatial and scale distributions of coherent structures are related to the dissipation rate variation. Most large-scale coherent structures, with the largest length scale of 13 m, are found to exist in stable fluid, such as the water column below 50 m. However, small-scale structures are found in chaotic fluids, such as the upper layer. Furthermore, we found that coherent structures of different scales coexist simultaneously in the same depth range, indicating a simultaneous multi-scale structure pattern for turbulent flow investigations.
... Therefore, understanding the power level decay law of turbulent coherent structures is essential for assessing their impact on the power characteristics of water turbines. 36,37 Studies of a single HATT interacting with onset turbulence have shown that the rotor behaves as a lowpass filter by ignoring the small-scale fluctuations and responding only to the larger coherent structures. 38 The characteristics of the power spectrum can be divided into three regions: 38 In the first region, it appears that the power output of the turbine is not sensitive to the flow scale smaller than the blade diameter. ...
Horizontal axis tidal turbines (HATTs) working in a complex flow environment will encounter unsteady streamwise flow conditions that affect their power generation and structural loads, where power fluctuations determine the quality of electricity generation, directly affecting the grid and reliability of the power transmission system; fatigue loads affect various structures and mechanical components of the turbine, directly determining the lifespan and reliability of the turbine. To gain insight into the generation mechanism and distribution of these excitations, a large eddy simulation is employed to analyze the inflow turbulence and unsteady forces excitations by a three-blade HATT. A spectral synthesizer was used to generate incoming turbulence flow. The strip method was applied on the HATT by dividing the blade into 20 strips. The thrust received by each strip and the flow velocity upstream and downstream of the blade's root, middle, and tip were monitored. The distribution of unsteady loads on the blades was analyzed, as well as the relationship between flow velocity upstream and downstream of the blade and the unsteady characteristics of the blades. The simulation results show that the unsteady hydrodynamic fluctuations of the HATT blades reach up to 57.44% under a turbulent intensity of 10%. Through intuitive analysis of flow separation on the suction surface of the blade at various moments under a low tip speed ratio, we can comprehend the variations in inflow velocity and flow separation on the blade surface. Analyzing the distribution of blade load from root to tip reveals that the maximum load values are concentrated in the 14th–16th strips, corresponding to the region from 0.7R to 0.8R. Moreover, the middle and tip sections of the blades predominantly contribute to the harmonics of the 3BPF (blade passing frequency) and broadband, with the middle section making a greater contribution. The tip section primarily contributes to harmonics above 3BPF. This research want to makes a valuable contribution to the comprehensive understanding of turbulence-induced exciting forces and the practical engineering design of HATT.
... The working section of the system is 4 m in width by 2 m in depth and 18 m in length. The streamwise flow velocity has a mean value of < U >= 0.8 m/s, a turbulence intensity (ratio of the standard deviation of the streamwise component to its mean value) of 20%, a dissipation rate value of = 1.79 × 0 −2 m 2 s −3 and a Taylor-based Reynolds number of R λ = 487 [24]. The data are freely available through the SEANOE depository [25]. ...
In fully developed turbulence, there is a flux of energy from large to small scales in the inertial range until the dissipation at small scales. It is associated with irreversibility, i.e., a breaking of the time reversal symmetry. Such turbulent flows are characterized by scaling properties, and we consider here how irreversibility depends on the scale. Indicators of time-reversal symmetry for time series are tested involving triple correlations in a non-symmetric way. These indicators are built so that they are zero for a time-reversal symmetric time series, and a departure from zero is an indicator of irreversibility. We study these indicators applied to two fully developed turbulence time series, from flume tank and wind tunnel databases. It is found that irreversibility occurs in the inertial range and has scaling properties with slopes close to one. A maximum value is found around the injection scale. This confirms that the irreversibility is associated with the turbulent cascade in the inertial range and shows that the irreversibility is maximal at the injection scale, the largest scale of the turbulent cascade.
... The frequency range where the RAO is defined is however twice as small as for I ∞ = 1.5 % with a maximum frequency of f ≃ 0.2 Hz, comparing to I ∞ = 15 %. This is explained by the coherence function which is smaller in amplitude for I ∞ = 1.5 % (not shown here) and by the cutoff which occurs for a lower frequency comparing to I ∞ = 15 %, as already observed by [18]. As previously observed, the phase angle stays constant and close to 0 • . ...
A transfer function determination method is proposed in this study to predict the unsteady fluctuations of the performance of a tidal turbine model. This method is derived from the Response Amplitude Operator (RAO) applied in the offshore industry to predict linear wave-induced loads on large structures. It is based on a spectral approach and requires the acquisition of a turbine parameter (e.g. torque, thrust, power or root-blade force) in synchronization with an upstream flow velocity measurement. On the frequency range where the causality between these two signals is proven, the transfer function is established using the ratio between the cross-spectral density and the spectral density of the incoming velocity.
The linearity is verified using the coherence function, which shows validity for the turbine power in the lowest frequencies only. This transfer function is then used to reconstruct the power fluctuations which is compared to the recorded one for a particular flow condition with bathymetry generated turbulence. The result shows the dependence on the accurate location of the velocity measurement point used for the reconstruction. This point must exactly correspond to the expected turbine location, i.e. where the turbine response needs to be processed. Bearing in mind its limits, the method can be used to predict the loadings of extreme events on the turbine structure and the performance variations corresponding to the unsteady characteristics of a turbulent flow, for a better grid integration.
... This PSD appears to converge with respect to the variation σ(λ) as of σ(λ) = 75%, towards a quasi-linear energy decay. The quality of the PSD curves obtained with sufficient variation on the size of the turbulent structures is further validated by comparison with experimental spectra provided by Medina et al. [102]. While the correspondence between numerical and experimental data is not complete, the similarities between these curves is sufficient to conclude that the SEM is capable of accurately representing the energy behaviour of a real turbulent flow. ...
In the current context of diversification of renewable energies, tidal turbines are set to occupy an important niche, and numerical simulation is a crucial tool for their investigation. The in-house simulation code DOROTHY developed in collaboration between IFREMER and LOMC uses the Vortex Particle Method offering a good compromise between physical realism and and computational time. Some additional developments are required in order to make of this software a fully rounded numerical tool able to mimic advanced realistic configurations. Firstly, an important overhaul of its computation of loads has been undertaken, including a new framework to represent the previously simplified and now fully-rendered turbine blades. This endeavour includes the mathematical justification, investigation, and preliminary validation of additional integral methods accounting for the turbine body. Secondly, the importance of the impact of ambient turbulence on the wake interaction and power output within a turbine farm cannot be ignored. This element is introduced using a Synthetic Eddy Method uniquely adapted to the present Lagrangian framework. All aspects of this method as well as a promising alternative are closely examined, culminating in the demonstration of its capabilities for the simulation of the flow and prediction of detrimental interaction effects throughout a projected four turbine pilot farm configuration.
... A better knowledge of the turbine power variabilities related to temporal fine-scale fluctuations could potentially enable an improvement of the integration of these turbines into the 10 electricity grid (Lewis et al., 2019;Sentchev et al., 2020). It is now well recognized that turbine performance is mainly impacted by the turbulence intensity (Mycek et al., 2014;Chamorro et al., 2015c;Li et al., 2016;Blackmore et al., 2016;Bartl and Saetran, 2017;Ahmed et al., 2017;Durán Medina et al., 2017;Thiébaut et al., 2020;Allmark et al., 2021;Ebdon et al., 2021) as well as the mean-shear flow (Bahaj et al., 2007;Wagner et al., 2009;Gaurier et al., 2020a). Furthermore, the knowledge of the spectral content of turbine outputs is also necessary to improve the predictability of the produced power, for a better grid integration of wind or tidal farms (Gayme and Chakrabortty, 2012;Bandi, 2017;Lewis et al., 2019). ...
To improve the turbine operational life, the interaction between flow properties and turbine performance needs to be elucidated. We then propose to examine the physical origin of the power-law scaling in the inertial range of turbine power outputs by experimentally exploring the spectral content of a 1:20 scaled model of a three-bladed horizontal-axis turbine positioned in a 3D turbulent flow. First, measurements confirm that the turbine power frequency spectra exhibit a power law decay proportional to −11/3 in the inertial range. Knowing that the turbine power fluctuations are linearly dependent on the incoming velocity fluctuations, PIV measurements are carried out to study the effect of the spatially integrated velocity onto its resulted spectrum. It is demonstrated that in inhomogeneous anisotropic turbulent flow, the velocity spectrum of its spatial average along N direction(s) has an inertial slope of −5/3 − 2N/3. This information is used to physically interpret the power-law scaling in the inertial range of the turbine power spectra. The previously observed f−11/3 scaling results from a 2D-spatial average velocity field coupled with a spectral average over blades. This physical explanation confirms previous works in which a transfer function was developed between incoming turbulence and the turbine power outputs.
... Experimental testing of the Magallanes ATIR, Sabella D12, and IFREMER-LOMC turbine rotors are completed by University Le Havre Normandy, France at IFREMER wave and flume tank in Boulogne-Sur-Mer, France. Detailed description of the IFREMER wave and current flume tank laboratory facility can be found in previous works [17], [27]- [30]. Experimental data used for evaluation of the BEMT model in this work, for the IFREMER-LOMC turbine, was gathered from a study by P.Mycek et al. [27]. ...
Tidal energy converters have the potential to become significant contributors in the generation of low carbon energy. Operational cost, driven mostly by planned and unplanned maintenance, is one of the most significant barrier limiting their widespread adoption. Accurate numerical models can be used to predict structural loads and help improve their reliability, and thus reduced maintenance costs. Blade element momentum theory is a common numerical model that is used for design and performance evaluation of tidal energy converters. It offers acceptable accuracy for evaluation of turbine design iterations with significant computational saving. This paper will investigate the prediction improvements possible by allowing non-uniform blade properties in a previously-uniform model. The capability of assigning unique lift and drag characteristics to each element based on its geometry and Reynolds number will be implemented, resulting in improved blade geometry modelling in the numerical model, and thus the turbine performance predictions. Three turbine rotor blades are analysed: Magallanes ATIR, Sabella D12, and IFREMER-LOMC. Results from the improved numerical model are compared to laboratory data to quantify any improvements in its predictions. Prediction of optimum turbine performance from the numerical model has improved by an average of 8.1%, to an accuracy of 94.4%, which will directly enhance the design and evaluation of tidal energy converters.
... There have been many laboratory studies into differing aspects of the three flow artefacts discussed above. There have been many articles focused on quantifying and modelling HATT power production under turbulent flow cases with a focus on spectral characterisation and modelling of loading and power production [6,14,15,[18][19][20][21][22][23][24][25][26]. Many laboratory-scale experiments detail the effects of upstream devices both in low turbulence and high turbulence scenarios with results presented in a phenomenological sense [6,14,15,17,27,28]. ...
Tidal devices are likely to faced with shear flows and subjected to various wave climates. The paper presents an experimental study of the combined impacts of shear profile and irregular waves on the loading of a 1/20th scale device operating at peak power extraction. The experiments presented were conducted at various depths to facilitate analysis of the effects of the shear flow and wave impact on the device at various positions in the water column. The fluid field was measured at three different upstream positions and at three depths (top, middle and bottom of the rotor) for each experiment; in doing so, data from the device were captured three times. The fluid measurements were of a high quality and were analysed to present the structure flow upstream of the device, which contained velocity and turbulence profiles. The upstream measurement was utilised to understand the development of flow structures in the approach to the device, and the impact of the flow structures measured was confirmed via cross-covariance calculations. The long datasets gathered were used to produce full rotational probability density functions for the blade-root-bending moments for three blades. The spectral characteristics were also considered, and showed that rotor loading quantities are less reactive to smaller scale flow structures.
... This observation suggests that knowledge of the spectral properties of the turbulent flowfield is not sufficient to predict the spectral properties of the turbine loads for synthetic turbulence. This is similar to observations made of turbulent and turbine power spectra made in flume experiments [32], but there is an important distinction to be drawn. Experimental results indicate that as frequency increases, velocity and power fluctuations become less well correlated and so their spectra differ more significantly at higher frequencies. ...
Turbulence is a crucial flow phenomenon for tidal energy converters (TECs), as it influences both the peak loads they experience and their fatigue life. To best mitigate its effects we must understand both turbulence itself and how it induces loads on TECs. To that end, this paper presents the results of blade element momentum theory (BEMT) simulations of flume-scale TEC models subjected to synthetic turbulent flows. Synthetic turbulence methods produce three-dimensional flowfields from limited data, without solving the equations governing fluid motion. These flowfields are non-physical, but match key statistical properties of real turbulence and are much quicker and computationally cheaper to produce. This study employs two synthetic turbulence generation methods: the synthetic eddy method and the spectral Sandia method. The response of the TECs to the synthetic turbulence is predicted using a robust BEMT model, modified from the classical formulation of BEMT. We show that, for the cases investigated, TEC load variability is lower in stall operation than at higher tip speed ratios. The variability of turbine loads has a straightforward relationship to the turbulence intensity of the inflow. Spectral properties of the velocity field are not fully reflected in the spectra of TEC loads.
... Understanding the impact that the extreme marine environment has on the survivability of tidal energy converters is fundamental for the successful development and commercialisation of full scale devices. Recent efforts from the scientific community to drive the development of tidal technology forward include the estimation of wave loading: for example, Gaurier et al. [13], Galloway et al. [14], Martinez et al. [15], Ordonez-Sanchez et al. [16] and Draycott et al. [17], or turbulence effects: for example, McNaughton et al. [18], Durán Medina et al. [19] and Gaurier et al. [20,21], on tidal energy devices. These investigations cover a wide range of topics ranging from the influences of diverse waveforms, wave directionality, control strategies or turbulent flow characteristics and intensities. ...
This Round Robin Test program aims to establish the influence of the combined wave and current effect on the power capture and performance of a generic tidal turbine prototype. Three facilities offering similar range of experimental conditions have been selected on the basis that their dimensions along with the rotor diameter of the turbine translate into low blockage ratio conditions. The performance of the turbine shows differences between the facilities up to 25% in terms of average power coefficient, depending on the wave and current cases. To prevent the flow velocity increasing these differences, the turbine performance coefficients have been systematically normalized using a time-average disc-integrated velocity, accounting for vertical gradients over the turbine swept area. Differences linked to blockage effects and turbulence characteristics between facilities are both responsible for 5 to 10% of the power coefficient gaps. The intrinsic differences between the tanks play a significant role as well. A first attempt is given to show how the wave-current interaction effects can be responsible for differences in the turbine performance. In these tanks, the simultaneous generation of wave and current is a key part often producing disruptions in both of these flow characteristics.
... of the transportation of the turbulent kinetic energy in the wake of two-dimensional obstacle related to realistic bathymetry variations. The knowledge of the inflow turbulence that will potentially impact a turbine is of great interest for optimizing the turbine performance (Durán Medina et al., 2017). Indeed, the turbulent inflow condition plays a crucial role not only on both fatigue and mean loads acting on hydrodynamics turbine (Mycek et al., 2014) but also on the wake evolution behind the turbine. ...
In high flow velocity areas, turbulence intensity is high and flow variations may have a major impact on tidal turbines behaviour. Previous studies show that a square wall-mounted cylinder produces a very extended and energetic wake. In this work, two-component PIV measurements are conducted in the symmetry plane of the flow in order to evaluate the Turbulent Kinetic Energy (TKE) budget. This analysis enables to show how the TKE, produced in the shear-layer region, is re-distributed and dissipated within the flow. The Large-Eddy PIV method enables to obtain full spatial maps of dissipation and to assess the validity of the constant involved in the spectral method. Results show that the production occurs when the Reynolds shear stress is the more intense, so is the dissipation. Energy is then transported through turbulent motion into the outer flow and swept to the bottom due to the pressure gradient effects. Production is directly due to the periodic vortex shedding unlike for other terms. A better description of the energy exchanges in the turbulent wake flow is obtained. This knowledge is important for the performance determination of a tidal turbine positioned in such an obstacle wake which affects the flow-induced dynamic load on turbine.
... Turbines are deployed in harsh environments where their lifetime is not yet known. In particular, large eddies are expected to be harmful to the tidal turbines because they induce rapid loading fluctuations and generate extreme instantaneous flow velocities, as demonstrated in wave tanks experiments by [Blackmore et al. (2016)], [Medina et al. (2017)] or [Vinod and Banerjee (2019)]. Large-eddy simulation (LES), that resolves the greatest turbulent motions of the flow [Sagaut (2006)], showed that the ambient turbulence strongly a ects the characteristics of the turbine wake [Blackmore et al. (2014)], [Ahmadi (2019)], [Churchfield et al. (2013)], as well as turbine loads [Ahmed et al. (2017)]. ...
Strong tidal power flows are highly turbulent. The role of the seabed roughness on the generation of turbulence is not yet fully understood. Numerical simulation is a promising observation method of turbulent events in three dimensions. In this work, the lattice Boltzmann method is used to simulate turbulent tidal flow at a local scale. The capacity of the method to treat large simulation meshes is exploited to attenuate computational costs. Vertical profiles of velocity and velocity variance are generated from the simulation of a 0.23 km² domain. The simulation is performed with realistic tidal conditions and seabed morphology. Numerical results are in good agreement with in situ acoustic Doppler current profiler measurements. Large turbulent flow structures generated by seabed roughness are observed in the water column through λ2 criterion visualisation.
... These tests have provided valuable data at the small experimental scale and, in particular, provided indications on how current speed and current/wave interaction can affect the power production by the turbine (e.g., [22,25]). Other experimental works highlighted the impact of turbulence on turbine performance (e.g., [1,5,8]). ...
An experiment was performed to study the power production by a Darrieus type turbine of the Dutch company Water2Energy in a tidal estuary. Advanced instrumentation packages, including mechanical sensors, acoustic Doppler current profiler (ADCP), and velocimeter (ADV), were implemented to measure the tidal current velocities in the approaching flow, to estimate the turbine performance and to assess the effect of turbulence on power production. The optimal performance was found to be relatively high (Cp ∼ 0.4). Analysis of the power time history revealed a large increase in magnitude of power fluctuations caused by turbulence as the flow velocity increases between 1 and 1.2 m/s. Turbulence intensity does not alone capture quantitative changes in the turbulent regime of the real flow. The standard deviation of velocity fluctuations was preferred in assessing the effect of turbulence on power production. Assessing the scaling properties of the turbulence, such as dissipation rate, ε, the integral lengthscale, L, helped to understand how the turbulence is spatially organized with respect to turbine dimensions. The magnitude of power fluctuations was found to be proportional to L and the strongest impact of turbulence on power generation is achieved when the size of turbulent eddies matches the turbine size.
... In this kind of area, bathymetric variations are causing a high turbulence rate in the water column (Myers and Bahaj, 2005) and some very energetic turbulent events can rise up to the surface (Best, 2005). Velocity fluctuations can have a major impact on the tidal turbines behaviour, on their production (Duràn Medina et al., 2017;Mycek et al., 2014) and on the structural fatigue (Gaurier et al., 2013). It is therefore essential to characterize this kind of flow. ...
In high flow velocity areas like those suitable for marine energy application, bathymetry variations create strong velocity fluctuations in the water column. It is therefore essential to characterize the turbulence evolution in the wake of seabed elements which may impact the loads on tidal turbines. For that purpose, experiments are carried out in a flume tank with Re as high as achievable in Froude similitude, with bathymetry variations experimentally represented with various wall-mounted square elements of height H: a cylinder or a cube as unitary obstacles and combinations of these elements followed by an inclined floor to resemble smooth bathymetry changes. The onset flow is a simple boundary layer profile with height 1.3 H and a low turbulence intensity. PIV and LDV measurements are used to investigate the wake past all test cases in order to distinguish high floor elevation cases (unitary obstacles) from mean roughness effect (obstacle combinations). Results show that the obstacle combinations produce a wake less extended than for a single wide cylinder that produces an extended wake and very energetic turbulent events. With a single cube, no downstream development of large turbulent events exist and the wake reduces by a factor of 3 compared to the wake cylinder case. An inclined floor downstream of a single wall-mounted obstacle reduces its wake length but does not alter the turbulent structures shed. Turbulent velocity profiles extracted from every wake topology investigated are also compared. The general conclusion is that: for small aspect ratio cases, the obstacle will not affect the water column. On the contrary, strong energetic turbulent events are emitted from large aspect ratio obstacles. Combinations cases stand in-between.
... Some turbulent events can sometimes rise up to the surface and erupt to create a boil. Turbulence can have a major impact on the tidal turbines behaviour, on their production [Dur� an Medina et al. (2017)] and on the structural fatigue [Mycek et al. (2014)]. Before trying to reproduce complex bathymetric structures, we chose to introduce the topic by studying elementary obstacles representative of real life condition: i.e. with an aspect ratio (A R ¼ Width=Height) of the magnitude of the mean bathymetry variations encountered between France and Alderney. ...
In high flow velocity areas like those suitable for tidal applications, turbulence intensity is high and flow variations may have a major impact on tidal turbines behaviour. Large boils that can be observed at the sea surface are emitted from the sea floor and may interact with the tidal turbines. These boils have then to be characterized. The Reynolds number, based on the rugosity height and the mean flow velocity, is rather high in this context:
Re= 2.5 x10^7 . For that purpose, experiments are carried out in a flume tank with Re as high as achievable in Froude similitude (in the tank: Re=2.5x10^5 and Fr= 0.23) in order to study coherent flow structures emitted behind seabed obstacles. The obstacle is here a canonical square wall-mounted cylinder chosen to be representative of specific in-situ bathymetric variations. Using PIV and LDV measurements, the flow past the cylinder is investigated. Using a POD filter, large coherent structures are identified and their trajectories are analysed. By means of a Lamb-Oseen profile approximation, properties of these structures are determined. The formation mechanism of such structures is discussed in this paper and their behaviour is characterized. It is assumed that vortices periodically shed from the obstacle interact and generate hairpin structures.
... The flow regime upstream of the sampling mast of SUSANE (with and without the sampler, Fig. 4b and c) was determined by the use of a 2D Laser Doppler Velocimetry (LDV) that acquires point velocity values (u, v) in the X and Y directions from the mast (Fig. 5b) with a sampling rate of at least 100Hz that detects events of 10µm seeding particles (Duran Medina et al., 2017). ...
In aquatic environments, the benthic water column exhibits strong concentration gradients of various substances. They result from transfers and chemical reactions that may occur both within this layer, and at the sediment–water interface (SWI). Characterization of these gradients yields important information for the quantification of such processes and transfers. However, it is difficult to actually sample these gradients in the field, since turbulence decreases their vertical scale. This article describes a sampler designed to collect simultaneously 16 discrete water column samples at a centimeter‐scale vertical resolution. This small device (40 × 40 × 60 cm) is reliable, safe to handle, and easily deployed from a small boat using a cable or a Scuba diver. It is made of materials compatible with trace element and dissolved gases work, and simultaneously draws samples from various heights above the SWI into 60 mL syringes. The altitude of the sample inlets is field‐adjustable. Sampling artifacts are minimized by in situ flushing of tubing dead volumes, by rapid and simultaneous sample collection, and by sampling an undisturbed water‐column. Thus, this device can contribute to the characterization of vertical concentration gradients in benthic water‐columns. Such gradients of various compounds and metals from two coastal sites (Quiberon Bay and Berre Lagoon) are shown, illustrating the sampler's usefulness to describe and investigate processes in the benthic zone.
--> Susane is available for collaborative work : contact me! <--
... Such variations are causing a high turbulence rate in the water column, where marine current turbines are meant to be installed. Turbulence can have a major impact on the tidal turbines, on their production [1] and on the structural fatigue [2]. Before trying to reproduce complex structures, we chose to introduce the topic by studying elementary obstacles representative of real life condition: i.e. with an aspect ratio of the magnitude of the mean bathymetry variations encountered between France and Alderney. ...
There is a strong tidal energy potential in France, especially in the Alderney Race. In the area of study, where turbines will be installed, bathymetry variations are causing velocity fluctuations with a high turbulence rate in the water column: large coherent turbulent structures can be observed at the sea surface. Such events can have a major impact on the marine tidal turbines behaviour and structural fatigue. To reproduce and analyse these turbulent events, tests are carried out in the wave and current circulating flume tank of IFREMER in Boulogne-sur-Mer. Before trying to reproduce a complex bathymetry, we chose to introduce the topic by studying elementary obstacles representative of real seabed elements (with an aspect ratio of the magnitude of the mean bathymetry variations): a wide square cylinder and an inclined floor. Experiments are carried out with Reynolds number as high as achievable in Froude similitude: Re = 2.5 × 10^5 and Fr = 0.23. The impact of the aspect ratio is studied by comparing results obtained with PIV and LDV measurements on the cube and cylinder cases. The addition of an inclined floor is also investigated. Results show a significant increase of the wake with the aspect ratio. The inclined floor induces a reduction of the shear layer created by the obstacle and modifications on the shedding frequency.
... The results show that by using simultaneously high vellicates of the drag and lift the blades tend to provide the best performance. In [11] a multiscale correlation and turbulence analysis between marine turbine power production and the upstream and downstream of the wind flow has been shown.The study used two simultaneous measurements of the wind flows. An experimental study by using wind tunnels was conductedin [12] in order to find a solution to the unmatched Reynolds numbers for downscaled wind turbine using a laminar and turbulent flow wind turbine single model. ...
In this study, a new investigation is underway with the primary objective of the weather parameters impacton the wind turbine power. Therobust and straightforward dynamic model for accurate predicting wind turbine output power waspresented. The evaluation has been done for thewhole environmental components influencing on the model and output power. The selected wind turbine type (RW-5kW)and the predicted energy has been evaluated based onthe experimental weather data of the city of Hel in the north of Poland. The results show that the air density has a high influence on the wind turbine output power and this value is affected by several environmental factors such as pressure, humidity, and temperature. It turns out that the temperature is the most influential, while the pressure and humidity have a lower effect and if they are not available in measured data during modelling they can create only 0.5% error.
... When it is done for all the oscillations, the high frequency time series are called Intrinsic Mode Functions (IMF) and the trend is called the residual (RE). The input signal is finally written as the sum of all the IMF and the RE [13]. This decomposition is shown on figures 17 and 18 with all the 14 IMF and the RE for F y3 . ...
In highly energetic sites, pebbles and shingles may be suspended in the water column. Positioning obstacles like Marine Current Turbines in such an environment can result in deterioration of the structure by shock or erosion and an increase in the blade loading and fatigue. This paper describes an experimental modelling implemented to study the impacts of such macro-particles on blades. The experimental setup and protocol trials established to monitor the trajectories of macro-particles injected upstream of a 1:20 scale turbine is presented. The particles behaviour during impacts on a classical three-bladed horizontal axis turbine is measured with a 3D tracking motion system. A first evaluation of the impact locations and frequency of occurrence is given. We will be particularly attentive to the increase of the loads applied on the blade during particle impacts.
... The authors also documented a reduction in power coefficient by roughly 10% caused by the ambient turbulence intensity increase from 3% to 15%. This range of turbulence level variation is considered in experimental studies (Mycek et al. 2014a, Medina et al. 2016. ...
A series of tidal turbine tests were conducted in a tidal estuary (the Sea Scheldt, Belgium). Two in-stream vertical axis tidal turbines were tested in real conditions during several weeks in winter 2014 and late summer 2015. Tidal current velocity variations were continuously recorded by one ADCP, operating at 1 Hz and two ADV operating at 16 and 32 Hz. The measurements covered different tidal current regimes: strong flood and ebb flow with velocity above 1.2 m/s and also a flow reversal. Turbulence intensity and scaling properties of the turbulent flow such as dissipation rate (ε), integral scale (L), and Kolmogorov scale (η) were quantified. The coherency spectrum between rotor velocity and output power generated by a Darrieus type turbine was established. The results show high correlation in low frequency range and low correlation in high frequency range. The overall performance of the turbine was assessed by evaluating the power coefficient, Cp. The mean value of Cp was found to be 0.25.
After positioning a 1:20 scaled model of a three-bladed horizontal-axis turbine in the wake of a wall-mounted cylinder, synchronized turbine performance and flow measurements are carried out to investigate the relationship between the incoming flow field and the turbine power fluctuations. The Linear Stochastic Estimation (LSE) is used to predict the turbine output fluctuations from the knowledge of the Large Scale flow Structures (LSS) embedded in the incoming turbulent flow. LSS extraction by Fourier analysis or Proper Orthogonal Decomposition shows that LSS are responsible for the main unsteady variations of the power fluctuations, especially their highest amplitudes. The RMS of turbine output fluctuations are entirely due to the LSS. It is also demonstrated that whatever the nature of the incoming turbulent flow is, the low frequency filtering process coupled with the LSE method allows the recovering of at least 90% of the turbine power RMS. Furthermore, the low-frequency spectral content of the turbine power fluctuations is very well predicted, especially the frequency peaks. A preliminary LSE application is performed in order to predict the instantaneous turbine output fluctuations at more than 85% confidence level, from only three velocity signals measured in front of the turbine.
This chapter has attempted to give an introductory level overview of FSI, it has stated that there are obvious scenarios that require the implementation of FSI, but that there are also other less intuitive situations that have an effect over time and may only be identified via computational analysis or testing. More research is therefore required both in numerical modelling and experimental testing, this is more so the case as HATTs move into the array phase, with the interaction of both wakes and waves for near surface HATTs. For the time being though the computational requirement to closely model wakes and waves while incorporating a FE model along with system coupling remains a challenge. The computational expense remains an obstacle for the initial design phase, but FSI should play a vital role in studying operation of the whole HATT structure if fatigue and life expectances are to be realized.
Dans le contexte actuel du réchauffement climatique, l’extraction de l’énergie des courants marins par des hydroliennes se doit d’intégrer le mix énergétique de demain. Ces machines étant confrontées à un environnement marin difficile, il est nécessaire de connaître leur comportement dans des conditions de fonctionnement réaliste pour garantir une utilisation optimale. Dans ces travaux, les performances d’une hydrolienne sont étudiées, quand la machine est soumise à un courant turbulent ou aux effets combinés de la houle et du courant. La représentation physique des essais à échelle réduite est d’abord abordée, en se focalisant sur la caractérisation de l’écoulement incident et ses effets sur une maquette d’hydrolienne à l’échelle 1/20. La position et le type de mesure de la vitesse amont sont discutés afin de définir avec précision ses performances et sa réponse temporelle et spectrale. Ces éléments sont essentiels dans la définition des standards de certification des modélisations expérimentales. La représentation de variations bathymétriques permet de générer des écoulements turbulents, caractérisés par le passage de structures tourbillonnaires de la taille du rotor. La réponse de la machine est déterminée de manière globale et locale, pour plusieurs positions relatives de la machine par rapport aux obstacles. Les effets combinés de la houle et du courant sur le comportement de la machine sont étudiés à partir de résultats de mesures obtenues dans trois infrastructures d’essais. Des différences significatives entre les bassins apparaissent et une discussion sur leur origine est menée.
The present study experimentally investigated near-bed coherent structures of turbulence over rough surface in the presence of surface waves over the steady current by replicating it in a laboratory flume. The rough surface was modelled by using wooden ribs of square cross section extended across the whole channel width of flume. The instantaneous velocities at different positions in the flow field were measured by using an acoustic Doppler velocimeter. The recorded velocities data were examined to investigate the coherent structures of turbulence near rough surface using the spatio-temporal averaging approach. Pre-multiplied velocity spectra, co-spectral, and coherency were evaluated in frequency domain at different vertical positions under different flow conditions. Length scales were determined to quantify the eddy size within the flow domain, and the anisotropy invariant maps were also obtained to characterize the anisotropic flow under different roughness conditions in wave-current flow cases. Results show increased correlation between velocity fluctuations in the large-scale low-frequency region due to the addition of surface waves, which accounts for the major part of Reynolds stress. Furthermore, turbulence dissipation in the near-bed region was found to be strongly dependent on roughness characteristics.
Flows at tidal-stream energy sites are characterised by high turbulence intensities and by the occurrence of highly energetic large and coherent flow structures. The interaction of the flow with seabed roughness is suspected to play a major role in the generation of such coherent flow structures. The problem is introduced with canonical wall-mounted square obstacles representing abrupt changes of bathymetry, with high Reynolds number flow (Re = 250000). Two methods are used: a numerical model, based on the LBM (Lattice Boltzmann Method) combined with LES (Large Eddy Simulation) and an experimental set-up in a circulating tank. The numerical model is validated by comparison with experimental data. In the case of a wall-mounted square cylinder, large-scale turbulent structures are identified in experiments where boils at the free surface can be observed. LBM simulation allows their three-dimensional characterisation. The dynamic of such large-scale events is investigated by temporal, spatial and spectral numerical analysis. Results show that periodical Kelvin–Helmholtz vortices are emitted in the cylinder wake. Then, they merge to form larger and more coherent structures that rise up to the surface. A wavelet study shows that the emission frequency of the Kelvin–Helmholtz vortices is not constant over time.
An unsteady five-hole probe has been developed for the measurement of turbulent flow in tidal channels. Such measurements are vital for accurate prediction of unsteady loads on tidal turbines. Existing field-based velocimeters are either unable to capture the required range of frequencies or are too expensive to profile the variation of turbulence across a typical tidal power site, and thus the available data is inadequate for turbine design. This work adapts the traditional five-hole wind tunnel probe to achieve a low-cost device with sufficient frequency range for tidal turbine applications. The main issue in the marine environment is that the ambient hydrostatic pressure is much higher than the dynamic pressure. This has been overcome by using novel calibration coefficients and differential transducers. In flume tank tests against laser Doppler velocimeter measurements, the frequency response of the probe has been shown to be sufficient to capture all the frequencies necessary for tidal turbine design.
Flow field results are presented for the near-wake of an axial-flow hydrokinetic turbine in the presence of surface gravity waves. The turbine is a 1/25 scale, 0.8 m diameter, two bladed turbine based on the U.S. Department of Energy's Reference Model 1 tidal current turbine. Measurements were obtained in the large towing tank facility at the U.S. Naval Academy with the turbine towed at a constant carriage speed and a tip speed ratio selected to provide maximum power. The turbine has been shown to be nearly scale independent for these conditions. The selected wave form was intended to represent oceanic swell encountered off the U.S. eastern seaboard. The resulting model wave is a deep water wave, in terms of relative depth, traveling with the “current”, in the opposite direction of the towing carriage. Velocity measurements were obtained using a submersible, planar particle image velocimetry (PIV) system at streamwise distances of up to two diameters downstream of the rotor plane. PIV ensembles were obtained for phase locked conditions with the reference blade at the horizontal position. Phase averaged results for no-wave and wave conditions are presented for comparison showing further expansion of the wake and shear layer in the presence of waves as compared to the no-wave case. When ensembles are selectively sampled on the wave phase, a high degree of coherency is shown to remain and the wake width is shown to undulate with the passing of the wave, with the vertical displacement range on the same order as that of a particle under similar conditions. The impact of waves on turbine tip vortex helical structure is also examined. Waves are shown to change the location of adjacent helices. In the streamwise direction, this changes the pitch of the helix, shown in previous studies to affect the downstream wake recovery distance. In the vertical direction, depending on the wave-induced flow field at the time they were created, vortices are forced outward, into the mean flow or inward, into the wake core, potentially enhancing kinetic energy transport and accelerating the re-energization process.
La thèse présentée dans ce document concerne la caractérisation du comportement d’une hydrolienne à membrane ondulante. Ce dispositif novateur utilise les instabilités de flottement d'une membrane semi-rigide pré-contrainte dans un écoulement pour capter l’énergie des courants marins. \`A partir d’une certaine vitesse critique de l’écoulement, une onde se propage le long de la structure, ce qui actionne des convertisseurs linéaires fixés sur celle-ci. Un modèle expérimental à échelle réduite 1/20ème est développé et testé en bassin d’essai. Le système de conversion est simulé par des amortisseurs hydrauliques. Les mesures de trajectoire et d’efforts permettent d’analyser la dynamique de la membrane pour un grand nombre de configurations et d’aboutir à une étude paramétrique. Des conditions réalistes d’écoulement sont étudiées, notamment l’influence de la direction du courant et celle de la houle sur le fonctionnement du système. La caractérisation du sillage est effectuée par mesures de Particle Image Velocimetry en 2D. Les effets d'échelle sont présentés à travers une comparaison des essais en bassin et en mer.Un modèle analytique et un modèle numérique sont développés et comparés avec les résultats d'expériences. Le modèle analytique linéaire est basé sur la théorie des poutres de Euler-Bernouilli et la théorie des profils minces de Lighthill. Il est résolu dans le domaine fréquentiel et donne de bons résultats en termes de fréquence d’ondulation et de vitesse critique. Le modèle numérique est fondé sur le couplage fort entre un code fluide basé sur la méthode vortex et un code structure utilisant les éléments finis en co-rotationnel. Ce modèle est validé sur un cas expérimental.
Certification can help to reduce perceived risks of marine energy technologies in terms of performance and structural integrity, and thus helps to attract commercial financing and make export easier. At present no international certification scheme for marine energy has been developed and implemented by all main stakeholders in a consistent way, even if Certification Bodies developed their own certification scheme. The implementation schedule of International Standards and Certification Schemes are thus needed to increase the commercial uptake of marine energy technologies. Today, international best practice guidelines for tidal turbine testing are under development by the International Electrotechnical Commission (IEC). In order to improve the work already done and to propose adaptations and enhancement, four experimental trials will be undertaken on different kind of tidal energy devices (fixed and floating horizontal axis turbine as well as undulating membrane) under the Interreg 2 Seas Met-Certified project. The first results obtained under this project in the wave and current flume tank of Ifremer are presented. The experimental setup and protocol trials, taking into account the actual best practices and guidelines, are presented. The behaviour and the performance of a classical three-bladed horizontal axis turbine at a scale of 1:20 are evaluated from the obtained results.
The effects of steady state conditions (e.g. blade pitch, rotor yaw angle, blockage, velocity shear profile) on turbine performance has received much attention over the past few years. However, tidal flows are highly dynamic and their effects on turbine performance and reliability unknown. Previous work has shown how waves and uniform unsteady inflows increase the loadings acting on a turbine, but further investigation is required over a wider range of conditions. This work used static grids to generate turbulence with different turbulence intensities and length scales. Two different turbines were operated in these turbulent flows and their performance monitored and compared. It was found that increasing turbulence intensity reduces power and thrust coefficients by up to 10%, and increases the fluctuations in thrust and torque. Increasing the turbulent length scale showed an increase in power and thrust coefficient, for each turbine, with a significant increase in their fluctuations. Overall, it has been found that turbulence has a significant effect on turbine performance and work is ongoing to consider a broader range of conditions with real site data.
There is growing efforts for the development of wind energy. Yet, as for other renewable resources, a basic characteristic of wind energy is to deliver intermittent power. In this paper, we consider this topic by analyzing data provided by the wind industry. We discuss first the spectral properties of the data and the corresponding power curve. We estimate the scaling behavior in the inertial scale of wind input and the power output. We also study the rotor revolution scaling fluctuations. Finally the wind/power transfer function is studied, together with the wind/rotation and rotation/power intermediary transfer functions.
Time-varying thrust has been measured on a rotor in shallow turbulent flow at laboratory scale. The onset flow has a turbulence intensity of 12% at mid depth and a longitudinal turbulence length scale of half the depth, about 5 times the vertical scale, typical of shallow flows. The rotor is designed to have thrust and power coefficient variations with tip speed ratio close to that of a full-scale turbine. Three extreme probability distributions give similar thrust exceedance values with the Type 1 Pareto in mid range which gives 1:100, 1:1000 and 1:10 000 exceedance thrust forces of 1.38, 1.5 and 1.59 times the mean value. With opposing waves superimposed the extreme thrust distribution has a very similar distribution to the turbulent flow only. Exceedance forces are predicted by superposition of a drag force with drag coefficient of 2.0 based on the wave particle velocity only and with an unchanged mean thrust coefficient of 0.89. These values are relevant for the design of support structures for marine turbines.
Turbulent intermittency plays a fundamental role in fields ranging from combustion physics and chemical engineering to meteorology. There is a rather general agreement that multifractals are being very successful at quantifying this intermittency. However, we argue that cascade processes are the appropriate and necessary physical models to achieve dynamical modeling of turbulent intermittency. We first review some recent developments and point out new directions which overcome either completely or partially the limitations of current cascade models which are static, discrete in scale, acausal, purely phenomenological and lacking in universal features. We review the debate about universality classes for multifractal processes. Using both turbulent velocity and temperature data, we show that the latter are very well fitted by the (strong) universality, and that the recent (weak, log-Poisson) alternative is untenable for both strong and weak events. Using a continuous, space-time anisotropic framework, we then show how to produce a causal stochastic model of intermittent fields and use it to study the predictability of these fields. Finally, by returning to the origins of the turbulent "shell models" and restoring a large number of degrees of freedom (the Scaling Gyroscope Cascade, SGC models) we partially close the gap between the cascades and the dynamical Navier–Stokes equations. Furthermore, we point out that beyond a close agreement between universal parameters of the different modeling approaches and the empirical estimates in turbulence, there is a rather common structure involving both a "renormalized viscosity" and a "renormalized forcing". We conclude that this gives credence to the possibility of deriving analytical/renormalized models of intermittency built on this structure.
A laboratory experiment was performed to study the dynamically rich interaction of a turbulent open channel flow with a bed-mounted axial-flow hydrokinetic turbine. An acoustic Doppler velocimeter and a torque transducer were used to simultaneously measure at high temporal resolution the three velocity components of the flow at various locations upstream of the turbine and in the wake region and turbine power, respectively. Results show that for sufficiently low frequencies the instantaneous power generated by the turbine is modulated by the turbulent structure of the approach flow. The critical frequency above which the response of the turbine is decoupled from the turbulent flow structure is shown to vary linearly with the angular frequency of the rotor. The measurements elucidate the structure of the turbulent turbine wake, which is shown to persist for at least fifteen rotor diameters downstream of the rotor, and a new approach is proposed to quantify the wake recovery, based on the growth of the largest scale motions in the flow. Spectral analysis is employed to demonstrate the dominant effect of the tip vortices in the energy distribution in the near-wake region and uncover meandering motions.
In the marine environment, many fields have fluctuations over a large
range of different spatial and temporal scales. These quantities can be
nonlinear, nonstationary, and often interact with each other. A good
method to study the multiple scale dynamics of such time series, and
their correlations, is needed. In this paper we present an application
of an empirical mode decomposition based time dependent intrinsic
correlation, to two coastal oceanic time series: temperature and
dissolved oxygen (saturation percentage). The two time series are
recorded every 20 minutes during 7 years, from 2004 to 2011. We
illustrate the application of the Empirical Mode Decomposition on such
time series, and estimate the power spectra of the time series using the
Hilbert transform (Hilbert spectral analysis). We find power-law regimes
with slopes of 1.33 for dissolved oxygen and 1.68 for temperature at
high frequencies (between 1.2 and 12 hours) and both close to 1.9 for
lower frequencies (time scales from 2 to 100 days). We consider the time
evolution and scale dependence of cross correlations between both
series. The trends are perfectly anti-correlated. The modes of mean year
2.6 years and 1 year have also negative correlation, whereas higher
frequency modes have a much smaller correlation. The estimation of
time-dependent intrinsic correlations helps to show patterns of
correlations at different scales, for different modes.
The understanding of interaction effects between marine energy converters
represents the next step in the research process that should eventually lead to
the deployment of such devices. Although some a priori considerations have been
suggested recently, very few real condition studies have been carried out
concerning this issue. Trials were run on 1/30th scale models of three-bladed
marine current turbine prototypes in a flume tank. The present work focuses on
the case where a turbine is placed at different locations in the wake of a
first one. The interaction effects in terms of performance and wake of the
second turbine are examined and compared to the results obtained on the case of
one single turbine. Besides, a three-dimensional software, based on a vortex
method is currently being developed, and will be used in the near future to
model more complex layouts. The experimental study shows that the second
turbine is deeply affected by the presence of an upstream device and that a
compromise between individual device performance and inter-device spacing is
necessary. Numerical results show good agreement with the experiment and are
promising for the future modelling of turbine farms.
This study tests the ability of a neutrally buoyant float to estimate the dissipation rate of turbulent kinetic energy from its vertical acceleration spectrum using an inertial subrange method. A Lagrangian float was equipped with a SonTek acoustic Doppler velocimeter (ADV), which measured the vector velocity 1 m below the float's center, and a pressure sensor, which measured the float's depth. Measurements were taken in flows where estimates of varied from 108 to 103 Wk g1. Previous observational and theoretical studies conclude that the Lagrangian acceleration spectrum is white within the inertial subrange with a level proportional to . The size of the Lagrangian float introduces a highly reproducible spectral attenuation at high frequencies. Estimates of the dissipation rate of turbulent kinetic energy using float measurements float were obtained by fitting the observed spectra to a model spectrum that included the attenuation effect. The ADV velocity measurements were converted to a wavenumber spectrum using a variant of Taylor's hypothesis. The spectrum exhibited the expected 5/3 slope within an inertial subrange. The turbulent kinetic energy dissipation rate ADV was computed from the level of this spectrum. These two independent estimates, ADV and float, were highly correlated. The ratio float/ADV deviated from one by less than a factor of 2 over the five decades of measured. This analysis confirms that can be estimated reliably from Lagrangian float acceleration spectra in turbulent flows. For the meter-sized floats used here, the size of the float and the noise level of the pressure measurements sets a lower limit of float 10 8 Wk g1.
Field measurements of turbulence are presented from two sites in Puget Sound, WA, that are proposed for electrical power generation using tidal current turbines. Time series data from multiple acoustic Doppler instruments are analyzed to obtain statistical measures of fluctuations in both the magnitude and direction of the tidal currents. The resulting turbulence intensities (i.e., the turbulent velocity fluctuations normalized by the deterministic tidal currents) are typically 10% at the hub heights (i.e., the relevant depth) of the proposed turbines. Length and time scales of the turbulence are also analyzed. Large-scale, anisotropic eddies dominate the turbulent kinetic energy (TKE) spectra, which may be the result of proximity to headlands at each site. At small scales, an isotropic turbulent cascade is observed and used to estimate the dissipation rate of TKE, which is shown to balance with shear production. Data quality and sampling parameters are discussed, with an emphasis on the removal of Doppler noise from turbulence statistics. The results are relevant to estimating the performance and fatigue of tidal turbines.
Wind turbines generate electricity from turbulent wind. Large fluctuations, and, more importantly, frequent wind gusts cause a highly fluctuating electrical power feed into the grid. Such effects are the hallmark of high-frequency turbulence. Here we show evidence that it is the complex structure of turbulence that dominates the power output for one single wind turbine as well as for an entire wind farm. We illustrate the highly intermittent, peaked nature of wind power fed into the grid. Multifractal scaling is observed, as described initially by Kolmogorov's 1962 theory of turbulence. In parallel, we propose a stochastic model that converts wind speed signals into power output signals with appropriate multifractal statistics. As more and more wind turbines become integrated into our electric grids, a proper understanding of this intermittent power source must be worked out to ensure grid stability in future networks. Thus, our results stress the need for a profound understanding of the physics of turbulence and its impact on wind energy.
This paper analyses a set of velocity time histories which were obtained at a fixed point in the bottom boundary layer of a tidal stream, 5 m from the seabed, and where the mean flow reached 2.5 m s(-1). Considering two complete tidal cycles near spring tide, the streamwise turbulence intensity during non-slack flow was found to be approximately 12-13%, varying slightly between flood and ebb tides. The ratio of the streamwise turbulence intensity to that of the transverse and vertical intensities is typically 1 : 0.75 : 0.56, respectively. Velocity autospectra computed near maximum flood tidal flow conditions exhibit an f(-2/3) inertial subrange and conform reasonably well to atmospheric turbulence spectral models. Local isotropy is observed between the streamwise and transverse spectra at reduced frequencies of f>0.5. The streamwise integral time scales and length scales of turbulence at maximum flow are approximately 6 s and 11-14 m, respectively, and exhibit a relatively large degree of scatter. They are also typically much greater in magnitude than the transverse and vertical components. The findings are intended to increase the levels of confidence within the tidal energy industry of the characteristics of the higher frequency components of the onset flow, and subsequently lead to more realistic performance and loading predictions.
The power spectral density of the output of wind turbines provides information on the character of fluctuations in turbine output. Here both 1-second and 1-hour samples are used to estimate the power spectrum of several wind farms. The measured output power is found to follow a Kolmogorov spectrum over more than four orders of magnitude, from 30 s to 2.6 days. This result is in sharp contrast to the only previous study covering long time periods, published 50 years ago. The spectrum defines the character of fill-in power that must be provided to compensate for wind's fluctuations when wind is deployed at large scale. Installing enough linear ramp rate generation (such as a gas generator) to fill in fast fluctuations with amplitudes of 1% of the maximum fluctuation would oversize the fill-in generation capacity by a factor of two for slower fluctuations, greatly increasing capital costs. A wind system that incorporates batteries, fuel cells, supercapacitors, or other fast-ramp-rate energy storage systems would match fluctuations much better, and can provide an economic route for deployment of energy storage systems when renewable portfolio standards require large amounts of intermittent renewable generating sources.
A Time-Dependent Intrinsic Correlation (TDIC) method is introduced. This new approach includes both auto- and cross-correlation analysis designed especially to analyze, capture and track the local correlations between nonlinear and nonstationary time series pairs. The approach is based on Empirical Mode Decomposition (EMD) to decompose the nonlinear and nonstationary data into their intrinsic mode functions (IMFs) and uses the instantaneous periods of the IMFs to determine a set of the sliding window sizes for the computation of the running correlation coefficients for multi-scale data. This new method treats the selection of the sliding window sizes as an adaptive process determined by the data itself, not a "tuning" process. Therefore, it gives an intrinsic correlation analysis of the data. Furthermore, the multi-window approach makes the new method applicable to complicated data from multi-scale phenomena. The synthetic and time series from real world are used to demonstrate conclusively that the new approach is far more superior over the traditional method in its ability to reveal detailed and subtle correlations unavailable through any other methods in existence. Thus, the TDIC represents a major advance in statistical analysis of data from nonlinear and nonstationary processes.
We relate the second-order structure function of a time series with the power spectrum of the original variable, taking an assumption of statistical stationarity. With this approach, we find that the structure function is strongly influenced by the large scales. The large-scale contribution and the contribution range are, respectively, 79% and 1.4 decades for a Kolmogorov -5/3 power spectrum. We show numerically that a single scale influence range, over smaller scales is about 2 decades. We argue that the structure function is not a good method to extract the scaling exponents when the data possess large energetic scales. An alternative methodology, the arbitrary order Hilbert spectral analysis which may constrain this influence within 0.3 decade, is proposed to characterize the scaling property directly in an amplitude-frequency space. An analysis of passive scalar (temperature) turbulence time series is presented to show the influence of large-scale structures in real turbulence and the efficiency of the Hilbert-based methodology. The corresponding scaling exponents ζ(θ)(q) provided by the Hilbert-based approach indicate that the passive scalar turbulence field may be less intermittent than what was previously believed.
Multi-scale systems, involving complex interacting processes that occur over a range of temporal and spatial scales, are present in a broad range of disciplines. Several methodologies exist to retrieve this multi-scale information from a given time series; however, each method has its own limitations. This book presents the mathematical theory behind the stochastic analysis of scaling time series, including a general historical introduction to the problem of intermittency in turbulence, as well as how to implement this analysis for a range of different applications. Covering a variety of statistical methods, such as Fourier analysis and wavelet transforms, it provides readers with a thorough understanding of the techniques and when to apply them. New techniques to analyse stochastic processes, including empirical mode decomposition, are also explored. Case studies, in turbulence and ocean sciences, are used to demonstrate how these statistical methods can be applied in practice, for students and researchers.
Laboratory scale testing of tidal turbines has generated valuable datasets to support optimised turbine design and numerical model validation. However, tidal sites are highly turbulent with a broad range of length scales and turbulence intensities that are site specific. In this work we describe an experimental campaign using static grids to generate turbulence and investigate its impact on a model tidal turbine in a circulating water flume. Length scales, energy spectra and turbulence dissipation rates are first considered for centre point measurements before full flow characterisation of the ambient conditions across the turbine rotor area. Six different cases were chosen to observe the performance of a 1/20th scale 0.8 m diameter turbine subjected to these flows. The rotor thrust and torque, and flapwise and edgewise blade root bending moments were measured. It was found that the thrust and power coefficients were sensitive to the estimate of ambient velocity. In the most extreme case the Betz limit could be ‘exceeded’ depending on which estimate of ambient velocity was used. Overall variations in the peak power coefficient of over 10% were observed, demonstrating the significance turbulence has on turbine performance. It was also found that there is a strong correlation between fluctuations in blade root bending moments and the rotor loads. As a result we proposed that fatigue loads acting on the blades may be estimated from the fluctuations in power output of the turbine. Therefore maintenance operations maybe optimised from real-time fatigue monitoring of blade loads without the need to install additional instrumentation on the turbine blades. Under this proposed regime the cost of energy will be reduced due to reductions in turbine costs and following optimisation of the maintenance requirements and operational costs. This could also improve turbine reliability which would have significant implications for large multi turbine arrays.
High-frequency temperature fluctuations recorded in the English Channel are compared using two long-term autonomous underwater monitoring stations at less than 20-min time resolution. Measurements were taken from 2005 to 2011 from two systems 460 km apart in the western and eastern parts of the English Channel. Spectral analysis reveals similar statistical behaviors, with approximate 5/3 spectra and several forcing frequencies in relation to tidal and daily cycles. A co-spectra study reveals a transition scale of 11 days. The influence of this scale is also visible though Time-Dependent Intrinsic Correlation analysis (TDIC)–a recently introduced cross-correlation analysis based on Empirical mode decomposition. This helps to spatialize high-frequency temporal records at a fixed location.
We survey the newly developed Hilbert spectral analysis method and its applications to Stokes waves, nonlinear wave evolution processes, the spectral form of the random wave field, and turbulence. Our emphasis is on the inadequacy of presently available methods in nonlinear and nonstationary data analysis. Hilbert spectral analysis is here proposed as an alternative. This new method provides not only a more precise definition of particular events in time-frequency space than wavelet analysis, but also more physically meaningful interpretations of the underlying dynamic processes.
One key step of the industrial development of a tidal energy device is the testing of scale prototype devices within a controlled laboratory environment. At present, there is no available experimental protocol which addresses in a quantitative manner the differences which can be expected between results obtained from the different types of facilities currently employed for this type of testing. As a consequence, where differences between results are found it has been difficult to confirm the extent to which these differences relate to the device performance or to the test facility type.
The impact of tidal current, waves, and turbulence on particles re-suspension over the sea bottom is studied through eulerian high frequency measurements of velocity and particle size distribution (PSD) during 5 tidal cycles (65 hours) in a coastal environment of the eastern English Channel. High frequency variability of PSD is observed along with the velocity fluctuations. Power spectral analysis shows that turbulent velocity and PSD parameters have similarities in their spectral behaviour over the whole range of examined temporal scales. The low frequency variability of particles is controlled by turbulence (β≃−5/3)(β≃−5/3) and the high frequency is partly driven by dynamical processes impacted by the sea bottom interactions with turbulence (wall turbulence). Stokes number (St), rarely measured in situ, exhibits very low values, emphasizing that these particles can be considered as passive tracers. The effect of tide and waves on turbidity and PSD is highlighted. During slack tide, when the current reaches its minimum value, we observe a higher proportion of small particles compared to larger ones. To a lower extent, high significant wave heights are also associated with a greater concentration of suspended sediments and the presence of larger particles (larger Sauter's diameter DA, and lower PSD slope ξ).
Shellfish farming, particularly oyster farms, suffers from strong siltation phenomena which are harmful to the production of shellfish. The lack of knowledge about the impact of an oyster farm on the wave propagation and on the flow remains a significant difficulty for the comprehension of sediment transport processes in coastal zones. These effects are one of the possible sources of sedimentation. The study presented here focuses on describing finely hydrodynamical phenomena (velocity fields, turbulence parameters) around oyster tables. The analysis is based on experimental trials carried out in a flume tank on reduced models of oyster tables. Experiments highlight the flow perturbations in the near field and constitute a database for validation of numerical models developed at the same time. The model based on Navier–Stokes equations offers the possibility to study the impact of more realistic table lengths on the flow and to simulate the perturbation produced by a group of oyster tables. Both experimental and numerical results are presented in this paper.
Seven sets of 2D particle image velocimetry data obtained in the bottom boundary layer of the coastal ocean along the South Carolina and Georgia coast [at the South Atlantic Bight Synoptic Offshore Observational Network (SABSOON) site] are examined, covering the accelerating and decelerating phases of a single tidal cycle at several heights above the seabed. Additional datasets from a previous deployment are also included in the analysis. The mean velocity profiles are logarithmic, and the vertical distribution of Reynolds stresses normalized by the square of the free stream velocity collapse well for data obtained at the same elevation but at different phases of the tidal cycle. The magnitudes of <u'u'>, <w'w'>, and -<u'w'> decrease with height above bottom in the 25-160-cm elevation range and are consistent with the magnitudes and trends observed in laboratory turbulent boundary layers. If a constant stress layer exists, it is located below 25-cm elevation. Two methods for estimating dissipation rate are compared. The first, a direct estimate, is based on the measured in-plane instantaneous velocity gradients. The second method is based on fitting the resolved part of the dissipation spectrum to the universal dissipation spectrum available in Gargett et al. Being undervalued, the direct estimates are a factor of 2-2.5 smaller than the spectrum-based estimates. Taylor microscale Reynolds numbers for the present analysis range from 24 to 665. Anisotropy is present at all resolved scales. At the transition between inertial and dissipation range the longitudinal spectra exhibit a flatter than -5/3 slope and form spectral bumps. Second-order statistics of the velocity gradients show a tendency toward isotropy with increasing Reynolds number. Dissipation exceeds production at all measurement heights, but the difference varies with elevation. Close to the bottom, the production is 40%-70% of the dissipation, but it decreases to 10%-30% for elevations greater than 80 cm.
To test the local isotropy predictions of Kolmogorov's (1941) universal equilibrium theory, hot-wire measurements of the velocity fluctuations were made in the test section-ceiling boundary layer of the 80×120 foot Full-Scale Aerodynamics Facility at NASA Ames Research Center. The boundary layer develops over a rough surface, but the Reynolds stress profiles agree with canonical data sufficiently well. Spectral and structure-function relations for isotropic turbulence were used to test the local-isotropy hypothesis.
The Hilbert transform of a real-valued time-domain signal x(t) is another real-valued time-domain signal, denoted by x(t), such that z(t) = x(t) + jx(t) is an analytic signal. Like Fourier transforms, Hilbert transforms are linear operators. This chapter presents three equivalent mathematical definitions for the Hilbert transform, followed by examples and basic properties. The intrinsic nature of the Hilbert transform to causal functions and physically realizable systems is also shown. The chapter derives special formulas for the Hilbert transform of correlation functions and their envelopes. It outlines applications for both nondispersive and dispersive propagation. The computation of two envelope signals is discussed, and this is followed by correlation of the envelope signals. The Hilbert transform for general records can be defined in three ways namely, definition as convolution integrals, definition as phase shift system, and definition as imaginary part of analytic signal.
The dissipation of turbulent kinetic energy has been increasingly used as a scaling parameter to integrate microbiological accrual and metabolic rates with fluid-flow motion in natural and engineered aquatic ecosystems. The estimation of turbulent kinetic energy under field conditions and the generation of energy dissipation rates under controlled laboratory conditions with microbiological organisms are necessities required to integrate environmental/ecological laboratory protocols with a moving fluid in the environment. Turbulent fluid-flow conditions were generated in an oscillating grid setup, and turbulence variables were quantified using laser-Doppler velocimetry (LDV) and particle image velocimetry (PIV) measuring techniques. The rate of dissipation of the turbulent kinetic energy in the setup ranged from 10−9 to 10−4 m2/s3 and was similar to the levels of energy dissipation commonly reported in engineered and natural aquatic ecosystems. Time-averaged velocities were close to zero with the root-mean-square velocity ratios about 1, indicating nearly isotropic fluid-flow conditions in the setup. The velocity spectra, obtained by stationary LDV measurements for the vertical and horizontal velocity components across the setup revealed the existence of inertial subrange with the frequency power scaling law of “ω
−5/3.” The estimated Eulerian frequency spectrum followed the theoretical functional relation and confirmed the applicability of inertial dissipation method for the estimation of turbulent kinetic energy dissipation rates. PIV was used for a direct estimation of dissipation by evaluating spatially distributed velocity gradients. The direct dissipation estimate in conjunction with the estimated Eulerian frequency spectrum provided evaluation of a “universal” constant, α, commonly used for the estimation of an energy dissipation rate over the inertial subrange of the Eulerian spectrum. The results demonstrated a range of values, rather than a universal constant, of α with a lognormal probability distribution for vertical and horizontal velocity components. In order to encompass a 0.955 probability range under the lognormal distribution the universal constant, α, should be in the range 2.91 ≥ α
u
≥ 0.43 and 4.44 ≥ α
w
≥ 0.42 for horizontal and vertical velocity components, respectively.
Experimental results of tests carried out to investigate the hydrodynamics of marine current turbines are presented. The objective is to build an experimental database in order to validate the numerical developments conducted to characterise the flow perturbations induced by marine current turbines. For that purpose, we used a tri-bladed horizontal axis turbine. The work is dedicated to measuring the behaviour of the system and to characterising the wake generated by the turbine. The efficiency of the device is quantified by the measurement of the thrust and the amount of power generated by the rotor for various inflow conditions, whereas the wake is characterised by Laser Doppler Velocimetry. Particular attention is paid to the flow characteristic effects on the performance of a 0.70 m diameter turbine. The load predictions on the structure and the measured performance of the turbine over its working range of currents and rotational speeds are presented. The results showed that this kind of turbine is sensitive to the quality of the incoming flow. The turbulence intensity effects on turbine behaviour and on its wake are also characterised in order to study how the far wake decays downstream and to estimate the effect produced in downstream turbines.
Energy from marine currents offers the promise of regular and predictable electrical generation at higher power densities than other renewables. The marine current resource is potentially large but mainly concentrated in a number of sites around the world. The power density for a horizontal axis turbine operating in such currents has a similar form to that of a wind turbine and is dependent on the cube of the velocity and the fluid density which for water is about 1000 times that of air. These two factors imply that the power density for marine current energy converters will be appreciably higher than that of wind generators resulting in smaller and hence more manageable size turbines.
Hilbert-Huang transform is a method that has been introduced recently to decompose nonlinear, nonstationary time series into a sum of different modes, each one having a characteristic frequency. Here we show the first successful application of this approach to homogeneous turbulence time series. We associate each mode to dissipation, inertial range and integral scales. We then generalize this approach in order to characterize the scaling intermittency of turbulence in the inertial range, in an amplitude-frequency space. The new method is first validated using fractional Brownian motion simulations. We then obtain a 2D amplitude-frequency representation of the pdf of turbulent fluctuations with a scaling trend, and we show how multifractal exponents can be retrieved using this approach. We also find that the log-Poisson distribution fits the velocity amplitude pdf better than the lognormal distribution.
We survey the newly developed Hilbert spectral analysis method and its applications to Stokes waves, nonlinear wave evolution processes, the spectral form of the random wave field, and turbulence. Our emphasis is on the inadequacy of presently available methods in nonlinear and nonstationary data analysis. Hilbert spectral analysis is here proposed as an alternative. This new method provides not only a more precise definition of particular events in time-frequency space than wavelet analysis, but also more physically meaningful interpretations of the underlying dynamic processes.
A syntheticeddy-method to represent the ambient turbulence in numerical simulation of marine current turbine
- C Carlier
- G Pinon
- B Gaurier
- G Germain
C. Carlier, G. Pinon, B. Gaurier, G. Germain, and Rivoalen E. A syntheticeddy-method to represent the ambient turbulence in numerical simulation
of marine current turbine. In Proceedings of the 11th European Wave and
Tidal Energy Conference (EWTEC), 2015.
Distribution of energy spectra, reynolds stresses
- L Luznik
- W Zhu
- R Gurka
- J Katz
- W A M Nimmo
- T R Smith
- Osborn
L. Luznik, W. Zhu, R. Gurka, J. Katz, W.A.M. Nimmo Smith, and T. R.
Osborn. Distribution of energy spectra, reynolds stresses, turbulence pro-720
duction, and dissipation in a tidally driven bottom boundary layer
duction, and dissipation in a tidally driven bottom boundary layer. Journal
of Physical Oceanography, 37(6):1527-1550, 2007.