# Flow Turbulence and Combustion

Online ISSN: 1573-1987
Recent publications
Article
• Julia Steiner
• Axelle Viré
• Richard P. Dwight
Data-driven Reynolds-averaged Navier–Stokes (RANS) turbulence closures are increasing seen as a viable alternative to general-purpose RANS closures, when LES reference data is available—also in wind-energy. Parsimonious closures with few, simple terms have advantages in terms of stability, interpret-ability, and execution speed. However experience suggests that closure model corrections need be made only in limited regions—e.g. in the near-wake of wind turbines and not in the majority of the flow. A parsimonious model therefore must find a middle ground between precise corrections in the wake, and zero corrections elsewhere. We attempt to resolve this impasse by introducing a classifier to identify regions needing correction, and only fit and apply our model correction there. We observe that such classifier-based models are significantly simpler (with fewer terms) than models without a classifier, and have similar accuracy, but are more prone to instability. We apply our framework to three flows consisting of multiple wind-turbines in neutral conditions with interacting wakes.

Article
• Angela Busse
• Oleksandr Zhdanov
The influence of the orientation of ratchet-type rough surfaces on their fluid dynamic roughness effect is investigated using direct numerical simulations of turbulent channel flow at Reτ=395\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$Re_{\tau }=395$$\end{document}. The ratchet length-to-height ratio is varied from ℓ/k=2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\ell /k=2$$\end{document} to 16 for a fixed ratchet height of k/δ=0.1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$k/\delta =0.1$$\end{document} where δ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\delta$$\end{document} is the mean channel half-height. The results show that both roughness function and mean flow and turbulence statistics strongly depend on the ratchet orientation. Existing empirical formulae, which estimate the roughness function ΔU+\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta U^+$$\end{document} or the equivalent sand-grain roughness ks\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$k_s$$\end{document} based on surface-slope related parameters such as the effective slope or the Sigal-Danberg parameter, fail to accurately predict the differences between ratchet surfaces with high windward slopes and ratchet surfaces with high leeward slopes.

Article
• Wei Zhang
The laminar flow on a curved surface transits to turbulent induced by streamline curvature which generates pressure gradient field and separated shear layer flow. We performed a direct numerical simulation investigation on transitional flow through a linear cascade consist of S-shaped S3525 hydrofoil which has different curvature variations on the two surfaces, i.e., concave-to-convex and convex- to-concave in the streamwise direction. The objectives are to quantitatively assess the effects of streamline curvature of the hydrofoil surface on the three-dimensionality of the separated and transitional flow, including the patterns of separation and reattachment, formation and development of three-dimensional boundary layer flow, and statistics on non-homogeneous turbulent near-wall flow. Comparisons between the near-wall flows of the two surfaces demonstrate the effect of streamline curvature and its associated influential mechanisms such as pressure gradient field. Numerical data reveal that transition and occurrence of three-dimensional flow are observed earlier for the concave-to-convex surface; intermittent flow is generated in the concave section near the leading edge and convex section near the trailing edge where three-dimensionality of flow and turbulent fluctuations are the most pronounced. However, the boundary layer and near-wall flow for the convex-to-concave surface is quite stable until the concave section, thus three-dimensionality of separation and reattachment, boundary layer flow and turbulent behaviors are only notable near the trailing edge.

Article
Particulate flow in closed space is involved in many engineering applications. In this paper, the prediction of particle removal is investigated in a thermally driven 3D cavity at turbulent Rayleigh number Ra = 1E9 using Coarse Large Eddy Simulation (CLES). The depletion dynamics of SiO2 aerosol with aerodynamic diameters between 1.4-14 µm is reported in an Euler/Lagrange framework. The main focus of this work is therefore to assess the effect of the subgrid-scale motions on the prediction of the particulate flow in a buoyancy driven 3D cavity flow when the mesh resolution is coarse and below optimal LES standards. The research is motivated by the feasibility of modeling more complex particulate flows with reduced CPU cost. The cubical cavity of 0.7 m side-length is set to have a temperature difference of 39 K between the two facing cold and hot vertical walls. As a first step, the carrier fluid flow was validated by comparing the first and second-moment statistics against both previous well-resolved LES and experimental databases [24, 25]. First moment Eulerian statistics show a very good match with the reference data both qualitatively and quantitatively, whereas higher moments show underprediction due to the lesser spatial resolution. In a second step, six particle swarms spanning a wide range of particle Stokes numbers were computed to predict particle depletion. In particular, predictions of 1.4 µm and 3.5 µm particles were compared to LES and available experimental data. Particles of low inertia i.e. dp < 3.5 µm are more affected by the SGS effects, while bigger ones i.e. dp = 3.5-14 µm exhibit much less grid-dependency. Lagrangian statistics reported in both qualitative and quantitative fashions show globally a very-Flow, Turbulence and Combustion-2-good agreement with reference LES and experimental databases at a fraction of the CPU power needed for optimal LES.

Article
The occurrence of knocking combustion is limiting the efficiency of modern spark ignition engine operation. Thus, an understanding of the processes at the knock limit is required for further optimization of the combustion process. In this work, the combustion of a multicomponent Toluene Reference Fuel (TRF) in a single-cylinder research engine is investigated under knocking conditions. The fuel exhibits a negative temperature coefficient (NTC) regime for thermodynamic conditions relevant to the engine operation. A precursor model is used to capture the auto-ignition process. Under homogeneous conditions, a two-stage auto-ignition is observed. Inside the NTC regime, the temperature affects both first-stage and second-stage auto-ignition delay times. With a subsequently conducted multi-cycle engine LES, the effects of temperature stratification and turbulent flame propagation on the local auto-ignition process are investigated. It is observed, that the NTC behavior leads to a widespread two-stage auto-ignition. The knock intensity observed in the experiments is directly related to the mass consumed by auto-ignition. This is due to the fast consumption of the auto-ignited mass by the flame front. With that, the NTC behavior affects the local auto-ignition process in the unburned mixture while the flame propagation determines the knock intensity for the operating conditions at the knock limit.

Article
A comprehensive study of direct-contact condensation heat transfer for turbulent, counter-current, liquid/vapour flow in a nearly horizontal channel at high pressure (i.e. 5 MPa) has been carried out based on Direct Numerical Simulation (DNS) and highly-resolved Large Eddy Simulation (LES) approaches. To simulate the two-phase flow situation, driven in this case by a constant pressure gradient, a single set of Navier–Stokes equations, coupled with an enthalpy conservation equation, have been employed. The interfacial mass transfer, seen in this case to be dominated by condensation, has been calculated directly from the heat flux at the liquid/vapour interface. To investigate the effect of condensation on the turbulence phenomena, and vice versa, cases have been considered involving two friction Reynolds numbers: namely Re∗=u∗h/ν=178\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$Re_{*}=u_{*}h/\nu =178$$\end{document} and Re∗=u∗h/ν=590\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$Re_{*}=u_{*}h/\nu =590$$\end{document} (u∗=(hΔP/ρ)1/2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$u_{*}=(h\varDelta P/\rho )^{1/2}$$\end{document}). At the lower Reynolds number, three levels of water subcooling—0 K, 10 K and 40 K—have been investigated. The use of water subcooling of 0 K has enabled the validation and verification procedures associated with the numerical approach to be compared against experimental and numerical data reported in the literature. The choice of the maximum degree of water subcooling is dictated by the need to justify the periodic boundary conditions applied in this numerical study. In the simulation for the higher Reynolds number, only the case of 10 K subcooling has been included, as a consequence of the very high computation effort involved. A detailed statistical analysis of the DNS and LES data obtained from the application of the well-known wall laws has also been assessed. In the vicinity of the liquid/vapour interface, the characteristics of the turbulent motions appear somewhat diverse, depending on whether the interface is basically flat or wavy in character. For a flat interface, some damping effect of the presence of the interface on the turbulence intensity has been observed, a feature which becomes enhanced as the level of liquid subcooling is increased. In the case of a wavy interface, the damping effect is predicted as considerably less pronounced.

Article
This paper reports on particle image velocimetry (PIV) measurements in compressible accelerated wake flows generated by two different central injector types, which are mounted in a convergent-divergent nozzle. The injectors differ by the extent of their trailing edge located either in the subsonic (injector A) or supersonic flow region (injector B). In addition, the undisturbed nozzle flow without injector is studied as a reference case. The PIV results reveal typical wake flow structures expected in subsonic (injector A) and supersonic (injector B) wake flows. They further show that the Reynolds stresses $$\mathrm {Re_{xx}}$$ Re xx and $$\mathrm {Re_{yy}}$$ Re yy significantly decay in all three cases due to the strong acceleration throughout the nozzle. Interestingly, in the case of injector A, the flow stays non-isotropic with $$\mathrm {Re_{yy}}>\mathrm {Re_{xx}}$$ Re yy > Re xx also far downstream in the supersonic flow region. These measurements were motivated by the lack of velocity data needed to validate numerical simulations. That is why this paper additionally contains results from (unsteady) Reynolds-averaged Navier-Stokes ((U)RANS) simulations of the two wake flows investigated experimentally. The URANS simulation of the injector A case is able to accurately predict the entire flow field and periodic fluctuations at the wake centerline. However, in the case of injector B, the RANS simulation underestimates the far wake centerline velocity by about $$4\%$$ 4 % .

Article
Cyclic variability is investigated in an optically accessible single-cylinder spark-ignition research engine by introducing artificial exhaust gas in controlled amounts to the homogenous air–fuel mixture before ignition. A skip-fire scheme ensures the absence of internal exhaust gas recirculation (EGR) and allows the engine to be fired continuously for acquisition of large statistics. Four operating conditions ranging from a stable 0% EGR case up to a highly unstable extreme EGR case are analyzed to examine the increasing effects of homogeneous EGR on the cycle performance. To that end, high-speed measurements of the velocity field via particle image velocimetry and flame imaging in the tumble plane allow the determination of phenomena leading to various flame positions and sizes as well as faster and slower combustion cycles. Through extensive conditional statistical and multivariate correlation techniques, flames are found to be heavily influenced by large-scale velocity motion, especially with the presence of greater EGR which leads to lower flame speeds. The greater sensitivity of slower flames to variations in the velocity field manifests itself in an exponential increase in cyclic variability of the maximum in-cylinder pressure and causes misfire cycles where the flame is blown off or quenched at the cylinder roof. In the most extreme cycles at the highest EGR level, the state of the large-scale velocity structures at the time of ignition determines whether the flame propagates towards the center of the cylinder (and is blown off or quenched) or if the flame sustains growth by propagating within the lingering tumble vortex.

Article
Ammonia ( $$\mathrm {NH_3}$$ NH 3 ) has attracted interest as a future carbon-free synthetic fuel due to its economic storage and transportation. In this study, quasi direct numerical simulations (quasi-DNS) with detailed-chemistry have been performed in 3-D to examine the flame thickness and assess the validity of Damköhler’s first hypothesis for premixed turbulent planar ammonia/air and methane/air flames under different turbulence levels. The Karlovitz number is systematically changed from 4.26 to 12.06 indicating that all the test conditions are located within the thin reaction zones combustion regime. Results indicate that the ensemble average values of the preheat zone thickness deviate slightly from the thin laminar flamelet assumption, while the reaction zone regions remain relatively intact. Following the balance equation of reaction progress variable gradient, normal strain rate and the tangential diffusion component of flame displacement speed variation in the normal direction to the flame surface are found to be responsible for thickening the flame. However, the sum of reaction and normal diffusion components of flame displacement speed variation in the normal direction to the flame surface is in charge of flame thinning for ammonia/air and methane/air flames. In addition, the validity of Damköhler’s first hypothesis is confirmed by indicating that the ratio of the turbulent burning velocity to the unstrained premixed laminar burning velocity is relatively equal to the ratio of the wrinkled to the unwrinkled flame surface area. Furthermore, the probability density functions of the density-weighted flame displacement speed show that the bulk of flame elements propagate identical to the unstrained premixed laminar flame.

Article
Semi-closed supercritical CO2 (sCO2) gas turbine is a promising candidate for the next generation power cycles with high efficiency and almost 100% carbon capture. In this study, the multicomponent effects on the sCO2 systems are investigated. A real-fluid modeling framework based on the vapor-liquid equilibrium (VLE) theory is implemented to predict the phase boundary and real mixture critical point, and to capture the phase separation in computational fluid dynamics (CFD) simulations. A novel VLE-based tabulation method is developed to make the CFD solver computationally more affordable. VLE-based thermodynamic analyses show that a small amount of combustion-relevant impurities (e.g., H2O, CH4, and O2) can significantly elevate the mixture critical point of the sCO2 systems. As a result, the so-called “supercritical” CO2 systems might be in the subcritical two-phase zone where phase separation occurs. At the relevant conditions in this study (100–300 bar), phase separation only has a small influence on the CO2/H2O/CH4/O2 mixture density, but has a considerable influence on the heat capacity of the mixture. VLE-based CFD simulation of a laminar premixed sCO2 shock tube shows that expansion waves can trigger significant condensation in the systems and the latent heat of the condensation can change the temperature and density fields in the systems. To understand the phase separation during mixing, VLE-based large-eddy simulations (LES) of turbulent jet-in-crossflows in the sCO2 systems are conducted, and the results show that when two subcritical gas or supercritical gas-like streams mix, the mixture can partially condense to subcritical liquid phase. Higher pressure, lower temperature, and higher H2O concentration can enhance the phase separation phenomenon in the systems.

Article
The cross-scalar dissipation rate of reaction progress variable and mixture fraction εcξ~ plays an important role in the modelling of stratified combustion. The evolution and statistical behaviour of εcξ~ have been analysed using a direct numerical simulation (DNS) database of statistically planar turbulent stratified flames with a globally stochiometric mixture. A parametric analysis has been conducted by considering a number of DNS cases with a varying initial root-mean-square velocity fluctuation u′ and initial scalar integral length scale ℓϕ. The explicitly Reynolds averaged DNS data suggests that the linear relaxation model for εcξ~ is inadequate for most cases, but its performance appears to improve with increasing initial ℓϕ and u′ values. An exact transport equation for εcξ~ has been derived from the first principle, and the budget of the unclosed terms of the εcξ~ transport equation has been analysed in detail. It has been found that the terms arising from the density variation, scalar-turbulence interaction, chemical reaction rate and molecular dissipation rate play leading order roles in the εcξ~ transport. These observations have been justified by a scaling analysis, which has been utilised to identify the dominant components of the leading order terms to aid model development for the unclosed terms of the εcξ~ transport equation. The performances of newly proposed models for the unclosed terms have been assessed with respect to the corresponding terms extracted from DNS data, and the newly proposed closures yield satisfactory predictions of the unclosed terms in the εcξ~ transport equation.

Article
Micro Aerial Vehicles (MAVs) are state of the art in the aerospace industry and are involved in many operations. The reduced dimensions of these vehicles generate very low Reynolds number conditions in which separation-induced transition typically occurs. The extremely large computational cost of scale resolving simulations, which are capable of capturing laminar to turbulent transition, is prohibitive for most engineering and design applications. Therefore, it becomes very interesting to couple transition models with conventional Reynolds Averaged Navier–Stokes (RANS) simulations to allow the prediction of transition to turbulence at a reduced computational cost. This paper performs an investigation of the application of the γ-Reθ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\gamma \text{-Re }_{\theta }$$\end{document} transition model analysing different empirical correlations available in literature and studying the influence of the relevant model parameters using the commercial Computational Fluid Dynamics (CFD) code STAR-CCM+. The flow around the Eppler 387, Selig/Donovan 7003 and Ishii airfoils has been studied for different Reynolds numbers and angles of attack comparing the drag and lift forces and separation bubble characteristics with experimental and numerical results reported in literature.

Article
The influence of surface roughness on overlying turbulence is represented by a drag layer, with a quadratic drag law. The drag coefficient is considered to be a function of effective sandgrain roughness. The challenge to hybrid modeling is posed as being to calibrate that drag law, such that the calibration be nearly the same for either RANS or LES. To that end the coefficient is first calibrated for LES. With roughness represented by the drag formula, the RANS model and boundary condition are adjusted to provide consistency in the log layer. Then, it is found that RANS and LES calibrations become nearly the same. The model is validated by simulations of a rough wall boundary layer, of rough to smooth junction flow, of a rough ramp, and of streamwise roughness strips.

Article
Regulations to reduce greenhouse gas emissions of passenger vehicles are becoming increasingly stringent. The aerodynamic drag is a major contributor to the vehicle’s total energy consumption where a large portion is attributed to the base wake. This paper optimises the angles of small trailing edge flaps on a base cavity of a full-scale sports utility vehicle placed in a wind tunnel. The trailing edge flaps are controlled using servos mounted inside the cavity. The flap angles are optimised using a surrogate model based optimisation algorithm with the objective of reducing the aerodynamic drag at different yaw angles and to create a yaw-insensitive geometry by considering several weighted yaw angles to form the driving cycle averaged drag. Low drag designs are further investigated using base pressures and wake measurements. The results show that the base pressures are symmetrised by reducing the crossflow in the wake. As the model is yawed the wake becomes increasingly downwash dominated by a large rotating windward structure which is reduced by the optimised flaps. The cycle averaged drag optimised design has a smaller increase in drag when yawed compared to a design optimised without considering yaw.

Article
A filtered reaction rate model driven by deep learning is proposed and analyzed a priori in the context of large eddy simulation (LES). A deep artificial neural network (ANN) is trained on the explicitly filtered reaction rate source term extracted from a database comprised of turbulent premixed planar flame direct numerical simulations (DNSes) employing single-step chemistry. The filtered DNS database to be used for the training of the ANN covers a wide range of turbulence intensities and LES filter widths. An interpretation technique of deep learning is employed to search the principal input parameters in the high dimensional database to alleviate the model complexity. The deep learning filtered reaction rate model is then tested on the unseen filtered planar flames featuring untrained turbulence intensities and LES filter widths, in conjunction with another canonical type of flame configuration that it has not been trained on. The deep learning filtered reaction rate model achieves good agreement with the filtered DNS results and also provides a quantitatively accurate surrogate model when compared to existing algebraic models and other combustion models from the literature.

Article
In this study, we apply particle image velocimetry (PIV), hot-wire anemometry (HWA), and large-eddy simulation (LES) to identify and characterize a key mechanism by which high-intensity turbulence measured in the “Hi-Pilot” burner is generated. Large-scale oscillation of the high-velocity jet core about its own mean axial centerline is identified as a dominant feature of the turbulent flow field produced by this piloted Bunsen burner. This oscillation is linked to unsteady flow separation along the expanding section of the reactant nozzle and appears stochastic in nature. It occurs over a range of frequencies (100–300 Hz) well below where the turbulent kinetic energy (TKE) spectrum begins to follow a – 5/3 power law and results in a flow with significant scale separation in the TKE spectrum. Although scale separation and intermittency are not unusual in turbulent flows, this insight should inform analysis and interpretation of previous, and future studies of this unique test case.

Article
Different combustion models for large eddy simulation (LES), including the quasi-laminar (QL), Eddy Dissipation Concept (EDC), and Partially Stirred Reactor (PaSR) models, are assessed at various filter widths using direct numerical simulation (DNS). The DNS database is lean hydrogen-air turbulent flame across a wide range of Karlovitz numbers (5 to 239). Overall, the PaSR model performs best, except for small filter width and medium Karlovitz number conditions. The performance of the EDC model is very similar to the QL model at a relatively low turbulent Reynolds number. It is highlighted that both the EDC and PaSR models are suitable for high turbulent Reynolds number and medium Karlovitz number conditions. Theoretical analysis is carried out to explain the current observations and predict the models' behaviors with the variation of turbulent intensity, combustion intensity, and grid resolution. Implications of the present results for modeling are highlighted.

Article
This paper reports on the effect of DC electric fields on the dynamics of a premixed methane-air laminar flame, in a buoyant environment. DC electric fields can be capable of affecting both the buoyancy-driven flickering oscillation of the flame and the response of the flame to acoustic modulation of the flow. We conduct fast visualization of the emission of excited methylidyne radicals (CH*), representing the heat release rate of the flame. Such visualizations are also synchronized with electric current and voltage measurements. We notice that the suppression of buoyancy-driven flickering oscillations can be obtained by applying sub-critical negative DC voltages. Moreover, the current measured in the inter-electrodes area is analyzed for positive and negative DC applied voltages and we find that this quantity cannot be used as a tracer of heat release rate in a configuration where the flame location in the inter-electrodes gap varies with sub-critical electric fields. In addition, the effect of DC electric fields on the flame transfer function for acoustic modulation of the flow is reported and discussed.

Article
This research deals with the oscillation frequency of a classical and simple flip-flop jet nozzle with one feed-back loop based on the measurements of pressure and velocity. From these measurements, the traces of pressure difference between both ends of a connecting tube are modeled by a triangular wave, and the flow velocities in the connecting tube are calculated numerically. The resulting accumulated flow work, which consists of the time integral of mass flux into the re-circulation region on the low pressure side wall from the re-circulation region on the opposite high-pressure side wall through the connecting tube, is more adequate to determine jet-oscillation frequency than any other time integrals. This includes those of momentum flux and kinetic-energy flux. It is confirmed that jet-oscillation frequency predicted on the basis of this accumulated flow work agrees well with experiment.

Article
The cycle-to-cycle variations (CCV) of in-cylinder flow make critical impact on internal combustion engine performance. The flow structure evolution and its cyclic variations need to be fully understood. In this study, a novel approach which couples proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) is proposed to identify the CCV features of in-cylinder flow under various swirl conditions. The methodology is applied to a time-resolved high-speed particle image velocimetry (PIV) dataset which measures the crank angle resolved flow field on a swirl plane 30 mm below the injector tip in the cylinder. Phase-invariant POD and DMD analyses are conducted on 121 phase angles which cover the majority of in-cylinder flow evolution stroke from -300 crank angle degree (CAD) after top dead center (TDC) to -60 CAD after TDC with a 2-CAD resolution. The modal decomposition analyses are applied on 100 engine cycles individually to investigate the cyclic variation. Phase-invariant POD identifies the steady structure and coherent structure of the in-cycle engine flow evolution features. A novel stepwise traversal correlation search method (STCSM) is proposed to connect the DMD modes with POD decomposed coherent structure. This analysis approach is capable of identifying the characteristic frequencies and correlating them with the dynamical decay rates of underlying flow coherent structures with regards to CCV. In summary, the CCV of dynamical features for all swirl ratio conditions show similar variation level at intake stroke. Inducing a higher swirl ratio can minimize the effect of intake flow dynamics and suppress the cyclic variability of engine flow at compression stroke.

Article
Effects of three-dimensional (3-D) distributed roughness elements on the flow characteristics within a cavity are investigated using a series of high-fidelity eddy-resolving simulations. The cavity flows generate undesirable low-frequency pressure fluctuations due to the vortex impingement over the trailing edge of the cavity. We explore the possibility of employing distributed hemispherical roughness elements as a passive flow control strategy towards suppressing these pressure fluctuations. A rectangular cavity with a length to depth ratio, L/D, of 3 is considered. Simulations are carried out at a Mach number of 0.2 and Reynolds numbers of 7000 and 19300, based on the free-stream velocity and the depth of the cavity. The effect of sparsely and densely packed roughness elements on the stability of shear layer separating from the cavity are brought out. Pre-transitional fluctuations generated by the roughness elements (a) resulted in transitional/turbulent flow at the cavity leading edge for low/high Reynolds numbers (b) promoted an earlier breakdown of the large-scale coherent structures in the shear layer (c) are beneficial in decreasing the ‘cavity tones’ and the associated sound pressure levels (SPL) by 5-13 dB. Reduction in SPL is observed to be prominent at higher Reynolds numbers and with dense spacing between the roughness elements. At low Reynolds numbers, the benefit obtained by suppressing the ‘cavity tones’ can be eclipsed with an increase in the broadband noise.

Article
A vortex generator (VG) can effectively prevent the airflow from separating prematurely and transfer the energy to the low-energy airflow at the bottom of the boundary layer so that it will not separate after obtaining energy and reduce the strength of the reverse vortex in the wake area to reduce the aerodynamic drag of the train. In this study, transient numerical simulations of VGs on China Railway CRH5 Size trains were conducted. The SST K–ω two-equation improved delayed detached eddy simulation turbulence model was used to solve the incompressible three-dimensional Navier–Stokes equations. The flow in the boundary layer was found, and the structure of the wake flow field at the rear of the train under different operating conditions was compared. The results showed that the arrangement of VGs at different locations in the tail car had a significant effect on the slipstream around the train, the train aerodynamics, and the flow field structure in the tail section. The placement of VGs at the bottom of the train reduced the slipstream, while the VGs placed on both sides of the glass and at the nose tip point increased the slipstream significantly. The VG could effectively reduce the pressure drag of the train, but had little influence on the shear drag of the train.

Article
High fidelity spray flame modelling without ad-hoc tuning of the injection parameters is proposed for an Euler-Lagrangian LES description of turbulent combustion in swirled two-phase flow chambers. It is tested in a laboratory burner (SICCA-Spray rig from EM2C), which comprises both a simplex pressure swirl and an airblast atomiser. Relevant phenomena controlling the liquid spray inside the injector including primary and edge atomisation from a pressure swirl and an airblast atomiser respectively, secondary atomisation, evaporation and the formation and dynamics of the liquid film that forms on the inner injector wall are taken into account. Particular attention has been applied to understanding the shape of the droplet diameter probability distribution function and how each atomisation model contributes to it. Results show excellent agreement of the spray statistics measured experimentally with and without combustion. A sensitivity analysis of the model parameters shows that the models are sufficiently insensitive to user chosen parameters, which, together with their relatively low computational cost, make these models ideal candidates for industrial applications.

Article
Direct numerical simulations of a turbulent premixed stoichiometric methane-oxygen flame were conducted. The chosen combustion pressure was 20 bar, to resemble conditions encountered in modern rocket combustors. The chemical reactions followed finite rate detailed mechanism integrated into the EBI-DNS solver within the OpenFOAM framework. Flame geometry was thoroughly investigated to assess its interaction with the transport of turbulent properties. The resulting flame front was remarkably thin, with high density gradients and moderate Karlovitz and Damköhler numbers. At mid-flame positions, the variable-density transport mechanisms dominated, leading to the generation of both vorticity and turbulence. A reversion of this trend towards the products was observed. For intermediate combustion progress, vorticity transport is essentially a competition between the baroclinic torque and vortex dilatation. The growth of turbulent kinetic energy is strongly correlated to this process. A geometrical analysis reveals that the generation of enstrophy and turbulence is restricted to specific topologies. Convergent and divergent flame propagation promote turbulence creation due to pressure fluctuation gradients through different physical processes. The possibility of modeling turbulence transport based on curvature is discussed along with the inherent challenges.

Article
In this study, a single-cylinder direct-injection spark-ignition research engine with full optical access was used to investigate the influence of the flow field and fuel/air mixing on cyclic variability, in particular in the early flame propagation. The engine was operated under lean-burn conditions at 1500 rpm. Two different injection strategies were compared, port-fuel injection (PFI) and direct injection (DI), the latter with early and late injection split about 2:1 in fuel mass. High-speed particle image velocimetry captured the flow in the tumble plane in the compression stroke. The velocity fields and the movement of the tumble vortex are analyzed. Simultaneously, a second camera detected the chemiluminescence of the flame, and the projected area of the line-of-sight-integrated flame luminosity was extracted through morphological image processing. By combining pressure-based combustion analysis and high-speed optical diagnostics, the early flame propagation and the flow field are correlated. In separate experiments the equivalence ratio was imaged for the DI at selected crank angles and correlated with CA10 to learn about the influence of mixture inhomogeneity on early flame propagation. With PFI, the flow near the spark plug just before ignition is closely related to the subsequent speed of combustion. The combustion-relevant flow features can be traced back in time to about –90 °CA. In contrast, the chosen DI scheme results in a decorrelation of flow and flame, and the equivalence ratio distribution at ignition becomes more important. For both flow and mixture fields, regions of high correlation with early-combustion metrics are typically associated with gradients in the multi-cycle average fields.

Article
We perform direct numerical simulations of turbulent flow at friction Reynolds number Reτ≈500-2000\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$Re_\tau \approx 500{-}2000$$\end{document} grazing over perforates plates with moderate viscous-scaled orifice diameter d+≈40-160\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$d^+\approx 40-160$$\end{document} and analyse the relation between permeability and added drag. Unlike previous studies of turbulent flows over permeable surfaces, we find that the flow inside the orifices is dominated by inertial effects, and that the relevant permeability is the Forchheimer and not the Darcy one. We find evidence of a fully rough regime where the relevant length scale is the inverse of the Forchheimer coefficient, which can be regarded as the resistance experienced by the wall-normal flow. Moreover, we show that, for low porosities, the Forchheimer coefficient can be estimated with good accuracy using a simple analytical relation.

Article
A new algebraic RANS model for laminar–turbulent transition will be presented. The model follows the Local-Correlation-based Transition Modeling concept, is Galilean invariant and can handle natural, bypass and separation-induced transition. The model formulation is discussed in detail. A substantial number of test cases have been computed to evaluate the different transition mechanisms of the model.

Article
Direct numerical simulation of a fully developed turbulent channel flow controlled using a streamwise traveling wave having a periodicity not only in the streamwise direction but also in the spanwise direction, referred to as wave-machine-like traveling wave, is performed to investigate the impact of the spanwise variation of the streamwise traveling wave on the drag reduction effect. The maximum drag reduction rate attained in the present study is smaller than that in the case of the spanwise-uniform traveling wave. The drag reduction rate increases as the spanwise wavelength increases, and the drag reduction effect can be obtained in the range of λz+>400\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\lambda _z^+ > 400$$\end{document}. According to the analysis of the phased-averaged Reynolds shear stress (RSS), the wave-machine-like traveling wave makes the flow field more uniform in the streamwise direction and non-uniform in the spanwise direction, as compared to the case of spanwise-uniform traveling wave. While the turbulent component of RSS in the antinode plane is suppressed near the wall, that in the node plane is significantly suppressed far from the wall. An analysis using the Fukagata–Iwamoto–Kasagi identity shows that the drag reduction effect, which was primary due to the significant decrease in the contribution from the turbulent component of the RSS in the case of spanwise-uniform traveling wave, is partly deteriorated by the contribution from the periodic (i.e., dispersive) component of RSS in the case of wave-machine-like traveling wave.

Article
Experiments were carried out to assess the influence of spanwise spacing between adjacent orifices of an air-jet vortex-generator (AJVG) array on their separation-control effectiveness. The array was applied to a 24° compression-ramp-induced shock-wave / turbulent boundary-layer interaction at M∞=2.52 and Reθ=8225. Three spanwise oriented AJVG arrays of small, intermediate, and large jet spacings were studied. Their influence on the mean-flow organisation and turbulence quantities was assessed using flow visualisations and planar particle image velocimetry across multiple measurement planes. The streamwise vortices induced by the AJVGs incited different control effects depending on the degree of interaction between adjacent vortices. The array with intermediate spacing achieved the most favourable effects with reductions in separation length and area of about 25% and 52%, respectively. This reduction was brought about by the formation of stable, interacting streamwise-elongated coherent vortices downstream of jet injection and the associated entrainment of high-momentum fluid. The smallest jet spacing incites vortex interactions to adverse strength, breaking up the coherent structures and even increasing separation. The AJVGs with the largest spacing display characteristics similar to single jets in crossflow, with only a local modification of the separation region. Turbulent quantities are amplified both by the jets and the separation-inducing shock; AJVG control reduces the amplification across the shock-wave/boundary-layer interaction and the intermediate jet spacing is most effective also here.

Article
In this paper, an experimental study, aimed at delaying flow separation on a high-lift device using a pulsed blowing excitation method, is reported. The main objective of this investigation was to evaluate a new pulsed jet generation strategy to enhance flow control performance. In these experiments, two types of signal waveform were implemented to produce the unsteady blowing; a simple square-wave excitation signal for the first case, and a burst modulated excitation signal for the second case. The signal modulation was the first time to be used for a fast-switching solenoid valve actuator. Another objective of this study was to evaluate a new arrangement of the jet exit slots, in the form of a vortex generator which was employed for the first time on the high-lift device. For this purpose, a NASA SC(2)-0714 airfoil with a single slotted flap was employed. The vortex generator jets emanated from the shoulder of the trailing-edge flap with excitation frequencies from 40 to 1000 Hz. Pressure distribution around the model and wake total pressure deficit were measured. The results indicated that ejection from vortex generator slot pairs was able to prevent flow separation completely in most conditions. These measurements revealed that the burst modulated excitation signal was accompanied by more aerodynamic improvements and less air consumption relative to the simple pulsed jet excitation signal. In the best flow control mode, the results showed about a 53% increase in the value of the suction pressure peak on the flap and a 38% decrease in drag with a reduction in total pressure loss.

Article
The paper studies the dissociation and combustion of a layer of methane hydrate powder at a forced air flow over the upper surface of the layer (the air velocity is directed parallel to the upper surface of the layer). The influence of the layer thickness and air velocity on the combustion of gas hydrate is investigated. The calculated curves for the effect of the heat transfer coefficient, external convection and vapor concentration on the combustion temperature are obtained. The layer thickness and the air velocity significantly affect the dissociation rate of methane hydrate.

Article
Data-driven turbulence modelling is becoming common practice in the field of fluid mechanics. Complex machine learning methods are applied to large high fidelity data sets in an attempt to discover relationships between mean flow features and turbulence model parameters. However, a clear discrepancy is emerging between complex models that appear to fit the high fidelity data well a priori and simpler models which subsequently hold up in a posteriori testing through CFD simulations. With this in mind, a novel error quantification technique is proposed consisting of an upper and lower bound, against which data-driven turbulence models can be systematically assessed. At the lower bound is models that are linear in either the full set or a subset of the input features, where feature selection is used to determine the best model. Any machine learning technique must be able to improve on this performance for the extra complexity in training to be of practical use. The upper bound is found by the stable insertion of the high fidelity data for the Reynolds stresses into CFD simulation. Three machine learning methods, Gene Expression Programming, Deep Neural Networks and Gaussian Mixtures Models are presented and assessed on this error quantification technique. We further show that for the simple canonical cases often used to develop data-driven methods, lower bound linear models can provide very satisfactory accuracy and stability with limited scope for substantial improvement through more complex machine learning methods.

Article
The effects of buoyancy on turbulent premixed flames are expected to be strong due to the large changes in density between the unburned and fully burned gases. The present work utilises three-dimensional direct numerical simulations of statistically planar turbulent premixed flames under decaying turbulence to study the influence of buoyancy on the evolution of turbulent kinetic energy within the flame brush. Four sets of turbulence parameters have been studied, with four different values of Froude number for each set. It is found that for a given set of turbulence parameters, flame wrinkling increases with an increase in body force magnitude in the case of unstable stratification, which is also reflected in the increased values of turbulent burning velocity and flame surface area. An increase in body force magnitude in the case of stable stratification acts to reduce the extent of flame wrinkling. Turbulent kinetic energy and its dissipation rate are found to be affected by both the magnitude and direction of the body force. For low turbulence intensities considered here, turbulent kinetic energy increases from the leading edge of the flame brush before decaying eventually towards the product side of the flame brush. For high turbulence intensities, the turbulent kinetic energy is found to decay across the flame brush, and it is also found that the effect of body force on the evolution of turbulent kinetic energy is marginal in the case of high turbulence intensities. The effects of body force magnitude and direction on the statistical behaviours and closures of the various terms of the turbulent kinetic energy transport equation are analysed, and existing models have been modified to account for Froude number effects, where necessary.

Article
This paper provides a numerical study on n-dodecane flames using Large-Eddy Simulations (LES) along with the Flamelet Generated Manifold (FGM) method for combustion modeling. The computational setup follows the Engine Combustion Network Spray A operating condition, which consists of a single-hole spray injection into a constant volume vessel. Herein we propose a novel approach for the coupling of the energy equation with the FGM database for spray combustion simulations. Namely, the energy equation is solved in terms of the sensible enthalpy, while the heat of combustion is calculated from the FGM database. This approach decreases the computational cost of the simulation because it does not require a precise computation of the entire composition of the mixture. The flamelet database is generated by simulating a series of counterflow diffusion flames with two popular chemical kinetics mechanisms for n-dodecane. Further, the secondary breakup of the droplet is taken into account by a recently developed modified version of the Taylor Analogy Breakup model. The numerical results show that the proposed methodology captures accurately the main characteristics of the reacting spray, such as mixture formation, ignition delay time, and flame lift-off. Additionally, it captures the “cool flame" between the flame lift-off and the injection nozzle. Overall, the simulations show differences between the two kinetics mechanisms regarding the ignition characteristics, while similar flame structures are observed once the flame is stabilised at the lift-off distance.

Article
A model to predict soot evolution during the combustion of complex fuels is presented. On one hand, gas phase, $$\hbox {polycyclic aromatic hydrocarbon (PAH)}$$ polycyclic aromatic hydrocarbon (PAH) and soot chemistry are kept large enough to cover all relevant processes in aero engines. On the other hand, the mechanisms are reduced as far as possible, to enable complex computational fluid dynamics (CFD) combustion simulations. This is important because all species transport equations are solved directly in the $$\hbox {CFD}$$ CFD . Moreover, emphasis is placed on the applicability of the model for a variety of fuels and operating conditions without adjusting it. A kinetic scheme is derived to describe the chemical breakdown of short- and long-chain hydrocarbon fuels and even blends of them. $$\hbox {PAHs}$$ PAHs are the primary soot precursors which are modeled by a sectional approach. The reversibility of the interaction between different $$\hbox {PAH}$$ PAH classes is achieved by the introduction of $$\hbox {PAH}$$ PAH radicals. Soot particles are captured by a detailed sectional approach too, which takes a non-spherical growth of particles into account. In this way the modeling of surface processes is improved. The applicability and validity of the gas phase, $$\hbox {PAH}$$ PAH , and soot model is demonstrated by a large number of shock tube experiments, as well as in atmospheric laminar sooting flames. The presented model achieves excellent results for a wide range of operating conditions and fuels. One set of model constants is used for all simulations and no case-dependent optimization is required.

Article
The flame pocket formation, including reactant pocket, product pocket, soot pocket, and fluid parcel, is a common phenomenon in turbulent combustion occurred as a response of the flame to flow straining and shearing. Understanding pocket behavior is vital to study the flames in such a regime. This work addresses the research need to experimentally measure and track multiple flame pockets in 3D. For this purpose, volumetric measurements were performed to measure the high-speed turbulent flame structure at 15 kHz based on emission tomography. With the 3D flame structures, a new tracking algorithm was developed to identify and track the multiple flame pockets simultaneously in 3D. The instantaneously tracked 3D flame pockets enabled the extraction of key properties of pocket dynamics, including the favorable formation location, 3D3C movement speed, and pocket expanding/shrinking speed. The developed methods were evidently able to resolve the detailed behavior of flame pockets in highly turbulent flames.

Article
The effects of varying turbulence intensity and turbulence length scale on premixed turbulent flame propagation are investigated using Direct Numerical Simulation (DNS). The DNS dataset contains the results of a set of turbulent flame simulations based on separate and systematic changes in either turbulence intensity or turbulence integral length scale while keeping all other parameters constant. All flames considered are in the thin reaction zones regime. Several aspects of flame behaviour are analysed and compared, either by varying the turbulence intensity at constant integral length scale, or by varying the integral length scale at constant turbulence intensity. The turbulent flame speed is found to increase with increasing turbulence intensity and also with increasing integral length scale. Changes in the turbulent flame speed are generally accounted for by changes in the flame surface area, but some deviation is observed at high values of turbulence intensity. The probability density functions (pdfs) of tangential strain rate and mean flame curvature are found to broaden with increasing turbulence intensity and also with decreasing integral length scale. The response of the correlation between tangential strain rate and mean flame curvature is also investigated. The statistics of displacement speed and its components are analysed, and the findings indicate that changes in response to decreasing integral length scale are broadly similar to those observed for increasing turbulence intensity, although there are some interesting differences. These findings serve to improve current understanding of the role of turbulence length scales in flame propagation.

Article
In free-field operation, many aerodynamic systems are confronted with changing turbulent inflow conditions. Wind turbines are a prominent example. Here, the rotation of the rotor blades causes incoming wind gusts to result in a local change in the angle of incidence for the blade segments, which changes the effective angle of attack and can lead to dynamic non-linear effects like dynamic stall. Dynamic stall is known to produce a significant overshoot in the acting forces and thus an increase in loads acting on the wind turbine, leading to long-term fatigue. To gain a better understanding, it is necessary to perform wind tunnel experiments under realistic and reproducible inflow with defined conditions. In this study, a so-called 2D active grid is presented, which allows the generation of defined two-dimensional inflow conditions for wind tunnel experiments. The focus is on generating sinusoidal transversal and longitudinal gusts with high amplitudes and frequencies. Different grid configurations and sizes are tested to investigate differences in the generated flow fields. Transversal gusts imposed in this way can be used to study dynamic phenomena without having to move the object under investigation itself. Inertial effects during force measurements and a changing shadow casting due to moving airfoils in particle image velocimetry measurements are thus avoided. The additional possibility to generate defined longitudinal gusts allows to generate a broad range of reproducible inflow situations like yaw or tower shadow effects during experimental investigations.

Article
Scalar forcing in the context of turbulent stratified flame simulations aims to maintain the fuel-air inhomogeneity in the unburned gas. With scalar forcing, stratified flame simulations have the potential to reach a statistically stationary state with a prescribed mixture fraction distribution and root-mean-square value in the unburned gas, irrespective of the turbulence intensity. The applicability of scalar forcing for Direct Numerical Simulations of stratified mixture combustion is assessed by considering a recently developed scalar forcing scheme, known as the reaction analogy method, applied to both passive scalar mixing and the imperfectly mixed unburned reactants of statistically planar stratified flames under low Mach number conditions. The newly developed method enables statistically symmetric scalar distributions between bell-shaped and bimodal to be maintained without any significant departure from the specified bounds of the scalar. Moreover, the performance of the newly proposed scalar forcing methodology has been assessed for a range of different velocity forcing schemes (Lundgren forcing and modified bandwidth forcing) and also without any velocity forcing. It has been found that the scalar forcing scheme has no adverse impact on flame-turbulence interaction and it only maintains the prescribed root-mean-square value of the scalar fluctuation, and its distribution. The scalar integral length scale evolution is shown to be unaffected by the scalar forcing scheme studied in this paper. Thus, the scalar forcing scheme has a high potential to provide a valuable computational tool to enable analysis of the effects of unburned mixture stratification on turbulent flame dynamics.

Article
Prediction of differential molecular diffusion remains a great challenge for flamelet modeling of turbulent non-premixed combustion. This work addresses this challenge through a priori and a posteriori testing of flamelet models by using DNS databases to enable a detailed examination of the model capability and limitation for the prediction of differential molecular diffusion under different combustion conditions characterized by different Reynolds numbers and Damköhler numbers. The emphasis is on the effect of unsteadiness and scalar dissipation models on the flamelet modeling. Two sets of Sandia DNS of temporally evolving turbulent non-premixed jet flames are used for the study, including six different cases. Three of the DNS cases are based on the CO/$$\hbox {H}_2$$ fuel with different Reynolds numbers, and the other three cases are based on the $$\hbox {C}_2\hbox {H}_4$$ fuel with different Damköhler numbers. The unsteady effect is examined by considering both steady and unsteady flamelet models in the context of Reynolds averaged Navier–Stokes simulations for the model examination. Different differential molecular diffusion models are incorporated in the flamelet models such as the linear differential diffusion model and the non-linear differential diffusion model (Wang, Phys Fluids 28:035102, 2019). It is found that the use of unsteady flamelet models can generally improve the model prediction for differential molecular diffusion when compared with the steady flamelet models. This suggests the importance of considering the unsteady effect in flamelet modeling of differential molecular diffusion. In the unsteady flamelet modeling, different models for the representative scalar dissipation rate are examined and compared. It is found that, in order to adequately capture differential molecular diffusion, an appropriate model for the scalar dissipation rate is needed in addition to the consideration of unsteadiness.

Article
The unsteady effects of buoyancy-induced instabilities on jet diffusion flames are investigated experimentally under normal gravity conditions. Methane and propane are used as test fuels that are lighter and heavier than ambient air, respectively. A similar Froude (Fr) and Reynolds (Re) number relationship is realized in both hydrocarbon fuels with different tube diameters ranging from 6 to 24.2 mm. The Schlieren visualization technique and high-speed imaging synchronized with chemiluminescence signal measurement are used to identify changes in global flame shape and dominant frequency. Buoyancy-induced instabilities generate two forms of diffusion flames with varying frequencies in space. Both laminar and turbulent jet flames exhibit natural and subharmonic frequencies, as well as a shift between them. The methane-propane Re–Fr relationship confirms the instability mode transition. In addition, Strouhal (St) and Froude number relations are obtained as St∝Fr-0.50\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$St \propto \ Fr^{-0.50}$$\end{document}, with a slope difference between natural and subharmonic modes in both fuels.

Article
This paper presents an experimental study on dynamic response of a forced low-swirl methane/air premixed flame with external acoustic excitation over a wide range of driving frequency. Global flame response in terms of gain and phase delay between flame intensity and incoming velocity perturbation is determined. Local flame response is investigated in detail at three typical frequencies: 55 Hz, 105 Hz and 155 Hz. The effect of swirl number on the flame response is also discussed. Proper orthogonal decomposition is applied to identify the large coherent structures in the forced flame. Experimental results show flame response gain exhibits a successive of valleys and peaks which is dependent on swirl number. Time delay decreases as swirl number is increased. The low-swirl flame oscillates back and forth mainly in the axial direction at low excitation frequency and it turns into radially dominated direction at high frequency. Flame intensity fluctuation is mainly dominated by the tail of the flame at 55 Hz and 155 Hz while the flame response is controlled by a combined effect of the base and tail region at 105 Hz. Further POD analysis shows symmetric, anti-symmetric and helical modes in the flame. The most energetic modes (mode 1 and mode 2) feature a symmetric wave-like structure at low excitation frequency while it tends to be in antisymmetric modes at high frequency.

Article
The physical mechanism leading to flame local extinction remains a key issue to be further understood. An analysis of large eddy simulation (LES) data with presumed probability density function (PDF) based closure (Chen et al. , 2020, Combust. Flame , vol. 212, pp. 415) indicated the presence of localised breaks of the flame front along the stoichiometric line. These observations and their relation to local quenching of burning fluid particles, together with the possible physical mechanisms and conditions allowing their appearance in LES with a simple flamelet model, are investigated in this work using a combined Lagrangian-Eulerian analysis. The Sidney/Sandia piloted jet flames with compositionally inhomogeneous inlet and increasing bulk speeds, amounting to respectively 70 and 90% of the experimental blow-off velocity, are used for this analysis. Passive flow tracers are first seeded in the inlet streams and tracked for their lifetime. The critical scenario observed in the Lagrangian analysis, i.e., burning particles crossing extinction holes on the stoichiometric iso-surface, is then investigated using the Eulerian control-volume approach. For the 70% blow-off case the observed flame front breaks/extinction holes are due to cold and inhomogeneous reactants that are cast onto the stoichiometric iso-surface by large vortices initiated in the jet/pilot shear layer. In this case an extinction hole forms only when the strain effect is accompanied by strong subgrid mixing. This mechanism is captured by the unstrained flamelets model due to the ability of the LES to resolve large-scale strain and considers the SGS mixture fraction variance weakening effect on the reaction rate through the flamelet manifold. Only at 90% blow-off speed the expected limitation of the underlying combustion model assumption become apparent, where the amount of local extinctions predicted by the LES is underestimated compared to the experiment. In this case flame front breaks are still observed in the LES and are caused by a stronger vortex/strain interaction yet without the aid of mixture fraction variance. The reasons for these different behaviours and their implications from a physical and modelling point of view are discussed in this study.

Article
In this paper, a simple method to locally compute the model coefficient C DES in Reddy et al. (Int J Heat Fluid Flow 50:103-113, 2014. https:// doi. org/ 10. 1016/j. ijhea tflui dflow. 2014. 06. 002) is presented. The formula for the coefficient is derived from the structural function B of Vreman (Phys Fluids 16(10):3670-3681, 2004. https:// doi. org/ 10. 1063/1. 17851 31). It, therefore, does not involve explicit filtering or averaging procedures. By virtue of the variable coefficient being based on B , the model is expected to retain the property of relatively small dissipation in transitional and near-wall regions. This property enables the present formulation to be a reasonable candidate to predict transitional flows. The formulation is validated in the canonical, fully developed turbulent channel and backward facing step flows, followed by simulations of orderly, bypass and separation induced laminar-to-turbulent transition in a spatially developing boundary layer over a flat plate.

Article
The mean and instantaneous flow separation of two different three-dimensional asymmetric diffusers is analysed using the data of large-eddy simulations. The geometry of both diffusers under investigation is based on the experimental configuration of Cherry et al. (Int J Heat Fluid Flow 29(3):803–811, 2008). The two diffusers feature similar area ratios of $$\mathrm{AR}=4.8$$ AR = 4.8 and $$\mathrm{AR}=4.5$$ AR = 4.5 while exhibiting differing asymmetric expansion ratios of $$\mathrm{AER}=4.5$$ AER = 4.5 or $$\mathrm{AER}=2.0$$ AER = 2.0 , respectively. The Reynolds number based on the averaged inlet velocity and height of the inlet duct is approximately $${\textit{Re}}=10{,}000$$ Re = 10 , 000 . The time-averaged flow in both diffusers in terms of streamwise velocity profiles or the size and location of the mean backflow region are validated using experimental data. In general good agreement of simulated results with the experimental data is found. Further quantification of the flow separation behaviour and unsteadiness using the backflow coefficient reveals the volume portion in which the instantaneous reversal flow evolves. This new approach investigates the cumulative fractional volume occupied by the instantaneous backflow throughout the simulation, a power density spectra analysis of their time series reveals the periodicity of the growth and reduction phases of the flow separation within the diffusers. The dominating turbulent events responsible for the formation of the energy-containing motions including ejection and sweep are examined using the quadrant analysis at various locations. Finally, isourfaces of the Q-criterion visualise the instantaneous flow and the origin and fate of coherent structures in both diffusers.

Article
The underlying mechanisms of three different flow-control strategies on drag reduction in a channel flow are investigated by direct numerical simulations at friction Reynolds numbers ranging from 65 to 85. These strategies include the addition of long-chain polymers, the incorporation of slip surfaces, and the application of an external body force. While it has been believed that such methods lead to a skin-friction reduction by controlling near-wall flow structures, the underlying mechanisms at play are still not as clear. In this study, a temporal analysis is employed to elucidate underlying drag-reduction mechanisms among these methods. The analysis is based on the lifetime of intermittent phases represented by the active and hibernating phases of a minimal turbulent channel flow (Xi and Graham, Phys Rev Lett 2010). At a similar amount of drag reduction, the polymer and slip methods show a similar mechanism, while the body force method is different. The polymers and slip surfaces cause hibernating phases to happen more frequently, while the duration of active phases is decreased. However, the body forces cause hibernating phases to happen less frequently but prolong its duration to achieve a comparable amount of drag reduction. A possible mechanism behind the body force method is associated with its unique roller-like vortical structures formed near the wall. These structures appear to prevent interactions between inner and outer regions by which hibernating phases are prolonged. It should motivate adaptive flow-control strategies to exploit the distinct underlying mechanisms for robust control of turbulent drag at low Reynolds numbers.

Article
An experiment-based closure framework for turbulent combustion modeling is further validated using the Sydney piloted turbulent partially premixed flames with inhomogeneous inlets. The flames are characterized by the presence of mixed mode combustion. The framework’s closure is “trained” on multi-scalar measurements to construct thermo-chemical scalar statistics parameterized in terms of principal components (PCs). Three flame conditions are used for this training, while an additional flame is used for validation. The results show that the leading PCs exhibit complex features near the jet inlet where effects of partial premixing and the presence of different burning modes are strong. These features may not be captured through a strict definition for the mixture fraction or measures of reaction progress. Further downstream, the first 2 PCs tend to be reasonably correlated with parameters that are characteristic of nonpremixed flames, including the mixture fraction and the progress variable. Comparisons of the model predictions for unconditional mean and RMS for the measured quantities show a very good qualitative and quantitative agreement with experimental statistics for all 4 flames using the same closure for the PCs governing equations.

Article
The hydrodynamic instability characteristics of non-adiabatic N2-diluted n-butane/air flames generated on McKenna burner were investigated experimentally under atmosphere pressure. In order to capture the quantitative structure of cellular flames, planar laser induced fluorescence technology (OH-PLIF and CH2O-PLIF) was employed, as well as the chemiluminescence imaging was used to record flame morphology directly. The results show that the hydrodynamic instability of stoichiometric (Φ = 1.0) n-butane/air flames can be significantly enhanced by N2 dilution. In addition, the increased mixture flow velocity and the reduced equivalence ratio of lean mixtures will enhance hydrodynamic instability. Moreover, the observed flame morphologies are connected wrinkles instead of independent-cells with lean and stoichiometric mixtures. It is probable that the wrinkled flames mainly caused by hydrodynamic instability cannot induce the extinction of high-temperature oxidant reaction in concave regions solely due to the weakened effect of preferential diffusion. The instability mechanism analysis shows that, the remarkably reduced local flame speed and the much deformed local flow field ahead of n-butane/air/N2 dilution flames by increasing N2 dilution ratio play an important role in enhancing hydrodynamic instability. It also indicates that the heat loss reduced more in concave regions than in convex regions toward unburnt mixtures is helpful to enhance the suppression effect of hydrodynamic instability.

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