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Dynamic stall on both horizontal axis and vertical axis wind turbine blades is accompanied by simultaneous changes in pitch and surge, but this simultaneous effect has never been documented. Using a unique unsteady wind tunnel, synchronous oscillations in angle of attack and flow speed were considered on two prototypical wind turbine blades. At a steady freestream, the concept of matched pitch rate was observed to be valid for large positive and negative pitch angles. In the presence of an unsteady stream, matching the flow speed as well as the pitch angle and its time derivative during pitch-up produced excellent correspondence between lift, drag and moment coefficients throughout the entire dynamic stall event.

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... The objective of this work is to explore the way in which rapid freestream oscillations, or periodic surging, affect the dynamic stall mechanism. In a previous study, the matched pitch rate concept introduced by McCroskey et al. [27] was extended to synchronous variations in freestream velocity and angle of attack [28]. The experimental results indicated that the timing of the shedding of the leading-edge dynamic stall vortex (DSV) and the amplitude of the associated load excursions were primarily determined by the inflow conditions just before separation. ...

... This is illustrated in Fig. 5, where it can be seen that at ϕ 45°, the flow speed attains its minimum value of U ∞ 5.6 m∕s τ 135°, its mean value of U ∞ 11.1 m∕s (τ 45°and τ 225°), or its maximum value of U ∞ 16.7 m∕s τ 315°, respectively. One scenario of practical relevance (τ 225°) relates to the idealized angle of attack and relative velocity magnitude profiles (V rel ) experienced by the blades of a VAWT operating at a tip-speed ratio of TSR 2 [28]. This is clearly a simplification because, although the turbine blade peak angles of attack are 30°with a 50% relative velocity amplitude, neither are harmonic. ...

Dynamic stall often occurs under conditions of simultaneous unsteady pitching and surging (e.g., rotorcraft and wind turbines), but many models employ a dimensionless time base that implicitly assumes that surging is superimposed, in a quasi-steady manner, on dynamic pitching. An unsteady wind tunnel was used to examine this assumption, where a technique was developed to quantify the unsteady effects of surging on a pitching NACA 0018 airfoil. The technique involved performing multiple harmonic pitching experiments under nominally steady freestream conditions that bracketed a corresponding 50% surging amplitude (125k ≤ Re ≤ 375k). By interpolating these data, unsteady-pitching/quasi-steady-surging data sets were constructed and compared with de facto synchronous pitch and surging experiments, thereby isolating the unsteady effects of surging on a pitching airfoil. Both large and small poststall maximum angles of attack (stall angle + 5°and stall angle + 15°) were considered at multiple pitch-surge phase differences. During deep dynamic stall (stall angle + 15°), with large-scale separation, surging was seen to have a secondary effect on the unsteady aerodynamics. However, at small poststall maximum angles of attack (stall angle + 5°), either light or deep dynamic stall behavior was observed depending upon the pitch-surge phase difference. This was attributed to Reynolds number history effects, exemplified by boundary-layer transition, and thus it can be referred to as "transitional" dynamic stall.

... If the angle of attack is increased fast enough to an angle above the static stall limit, flow will separate from the leading edge. [23][24][25][26][27][28][29] It thus seems natural to search for the indication of massive dynamic flow separation near the leading edge. As early as 1959, Evans and Mort 30 presented evidence of a correlation between the minimum value of the leading edge surface pressure coefficient and the steepness of the adverse pressure gradient at stall. ...

... The latter is defined as the dimensionless pitch rate when the static stall angle of attack is exceeded. It was found to be a suitable scaling parameter for the timing of stall onset by Mulleners and Raffel 14 and Müller-Vahl et al. 28 All curves are aligned in time with respect to the time when the static stall angle is exceeded, which is considered the start of the dynamic stall development stage. The lines are solid up to the occurrence of dynamic stall and dashed thereafter. ...

The dynamic stall development on a pitching airfoil at Re = 10 6 was investigated by time-resolved surface pressure and velocity field measurements. Two stages were identified in the dynamic stall development based on the shear layer evolution. In the first stage, the flow detaches from the trailing edge and the separation point moves gradually upstream. The second stage is characterized by the roll up of the shear layer into a large scale dynamic stall vortex. The two-stage dynamic stall development was independently confirmed by global velocity field and local surface pressure measurements around the leading edge. The leading edge pressure signals were combined into a single leading edge suction parameter. We developed an improved model of the leading edge suction parameter based on thin airfoil theory that links the evolution of the leading edge suction and the shear layer growth during stall development. The shear layer development leads to a change in the effective camber and the effective angle of attack. By taking into account this twofold influence, the model accurately predicts the value and timing of the maximum leading edge suction on a pitching airfoil. The evolution of the experimentally obtained leading edge suction was further analyzed for various sinusoidal motions revealing an increase in the critical value of the leading edge suction parameter with increasing pitch unsteadiness. The characteristic dynamic stall delay decreases with increasing unsteadiness, and the dynamic stall onset is best assessed by critical values of the circulation and the shear layer height which are motion independent. Published under license by AIP Publishing. https://doi.org/10.1063/1.5121312 ., s

... While VAWTs have shown appreciable performance, specifically under heavy gusts, the power coefficient, C P , associated with VAWTs is comparatively less than that of HAWTs [19][20][21][22][23][24], due to the complex, unsteady flow associated with VAWTs. Dynamic stall, blade-wake interaction, blade-shaft wake interaction, rotational flow, and flow curvatures are a few of the complex flow phenomena associated with VAWT aerodynamics [25][26][27][28]. The aforementioned flow phenomena are known to deteriorate the performance of VAWTs. ...

In recent days, enhancement of Vertical Axis Wind Turbines (VAWTs) by mitigating flow deteriorating effects like dynamic stalling, unsteady wake is given great importance. The following article focuses on implementing four different tubercles on the blades’ leading edge and studying its performance and flow characteristics using CFD techniques. Results indicate that the addition of tubercles generated counter-rotating vortices and delayed flow separation and helped control dynamic stalling. Between azimuth angles 70°–160°, the flow was seen to separate only along the trough regions of the blade and remained attached along the peak regions, thus providing more torque and power. In addition to the enhancements in the flow characteristics, a 28% increase in power coefficient was observed for the optimal configuration at the optimal tip speed ratio. Additionally, a 14% increase in maximum lift generated by the blade was observed. Preliminary aeroacoustics analysis revealed a 12% and 20% decrease in the noise emissions along the blade tip and mid-plane of the turbine, respectively. Hence, it can be shown that tubercles effectively control dynamic stall, reduce noise emissions, and increase the power output of VAWTs.

... For the impact of amplitude α 1 , Amandolese [6] shows that the flow regime changes from deep stalled to attached flow depending on the amplitude of the motion for a mean angle beyond α s , resulting in very different force coefficients for the same angle of attack. McCroskey [126] and later Müller-Vahl [142] develop the matched pitched rated concept. If α(t) matches for values above α s (which means that the pitching rate is the same) for specific combinations of α 1 ,α 2 and k, then the force coefficient matches too. ...

The size of modern offshore wind turbine rotors has reached very large dimensions and keeps increasing in order to reduce the cost of electricity. More challenging designs are thus needed to improve the aerodynamic performances and reduce the structural loads. The state-of-the-art tools such as Blade Element Momentum Theory (BEMT) used to predict the loads and performances of wind turbines have been designed for much smaller rotors in standard operating conditions. Load cases in specific conditions such as yaw misalignment are a priori out of the validity range for such tools. The goal of the thesis is to investigate more advanced aerodynamic models in order to assess the differences in load predictions compared to state-of-the-art tools. In particular, this work focuses on unsteady flows which represent a challenge for engineering tools. For this purpose, a panel method code including viscous effects such as dynamic stall is compared to a BEMT code in realistic wind conditions with large yaw misalignment. The calculations are performed in the framework of aero-servo-elasto coupling in order to be represen¬tative of the load calculations performed in industry following certification standards. The impact of the dynamic stall model is investigated in particular for both BEMT and panel method, for extreme and fatigue loading in cases of yaw misalignment. Differences have been observed between both codes and for several parametrizations of dynamic stall model. In addition, it has been noticed that including the servo-elasto coupling changes a lot the observations regarding aerodynamic loading. Large angles of at¬tack are observed on wind turbine blades in yaw misalignment cases, and the flow around blade sections in such conditions is particularly affected by viscous effects such as dynamic stall or vortex shedding which are not inherently solved by panel methods nor BEMT but modeled with semi-empirical models. Alternative models such as Large Eddy Simulation (LES) that would capture these effects have to be considered. Wall-modeled LES (WMLES) is thus used in the second part of this thesis to investigate the flow around wind turbine dedicated airfoils, much thicker than airfoils used in aeronautics. Several cases are simulated, for attached and detached flows and in steady or oscillating cases. Angles of attack up to 90° are investigated at realistic Reynolds number. It appears that WMLES is able to capture correctly the main flow features in attached conditions and at very high angle of attack with coarse meshes. However, the near stall cases are more challenging to capture even with appropriate wall laws and require very fine meshes to be correctly solved. A comparison is also performed for motions with high reduced frequency and compared to other models, revealing the promising capacities of WMLES in such cases.

... A full description of dynamic stall would be extraneous here but excellent reviews can be found in McAlister et al. (1978), McCroskey (1982), McCroskey (1981, Leishman (2002) and Carr (1987). More modern experimental works can be found in Granlund et al. (2014), Mulleners and Raffel (2013), , ), Müller-Vahl et al. (2017, Strangfeld et al. (2015), Balduzzi et al. (2019), andHolst et al. (2019). For the discussion here, it is sufficient to note that as the strength and phase of the leading vortex varies, so will the aeroelastic stability. ...

Once stall has set in, lift collapses, drag increases and then both of these forces will fluctuate strongly. The result is higher fatigue loads and lower energy yield. In dynamic stall, separation first develops from the trailing edge up the leading edge. Eventually the shear layer rolls up, and then a coherent vortex forms and then sheds downstream with its low-pressure core causing a lift overshoot and moment drop. When 50+ experimental cycles of lift or pressure values are averaged, this process appears clear and coherent in flow visualizations. Unfortunately, stall is not one clean process but a broad collection of processes. This means that the analysis of separated flows should be able to detect outliers and analyze cycle-to-cycle variations. Modern data science and machine learning can be used to treat separated flows. In this study, a clustering method based on dynamic time warping is used to find different shedding behaviors. This method captures the fact that secondary and tertiary vorticity vary strongly, and in static stall with surging flow the flow can occasionally reattach. A convolutional neural network was used to extract dynamic stall vorticity convection speeds and phases from pressure data. Finally, bootstrapping was used to provide best practices regarding the number of experimental repetitions required to ensure experimental convergence.

... Fluctuations are conspicuously observed in the unsteady aerodynamic forces, which are attributed to the shedding of the USV and TEV and the formation of the dynamic stall vortex (DSV), which induces variations in the C p curve, as shown in Fig. 16d. In contrast to the airfoil with a thin or moderate leading edge [39,40], the DSV forms initially at the aft surface, which was called aft DSV (ADSV) [42]. ...

... Dynamic stall is of high relevance to several applications, such as helicopters [19][20][21][22][23][24], airplanes [11,12,25], micro-air vehicles [25,26], flapping wings [13,27], horizontal axis wind turbines (HAWTs) [28][29][30][31][32][33][34][35][36][37] and vertical axis wind turbines (VAWTs) [15,[38][39][40][41][42][43][44][45][46][47][48][49][50][51]. It should be noted that in the vast majority of these applications, dynamic stall occurs in a non-oscillating inflow condition. ...

The Scale-Adaptive Simulation (SAS) approach has emerged as an improved unsteady Reynolds-Averaged Navier-Stokes (URANS) formulation to bridge the gap between the less accurate commonly used URANS and the computationally expensive hybrid RANS/LES for highly separated unsteady flows, e.g. dynamic stall. However, while the SAS has been successfully used at several occasions, it has not yet been tested for the complex case of dynamic stall. Therefore, the present study analyzes the SAS predictions of dynamic stall on a vertical axis wind turbine at a chord Reynolds number of 5 × 10 4 and a reduced frequency of 0.125. The analysis is based on comparison of the SAS predictions of the blade aerodynamics and the turbine power performance against the corresponding URANS and hybrid RANS/LES predictions. The results show that the SAS predictions are closer to hybrid RANS/LES than URANS with respect to: (i) the instant of the bursting of the laminar separation bubble (LSB), the leading-edge suction collapse, the formation of the dynamic stall vortex (DSV) and the trailing-edge vortex (TEV) and the shedding of the TEV; (ii) the size and strength of the TEV; (iii) the DSV-TEV interaction; (iv) the drag prediction during the downstroke. On the other hand, both URANS and SAS fail to corroborate with hybrid RANS/LES with respect to: (i) the instant of the formation of the LSB and the shedding of the DSV (the stall angle); (ii) the drag jump at the stall angle; (iii) the lift values during the downstroke; and (iv) the chordwise extent of the LSB.

... Active-stall-based systems typically employ a mechanical element (e.g., springs) that causes the blades to rotate towards the plane of rotation with increasing wind speed. This phenomenon causes the pitch angle to increase negatively as there is a decrease in the lift coefficient as well as in the extracted power [24]. Fig. 7 presents the major differences among the aforementioned techniques. ...

... The corrections are implemented using a modified Beddoes-Leishmann model for wind turbines [39e41]), and thus capture the oscillations due to the unsteady inflow and/or fluctuations due to turbulence affecting the unsteady blade section loads. It is noted that recent studies [42,43] have suggested that a more accurate estimation of unsteady loads due to dynamic stall should consider synchronous oscillations in angle of attack and inflow velocity, even though it could be also be argued that quite complex dynamic stall models already exist [44] and that future efforts should address the coupling of the different aerodynamic models instead [45]. Nacelle and tower drag: aerodynamic drag for the tower and nacelle are calculated based on solution of potential flow. ...

Unsteady loads are a major limiting factor for further upscaling of HAWTs considering the high costs associated to strict structural requirements. Alleviation of these unsteady loads on HAWT blades, e.g. using active flow control (AFC), is of high importance. In order to devise effective AFC methods, the unsteady loading sources need to be identified and their relative contribution to the load fluctuations experienced by blades needs to be quantified. The current study investigates the effects of various atmospheric and operational parameters on the fluctuations of α and for a large HAWT. The investigated parameters include turbulence, wind shear, yawed inflow, tower shadow, gravity and rotational imbalances. The study uses the DTU's aeroelastic software HAWC2. The study identifies the individual and the aggregate effect of each source on the aforementioned fluctuations in order to distinguish the major contributing factors to unsteady loading. The quantification of contribution of each source on the total fatigue loads reveals of flapwise fatigue loads is a result of turbulence while gravity results in of edgewise fatigue loads. The extensive parametric study shows that the standard deviation of is 0.25. The results support to design active load control systems since they show the magnitude of and α variations experienced by HAWT, and thus the that needs to be delivered by an AFC system.

Accurate modeling of the dynamic stall remains a challenge for the design and construction of turbine blades and helicopter rotors. At the same time, wind turbines, for instance, are becoming steadily larger, further increasing the demands on their structure and necessitating even more detailed modeling of the forces at hand. The primarily used (semi-)empirical models today have a long research history and are invariably based on phase-averaged data from oscillating blade pitch experiments. However, much potential for more accurate modeling of uncertainties and force peaks is wasted here, since averaging blurs many features of the response signals. Even computational fluid dynamics can help little in this regard, since the Reynolds-averaged Navier–Stokes equations used in practice cannot account for cycle variations, and scale-resolving models require extremely large amounts of computational resources. This paper presents an approach for a fully stochastic machine learning model that can nevertheless simulate these critical properties. Aerodynamic coefficients are compared with experimental data for different test cases. It is shown that synthetic force profiles which cannot be distinguished from the experimental data visually and are very close to them in the frequency spectrum can be generated. Additionally, attention is drawn to the difficulty of evaluating such a model, as traditional error metrics are of little use. A combination of dynamic time warping and the Earth mover's distance provides a robust solution for this problem.

Accurate modeling of the dynamic stall remains a challenge for the design and construction of turbine blades and helicopter rotors. At the same time, wind turbines, for instance, are becoming steadily larger, further increasing the demands on their structure and necessitating even more detailed modeling of the forces at hand. The primarily used (semi-)empirical models today have a long research history and are invariably based on phase-averaged data from oscillating blade pitch experiments. However, much potential for more accurate modeling of uncertainties and force peaks is wasted here, since averaging blurs many features of the response signals. Even computational fluid dynamics can help little in this regard, since the Reynolds-averaged Navier-Stokes equations used in practice cannot account for cycle variations, and scale-resolving models require extremely large amounts of computational resources. This paper presents an approach for a fully stochastic machine learning model that can nevertheless simulate these critical properties. Aerodynamic coefficients are compared with experimental data for different test cases. It is shown that synthetic force profiles can be generated which cannot be distinguished from the experimental data visually and are very close to them in the frequency spectrum. Additionally, attention is drawn to the difficulty of evaluating such a model, as traditional error metrics are of little use. A combination of Dynamic Time Warping and the Earth Mover Distance provides a robust solution for this problem.

This paper presents the development and validation of a lightweight fluid solver based on the Navier–Stokes equations coupled to a special blade element method (BEM) for vertical axis turbines. This method is called Actuator Surface Model (ASM) and leads to a medium fidelity tool designed to bridge the gap between expensive blade resolving computational fluid dynamics (CFD) calculations and simple, often inaccurate momentum models such as the double-multiple streamtube (DMST) model. Since the simulation runs on coarse meshes, it is possible to use an explicit solver with a reasonable time step to significantly reduce the computing time to almost real-time speed. The fluid model is validated for the classical lid-driven cavity flow problem, while the coupled ASM code is compared with three different experimental measurements from the literature. For a more accurate description of blade specific phenomena, several higher order corrections were implemented, such as dynamic stall, parasitic drag and finite aspect ratio. The model is able to calculate the performance of all tested turbines reasonably well and can predict the maximum efficiency up to a few percent. Therefore it is an excellent alternative for a fast and precise initial design of vertical axis turbines compared to traditional BEM methods.

A turbine compressor package is used for pipeline gas transmission. When operating, compressor turbine blades develop vibration, which increases the number of dynamic stress cycles and results in the blade failure. The present study aims to determine the frequency of natural blade vibration and to consider it in the context of the blade repair process. In the first stage of the study, an oscillating contour is developed to generate standing oscillation wave which characteristics are used as experimental data. To process those data, a mathematical model is developed to calculate the blade resonant frequency. Finally, the boundaries of the assured quality area are determined. Blade operation capacity analysis method will allow us to reduce the number of environmentally dangerous experiments.

In this paper, the numerical simulation was used to investigate the effects of the leading-edge slat installation angles ( β for airfoils from 0° to 40° and β 1 for blades from −20° to 40°) on the aerodynamic characteristics of the airfoil and the wind turbine blade. The chord length of the leading-edge slat is 0.1c (the chord length of the clean airfoil). The horizontal and vertical distances from its center to the leading edge of the clean airfoil are 0.005c and 0.009c, respectively. The results indicated that the lift coefficient could be significantly improved by the leading-edge slat (except β = 40°) when the attack angle exceeded 10.2°. For β = 0°, the lift coefficient increased the most. The trailing vortex of the leading-edge slat played an important role at the process of flow control. It could transfer kinetic energy from the bounder layer to its out-flow region. Furthermore, the vorticities of trailing vortex generated by the leading-edge slat with different installation angles were different, promoting several effects on the airfoil at the different cases. The torque of the blade with leading-edge slat (except β 1 = −20°) was improved significantly as the leading-edge slat trailing-vortices became stronger with the higher wind-speeds.

Airfoil stall is bad for wind turbines. Once stall has set in, lift collapses, drag increases and then both of these forces will fluctuate strongly. The result is higher fatigue loads and lower energy yield. In dynamic stall, separation first develops from the trailing edge up the leading edge, eventually the shear layer rolls up and then a coherent vortex forms and then sheds downstream with it’s low pressure core causing a lift spike and moment dump. When 50+ experimental cycles of lift or pressure values are averaged, this process appears clear and coherent in flow visualizations. Unfortunately, stall is not one clean process, but a broad collection of processes. This means that the analysis of separated flows should be able to detect outliers and analysis cycle to cycle variations. Modern data science/machine learning can be used to treat separated flows. In this study, a clustering method based on dynamic time warping is used to find different shedding behaviors. This method captures that secondary and tertiary vorticity vary strongly and in static stall with surging flow; the flow can occasionally reattach. A convolutional neural network was used to extract dynamic stall vorticity convection speeds and phases from pressure data. Finally, bootstrapping was used to provide best practices regarding the number of experimental repetitions required to ensure experimental convergence.

A comparative analysis of seven commonly-used eddy-viscosity turbulence models for CFD simulations of VAWTs is presented. The models include one- to four-equations, namely the Spalart-Allmaras (SA), RNG k-ε, realizable k-ε, SST k-ω, SST k-ω with an additional intermittency transition model (SSTI), k-kl-ω and transition SST (TSST) k-ω models. In addition, the inviscid modeling is included in the comparison. The evaluation is based on validation with three sets of experiments for three VAWTs with different geometrical characteristics operating in a wide range of operational conditions, from dynamic stall to optimal regime and to highly-rotational flow regime. The focus is on the turbine wake, the turbine power performance, and the blade aerodynamics. High-fidelity incompressible unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations are employed. The extensive analysis reveals high sensitivity of the simulation results to the turbulence model. This is especially the case for the turbine power coefficient CP. The results show that the inviscid, SA, RNG k-ε, realizable k-ε and k-kl-ω models clearly fail in reproducing the aerodynamic performance of VAWTs. Only the SST model variants (SST k-ω, SSTI and TSST) are capable of exhibiting reasonable agreement with all the experimental data sets, where the transitional SST k-ω versions (SSTI and TSST) are recommended as the models of choice especially in the transitional flow regime.

A theory based upon linearized governing equations is developed that describes the operation principles and design parameters for low-speed wind tunnels with longitudinal freestream oscillations. Existing measurements made in unsteady wind tunnels are shown to be consistent with the theory and targeted validation experiments performed in a variable-geometry blowdown-type wind tunnel revealed excellent correspondence with the theoretical results. In particular, the tunnel frequency bandwidth is proportional to the mean tunnel freestream velocity and inversely proportional to the test-section length and the square of the exit area to test-section area ratio. The acoustics equations reveal a "Helmholtz damping ratio" that is not only dependent on the tunnel geometry and exit area but also proportional to the freestream Mach number. At appreciable reduced frequencies, dynamic stall on the louver vanes increases pressure losses, thereby reducing the mean flow speeds. Varying the exit area results in louver-vane vortex shedding that can excite resonances, resulting in an apparent increase in the turbulence level. To achieve large freestream amplitudes and high-frequency bandwidths, substantial blockage, and therefore pressure losses, will be incurred; thus, large increases in the tunnel power factor should be anticipated. © 2015 by David Greenblatt. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

The article demonstrates a small vertical-axis wind turbines (VAWT) wind-tunnel model that was constructed with leading-edge DBD plasma actuators configured to control dynamic stall on the upwind half of the turbine azimuth. Experiments were conducted in a wind tunnel at speeds of 7 m/s, where simultaneous torque and rotational speeds were measured for both baseline (no plasma pulsations) and controlled (plasma pulsations actuated) conditions. Limited phase-locked particle image velocimetry (PIV) measurements were performed under baseline conditions as well as two controlled conditions: namely higher rotational speed/constant torque and higher torque/constant rotational speed. A mirror was placed beneath the test section at a 45 deg angle, and the PIV camera was placed in front of the mirror. Use of the mirror was necessary to orient the camera axis horizontally and thereby avoid accumulation of seeding particles on the lens.

Dynamic stall was controlled on the blades of a small high-solidity vertical axis wind turbine, by means of dielectric barrier discharge plasma actuators installed on the blade leading-edges. A parametric study was conducted with the objective of increasing the net power output resulting from control in an open-loop manner. For the majority of experiments, the actuators were configured to control separation on the upwind half of the turbine azimuth while limited experiments were conducted on the downwind half. Turbine power was measured using a specially-designed dynamometer that allowed full characterization of its performance. Actuator duty cycle dependence observed previously on static airfoils was also observed on the turbine. However, optimum reduced frequencies showed substantially different dependence and this was traced to the importance of the plasma pulsation frequency relative to the turbine rotational frequency. Overall turbine power performance improvements of 38% and 20% were measured for upwind and downwind dynamic stall control, respectively. Based on the data acquired, up-scaling the turbine by a factor of 5 and 10, the percentage of plasma power required to produce comparable improvements was conservatively estimated at 3.3% and 1.7% respectively. A new switching-direction actuator was developed, together with high-speed plasma triggering, in order to produce combined upwind and downwind actuation that is phase-locked to turbine rotational frequencies. Flowfield measurements using particle image velocimetry are presently being performed. Copyright © 2012 by the American Institute of Aeronautics and Astronautics, Inc.

A theory based upon linearized governing equations is developed that describes the operation principles and design parameters for low-speed wind tunnels with longitudinal freestream oscillations. Existing measurements made in unsteady wind tunnels are shown to be consistent with the theory and targeted validation experiments performed in a variable-geometry blowdown-type wind tunnel revealed excellent correspondence with the theoretical results. In particular, the tunnel frequency bandwidth is proportional to the mean tunnel freestream velocity and inversely proportional to the test-section length and the square of the exit area to test-section area ratio. The acoustics equations reveal a "Helmholtz damping ratio" that is not only dependent on the tunnel geometry and exit area but also proportional to the freestream Mach number. At appreciable reduced frequencies, dynamic stall on the louver vanes increases pressure losses, thereby reducing the mean flow speeds. Varying the exit area results in louver-vane vortex shedding that can excite resonances, resulting in an apparent increase in the turbulence level. To achieve large freestream amplitudes and high-frequency bandwidths, substantial blockage, and therefore pressure losses, will be incurred; thus, large increases in the tunnel power factor should be anticipated. © 2015 by David Greenblatt. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

Separation control on NACA 0012 and NACA 0015 airfoils was compared under incompressible conditions, using leading-edge periodic excitation, in order to assess the effect of leading-edge curvature. Both lift and moment coefficients were considered to compare and analyse control effectiveness. In contrast to the relatively mild NACA 0015 trailing-edge stall, NACA 0012 stall was dominated by a leading-edge bubble-bursting mechanism that gave rise to alternating intervals of partial attachment and separation, but with no regular frequency. Low-amplitude excitation downstream of the bubble enhanced poststall lift and significantly attenuated the associated unsteadiness. In general, larger momentum coefficients were required for NACA 0012 separation control due to the large centrifugal acceleration of the flow around the leading edge. Because of the different stalling characteristics, relatively high- and low-excitation frequencies were effective for the NACA 0012 and NACA 0015 airfoils, respectively. However, the combination of high-excitation amplitudes with relatively low frequencies was effective on the NACA 0012, and this was believed to be associated with the large harmonic content of the evolving perturbations.

The article demonstrates a small vertical-axis wind turbines (VAWT) wind-tunnel model that was constructed with leading-edge DBD plasma actuators configured to control dynamic stall on the upwind half of the turbine azimuth. Experiments were conducted in a wind tunnel at speeds of 7 m/s, where simultaneous torque and rotational speeds were measured for both baseline (no plasma pulsations) and controlled (plasma pulsations actuated) conditions. Limited phase-locked particle image velocimetry (PIV) measurements were performed under baseline conditions as well as two controlled conditions: namely higher rotational speed/constant torque and higher torque/constant rotational speed. A mirror was placed beneath the test section at a 45 deg angle, and the PIV camera was placed in front of the mirror. Use of the mirror was necessary to orient the camera axis horizontally and thereby avoid accumulation of seeding particles on the lens.

Dynamic stall on an oscillating airfoil was investigated by a combination of surface pressure measurements and time-resolved particle image velocimetry. Following up on previous work on the onset of dynamic stall (Mulleners and Raffel in Exp Fluids 52(3):779–793, 2012), we combined time-resolved imaging with an extensive coherent structure analysis to study various aspects of stall development. The formation of the primary dynamic stall vortex was identified as the growth of a recirculation region and the ensuing instability of the associated shear layer. The stall development can be subdivided into two stages of primary and secondary instability with the latter being the effective vortex formation stage. The characteristic time scales associated with the primary instability stage revealed an overall decrease in dynamic stall delay with increasing effective unsteadiness of the pitching airfoil. The vortex formation stage was found to be largely unaffected by variations of the airfoil’s dynamics.

The primary objective of the insteady aerodynamics experiment was to provide information needed to quantify the full-scale, three-dimensional aerodynamic behavior of horizontal-axis wind turbines. This report is intended to familiarize the user with the entire scope of the wind tunnel test and to support the use of the resulting data.

An experimental investigation into the dynamic-stall process of a pitching and plunging airfoil at low Reynolds numbers has been carried out using direct force measurements and smoke visualization in an Eiffel-type wind tunnel. The strong influence of reduced frequency (k = pi fc/U(infinity)) on the vortical wake of both pure-plunging and pure-pitching airfoils is revealed. Here, a transition from a bluff body to a mushroom-type wake has been observed at approximately k = 0.2. Some associated lift and moment hysteresis curves for combined pitching and plunging motions are then presented with an accompanying discussion on the nature of the dynamic-stall process. For these complex motions, it is observed that both lift and moment phase lags grow with reduced frequency from k = 0.05 to 0.1. Despite substantial lift augmentation in the light- and deep-stall regimes, strong pitching-down moments are not avoided.

The aerodynamic behavior of a vertical axis wind turbine (VAWT) is analyzed by means of 2D particle image velocimetry (PIV),
focusing on the development of dynamic stall at different tip speed ratios. The VAWT has an unsteady aerodynamic behavior
due to the variation with the azimuth angle θ of the blade’s sections’ angle of attack, perceived velocity and Reynolds number.
The phenomenon of dynamic stall is then an inherent effect of the operation of a VAWT at low tip speed ratios, impacting both
loads and power. The present work is driven by the need to understand this phenomenon, by visualizing and quantifying it,
and to create a database for model validation. The experimental method uses PIV to visualize the development of the flow over
the suction side of the airfoil for two different reference Reynolds numbers and three tip speed ratios in the operational
regime of a small urban wind turbine. The field-of-view of the experiment covers the entire rotation of the blade and almost
the entire rotor area. The analysis describes the evolution of the flow around the airfoil and in the rotor area, with special
focus on the leading edge separation vortex and trailing edge shed vorticity development. The method also allows the quantification
of the flow, both the velocity field and the vorticity/circulation (only the results of the vorticity/circulation distribution
are presented), in terms of the phase locked average and the random component.

The two-dimensional characteristics of airfoil NACA 0018 have been measured for Reynolds numbers between 0.15x106 and 1.0x106 to establish the lift, drag and moment curves that serve as input to performance calculations of vertical axis wind turbines. At the lower surface laminar separation occurs at low to medium angles of attack, which is of significant influence on the characteristics and the radiated noise. For the situation with a lower surface laminar separation bubble, span wise wake rake traverse measurements showed an irregular three-dimensional pattern. Noise reduction could be achieved with zigzag tape at the 70% to 80% lower surface chord station. Significant post-stall hysteresis loops occurred showing a high loss in lift.

An aerodynamic load control concept termed “adaptive blowing” was successfully tested on a NACA 0018 airfoil model at Reynolds numbers ranging from 150,000 to 500,000. The global objective was to eliminate lift oscillations typically encountered on wind turbine blade sections. Depending on the jet momentum flux, steady blowing from a control slot in the leading-edge region can be utilized to either enhance or reduce lift by suppressing or inducing boundary layer separation respectively. Furthermore, high momentum blowing effectively eliminated the dynamic stall vortex during deep dynamic stall conditions. Based on these previous findings, the present work explores the feasibility of controlling unsteady aerodynamic loads by dynamically varying the jet momentum flux to compensate for transient changes of the inflow. Various scenarios including high amplitude pitching, rapid freestream oscillations and combinations of both were investigated in a custom-built unsteady wind tunnel facility. An iterative control algorithm was implemented which successfully identified the momentum coefficient time profiles required to minimize the lift excursions. The combination of fully suppressing dynamic stall and dynamically adjusting the lift coefficient provided an unprecedented control authority, producing virtually constant phase averaged lift in all cases.

A combined theoretical and experimental investigation was carried out with the objective of evaluating theoretical predictions relating to a two-dimensional airfoil subjected to high amplitude harmonic oscillation of the free stream at constant angle of attack. Current theoretical approaches were reviewed and extended for the purposes of quantifying the bound, unsteady vortex sheet strength along the airfoil chord. This resulted in a closed form solution that is valid for arbitrary reduced frequencies and amplitudes. In the experiments, the bound, unsteady vortex strength of a symmetric 18 % thick airfoil at low angles of attack was measured in a dedicated unsteady wind tunnel at maximum reduced frequencies of 0.1 and at velocity oscillations less than or equal to 50 %. With the boundary layer tripped near the leading edge and mid-chord, the phase and amplitude variations of the lift coefficient corresponded reasonably well with the theory. Near the maximum lift coefficient overshoot, the data exhibited an additional high-frequency oscillation. Comparisons of the measured and predicted vortex sheet indicated the existence of a recirculation bubble upstream of the trailing edge which sheds into the wake and modifies the Kutta condition. Without boundary layer tripping, a mid-chord bubble is present that strengthens during flow deceleration and its shedding produces a dramatically different effect. Instead of a lift coefficient overshoot, as per the theory, the data exhibit a significant undershoot. This undershoot is also accompanied by high-frequency oscillations that are characterized by the bubble shedding. In summary, the location of bubble and its subsequent shedding play decisive roles in the resulting temporal aerodynamic loads.

The utility of constant blowing as an aerodynamic load control concept for wind turbine blades was explored experimentally. A NACA 0018 airfoil model equipped with control slots near the leading edge and at mid-chord was investigated initially under quasi-static conditions at Reynolds numbers ranging from 1.25·10^5 to 3.75·10^5. Blowing from the leading-edge slot showed a significant potential for load control applications. Leading-edge stall was either promoted or inhibited depending on the momentum coefficient, and a corresponding reduction or increase in lift on the order of Δcl≈0.5 was obtained. Control from the mid-chord slot counteracted trailing-edge stall but was ineffective at preventing leading-edge separation. The impact of blowing from the leading-edge slot on dynamic stall was explored by means of unsteady surface pressure measurements and simultaneous particle image velocimetry above the suction surface. At a sufficiently high momentum coefficient, the formation and shedding of the dynamic stall vortex were fully suppressed. This led to a significant reduction in lift hysteresis and form drag while simultaneously mitigating moment coefficient excursions.
Read More: http://arc.aiaa.org/doi/abs/10.2514/1.J053090

The lift and separation bubble characteristics of a NACA 0018 airfoil are investigated experimentally. Surface pressure measurements are presented for Reynolds numbers from 80×103 to 200×103 and angles of attack from 0° to 18°. These data were used to characterize the separation bubble and determine lift coefficients. From these results, two distinct regions in the lift curves can be identified: a region of rapid and linear growth of the lift coefficients at low angles of attack and a region of more gradual and linear growth at higher pre-stall angles. Furthermore, the slope of the lift curve in each region is found to be linked to the rates of change in separation, transition, and reattachment locations with the angle of attack. These findings are substantiated by an analysis of the available experimental data for a NACA 0012 airfoil.

Accurate simulations of the aerodynamic performance of vertical-axis wind turbines pose a significant challenge for computational fluid dynamics methods. The aerodynamic interaction between the blades of the rotor and the wake that is produced by the blades requires a high-fidelity representation of the convection of vorticity within the wake. In addition, the cyclic motion of the blades induces large variations in the angle of attack on the blades that can manifest as dynamic stall. The present paper describes the application of a numerical model that is based on the vorticity transport formulation of the Navier–Stokes equations, to the prediction of the aerodynamics of a vertical-axis wind turbine that consists of three curved rotor blades that are twisted helically around the rotational axis of the rotor. The predicted variation of the power coefficient with tip speed ratio compares very favorably with experimental measurements. It is demonstrated that helical blade twist reduces the oscillation of the power coefficient that is an inherent feature of turbines with nontwisted blade configurations.

The effects of a periodic free-stream velocity on the unsteady aerodynamics of an airfoil in incompressible flow are examined. Existing theories are reviewed, and their simplifications and limitations are properly identified. A new general aerodynamic theory for an airfoil undergoing a combination of harmonic pitching, plunging and fore-aft motion is presented. An extension to arbitrary free-stream velocity variations and arbitrary airfoil motion is also given. The theoretical results are validated against numerical predictions made by a modern Euler code.

A lwo speed wing tunnel equipped with an axial gust generator to simulate the aerodynamic environmeent of a helicopter rotor was used to study the dynamic stall of a pitching blade. The objective of this investigation was to find out to what extent harmonic velocity perturbations in the freestream affect dynamic stall. The study involved making measurements of the aerodynamic moment on a two-dimensional, pitching blade model in both constant and pulsating airstreams. Using an operational analog computer to perform on-line data reduction, plots of moment versus angle of attack and work done by the moment were obtained. The data taken in the varying freestream were then compared to constant freestream data, and to the results of two analytical methods. These comparisons showed that the velocity perturbations had a significant effect on the pitching moment which could not be consistently predicted by the analytical methods, but had no drastic effect on the blade stability.

The dynamic stall characteristics of eight airfoils have been investigated in sinusoidal pitch oscillations over a wide range of two-dimensional unsteady flow conditions. The results provide a unique comparison of the effects of section geometry in a simulated rotor environment. Important differences between the various airfoils were observed, particularly when the stall regimes were penetrated only slightly. Under these circumstances, the profiles that stall gradually from the trailing edge appear to offer an advantage. However, all of the airfoils tended increasingly toward leading-edge stall when both the severity of dynamic stall and the free-stream Mach number increased. In all cases, the parameters of the unsteady motion appear to be more important than airfoil geometry for configurations that are appropriate for helicopter rotors.

The effects of simultaneous velocity and incidence fluctuations on the two-dimensional aerodynamic behavior of a NACA 0012 airfoil are investigated in this paper. A new mechanical system allows driving the airfoil in pitching and in fore and aft motions, as well as in a simultaneous combination of these two basic unsteady motions. In response to the simultaneous velocity and incidence variations, the time-dependent lift and drag fluctuations are measured for increasing values of the reduced frequency and amplitude parameters, including dynamic stall conditions. Complementary information on the dynamic stall occurring in combined motion is provided by skin friction and pressure measurements along the airfoil surface.

A 21-percent-thick, laminar-flow airfoil, the S809, for horizontal-axis wind-turbine applications, has been designed and analyzed theoretically and verified experimentally in the low-turbulence wind tunnel of the Delft University of Technology Low Speed Laboratory, The Netherlands. The two primary objectives of restrained maximum lift, insensitive to roughness, and low profile drag have been achieved. The airfoil also exhibits a docile stall. Comparisons of the theoretical and experimental results show good agreement. Comparisons with other airfoils illustrate the restrained maximum lift coefficient as well as the lower profile-drag coefficients, thus confirming the achievement of the primary objectives.

Dynamic loads must be predicted accurately in order to estimate the fatigue life of wind turbines operating in turbulent environments. Dynamic stall contributes to increased dynamic loads during normal operation of all types of horizontal-axis wind turbine (HAWTs). This report illustrates how dynamic stall varies throughout the blade span of a 10 m HAWT during yawed and unyawed operating conditions. Lift, drag, and pitching moment coefficients during dynamics stall are discussed. Resulting dynamic loads are presented, and the effects of dynamic stall on yaw loads are demonstrated using a yaw loads dynamic analysis (YAWDYN). 12 refs., 22 figs., 1 tab.

An analytical model is proposed for calculating the rotor performance
and aerodynamic blade forces for Darrieus wind turbines with curved
blades. The method of analysis uses a multiple-streamtube model, divided
into two parts: one modeling the upstream half-cycle of the rotor and
the other, the downstream half-cycle. The upwind and downwind components
of the induced velocities at each level of the rotor were obtained using
the principle of two actuator disks in tandem. Variation of the induced
velocities in the two parts of the rotor produces larger forces in the
upstream zone and smaller forces in the downstream zone. Comparisons of
the overall rotor performance with previous methods and field test data
show the important improvement obtained with the present model. The
calculations were made using the computer code CARDAA developed at IREQ.
The double-multiple streamtube model presented has two major advantages:
it requires a much shorter computer time than the three-dimensional
vortex model and is more accurate than multiple-streamtube model in
predicting the aerodynamic blade loads.

This work describes the present state-of-the-art in double-multiple
streamtube method for modeling the Darrieus-type vertical-axis wind
turbine (VAWT). Comparisons of the analytical results with the other
predictions and available experimental data show a good agreement. This
method, which incorporates dynamic-stall and secondary effects, can be
used for generating a suitable aerodynamic-load model for structural
design analysis of the Darrieus rotor.

The effect of periodic flow field velocity on unsteady aerodynamic lift coefficient is analytically derived. Results are shon for a wide variety of reduced frequencies at the example of a stationary airfoil as wel las on a sinusoidally pitching airfoil.

The flow over a NACA 0012 airfoil undergoing large oscillations in pitch was experimentally studied at a Reynolds number of and over a range of frequencies and amplitudes. Hot-wire probes and surface-pressure transducers were used to clarify the role of the laminar separation bubble, to delineate the growth and shedding of the stall vortex, and to quantify the resultant aerodynamic loads. In addition to the pressure distributions and normal force and pitching moment data that have often been obtained in previous investigations, estimates of the unsteady drag force during dynamic stall have been derived from the surface pressure measurements. Special characteristics of the pressure response, which are symptomatic of the occurrence and relative severity of moment stall, have also been examined.

A NACA 0015 semispan wing was placed in a low-speed wind tunnel, and measurements were made of the pressure on the upper and lower surface of the wing and of velocity across the vortex trailing downstream from the tip of the wing. Pressure data were obtained for both 2-D and 3-D configurations. These data feature a detailed comparison between wing tips with square and round lateral edges. A two-component laser velocimeter was used to measure velocity profiles across the vortex at numerous stations behind the wing and for various combinations of conditions. These conditions include three aspect ratios, three chord lengths, a square- and a round lateral-tip, presence or absence of a boundary-layer trip, and three image plane positions located opposite the wing tip. Both pressure and velocity measurements were made for the angles of attack 4 deg less than or equal to alpha less than or equal to 12 deg and for Reynolds numbers 1 x 10(exp 6) less than or equal to Re less than or equal to 3 x 10(exp 6).

The near Wake of the Vawt

- C J S Ferreira

C.J.S. Ferreira, The near Wake of the Vawt, Ph.D. thesis, Delft University of
Technology, 2009.

Examples of Three Representative Types of Airfoil-section Stall at Low Speed, NACA Technical Note 250

- G Mccullough
- Gault

McCullough. G, Gault. D, Examples of Three Representative Types of Airfoilsection Stall at Low Speed, NACA Technical Note 250.

- T Theodorsen

T. Theodorsen, General Theory of Aerodynamic Instability and the Mechanism
of Flutter, Tech. rep., NACA Report 496, US Nat., Advisory Committee for
Aeronautics, Langley, VA, 1935.

Airfoil in Sinusoidal Motion in Pulsating Stream

- J M Greenberg

J.M. Greenberg, Airfoil in Sinusoidal Motion in Pulsating Stream, Tech. Rep. No.
1326, National Advisory Committee for Aeronautics, 1947. Technical Notes.

Wind Turbine Blade Dynamic Stall and its Control (Doctoral dissertation), Technion -Israel Institute of Technology and Technische Uni-versit€ at Berlin

- H F Müller-Vahl

H.F. Müller-Vahl, Wind Turbine Blade Dynamic Stall and its Control (Doctoral
dissertation), Technion -Israel Institute of Technology and Technische Uni-versit€ at Berlin, 2015.

NREL/TP-500e29494, National Renewable Energy Fig. 17. Phase averaged pressure coefficient distributions measured with the S809 airfoil. Angle of attack and relative flow speed are matched for 18 + a amax

- D Simms
- S Schreck
- M Hand
- L J Fingersh

D. Simms, S. Schreck, M. Hand, L.J. Fingersh, Nrel Unsteady Aerodynamics
Experiment in the nasa-ames Wind Tunnel: a Comparison of Predictions to
Measurements, Tech. Rep. NREL/TP-500e29494, National Renewable Energy
Fig. 17. Phase averaged pressure coefficient distributions measured with the S809 airfoil. Angle of attack and relative flow speed are matched for 18 + a amax.
H.F. Müller-Vahl et al. / Renewable Energy 105 (2017) 505e519