Wall-modeled Large Eddy Simulation Research Articles

Numerical investigation into the blade and wake aerodynamics of an H-rotor vertical axis wind turbine

Blade aerodynamics is of critical importance in the design of an isolated vertical axis wind turbine (VAWT). In contrast, wake aerodynamics plays a crucial role in the optimum placement of multiple VAWTs. In this study, both the blade and the wake aerodynamics of a straight-bladed VAWT are investigated using a three-dimensional computational fluid dynamics (CFD) model. The algebraic wall-modeled large eddy simulation (LES) was used for turbulence modeling. The LES predictions were compared with and had good agreement with the published experimental data. A semi-empirical method was proposed to estimate the convergence time of CFD calculations. Further, guidelines for the LES modeling of VAWTs were provided. Relatively thick airfoils are recommended for VAWTs to reduce fatigue loading on blades. Torque analysis suggested that winds in built environments would benefit the self-starting performance. Comparisons between transient and time-averaged velocity fields indicated a statistically stable wake. Moreover, the blade speed ratio effect on the development of wake velocities was assessed. The underlying causes of the significant wake asymmetry were addressed. The periodical vortex-ring structures, counter-rotating vortical motions, and wake asymmetry were analyzed and concluded as the signature features of H-rotor VAWTs' wake.

Numerical Study of Iced Airfoils with Horn Features Using Large-Eddy Simulation

Separated flowfields around two iced airfoils with horn features near stall are numerically studied using wall-modeled large-eddy simulation. The iced airfoils are the GLC305 airfoil with rime ice 212 and the NLF0414 airfoil with glaze ice 623. A grid-quality-based hybrid central/upwind scheme is applied for numerical flux computation. The close agreement between the computed lift, drag, and surface pressure distribution and the experimental results illustrates the capability of the present method. In particular, the current method can accurately predict the Kelvin–Helmholtz instability of a free shear layer. The statistical results, instantaneous flowfields, pressure fluctuations, and characteristic frequencies are investigated. Both flowfields are dominated by separation bubbles. The bubble length increases with the angle of attack according to a quadratic relation. A strong-fluctuation region in the flowfield grows rapidly at high angles of attack, especially near stall. High- and low-frequency peaks are found in the initial and further downstream regions of the free shear layer, respectively. The high frequency is caused by the Kelvin–Helmholtz instability and vortex shedding. The low frequency might be caused by the vortex merging in the further downstream region of the free shear layer.

Dependence of small-scale energetics on large scales in turbulent flows

In a turbulent flow, small- and large-scale fluid motions are coupled. In this work, we investigate the small-scale response to large-scale fluctuations in turbulent flows and discuss the implications on large eddy simulation (LES) wall modelling. The interscale interaction in wall-bounded flows was previously parameterized in the predictive inner–outer (PIO) model, where the amplitude of the small scales responds linearly to the large-scale fluctuations. While this assumed linearity is valid in the viscous sublayer, it is an insufficient approximation of the true interscale interaction in wall-normal distances within the buffer layer and above. Within these regions, a piecewise linear response function (piecewise with respect to large-scale fluctuations being positive or negative) appears to be more appropriate. In addition to proposing a new response function, we relate the amplitude modulation process to the Townsend attached eddy hypothesis. This connection allows us to make theoretical predictions on the model parameters within the PIO model. We use these parameters to apply the PIO model to wall-modelled LES. Further, we present empirical evidence of amplitude modulation in isotropic turbulence. The evidence suggests that the existence of nonlinear interscale interactions in the form of amplitude modulation does not rely on the presence of a non-penetrating boundary, but on the presence of a range of viscosity-dominated scales and a range of inertial-dominated scales.

Wall modeling via function enrichment: Extension to detached-eddy simulation

We extend the approach of wall modeling via function enrichment to detached-eddy simulation. The wall model aims at using coarse cells in the near-wall region by modeling the velocity profile in the viscous sublayer and log-layer. In our approach however, unlike other wall models, the full Navier–Stokes equations are still discretely satisfied, including the pressure gradient and convective term. This is achieved by enriching the elements of the high-order discontinuous Galerkin method with the law-of-the-wall. As a result, the Galerkin method can “choose” the optimal solution among the polynomial and enrichment shape functions. The detached-eddy simulation methodology provides a suitable turbulence model for the coarse near-wall cells. The approach is applied to wall-modeled LES of turbulent channel flow in a wide range of Reynolds numbers. Flow over periodic hills shows the superiority compared to an equilibrium wall model under separated flow conditions.

Simulation of Laser-Induced Plasma Ignition in a Hypersonic Crossflow

A numerical study of a laser-induced plasma ignition system in a scramjet-like geometry is conducted. Simulations are based on experiments in which hydrogen is injected into a hypersonic air-crossflow, compressed by a ramp from a freestream Mach number of 8 to below hydrogen autoignition conditions. Ignition of the jet is attempted using a laser pulse, modeled in the simulations as a small region of high temperature and pressure corresponding to the energy input measured in the experiments. To capture the highly unsteady nature of the fuel-air interface and its coupling with the dynamics of the expanding laser spark, wall-modeled large-eddy Simulations are used to resolve the majority of turbulent motion. Thermal nonequilibrium and detailed finite-rate chemistry models are included in the calculations. The simulations indicate that the cloud of hydroxyl radicals detected in the experiments is created on the jet’s forward surface, as the expanding plasma kernel drives a blast wave into the bowshock surrounding the jet. This event raises the temperature and pressure in the upstream mixing layer, causing spontaneous ignition of the mixture. Within tens of microseconds the laser-induced combustion detaches from the jet and combustion ceases due to unfavorable conditions for flame anchoring in this area. Downstream of the jet, the simulations suggest that the plasma kernel remains can initiate radical formation in the wake region, but no stable flame forms due to nominally low pressures in the experimental condition studied.

A semi-locally scaled eddy viscosity formulation for LES wall models and flows at high speeds

We show that the mean wall-shear stresses in wall-modeled large-eddy simulations (WMLES) of high-speed flows can be off by up to $$\approx 100\%$$ with respect to a DNS benchmark when using the van-Driest-based damping function, i.e., the conventional damping function. Errors in the WMLES-predicted wall-shear stresses are often attributed to the so-called log-layer mismatch, which, albeit also an error in wall-shear stresses $$\tau _\mathrm{w}$$ , is an error of about $$15\%$$ . The larger error identified here cannot be removed using the previously developed remedies for the log-layer mismatch. This error may be removed by using the semi-local scaling, i.e., $$l_\nu =\mu /\sqrt{\rho \tau _\mathrm{w}}$$ , in the damping function, where $$\mu $$ and $$\rho $$ are the local mean dynamic viscosity and density, respectively.

Fast and accurate prediction of airflow and drag force for duct ventilation using wall-modeled large-eddy simulation

Rectangular ducts are commonly adopted in ventilation systems to push airflow through pipes, with Reynolds number reaching up to 500000. Computational cost for flows with such a Reynolds number is prohibitive. Moreover, accurate prediction of airflow and drag in duct flows is important for ventilation duct design to remove dust particles and reduce turbulence, which will be beneficial for pipes cleaning and energy consumption. Hence, this work proposed a fast and accurate technique to simulate high-Reynolds-number turbulent flows in a square duct, using improved wall-modeled large-eddy simulation (LES) models. The improved wall-modeled LES approach features an improved wall-stress model and a self-adaptive subgrid-scale (SGS) model, which can dynamically adjust the mixing length to better predict wall-bounded flows. A new filtering strategy (filtering in time and space) is proposed as well. Wall-modeled LES are then conducted for the duct flows at a Reynolds number of 250000 with y+ of the first point (away from the wall) larger than 300. Existing experiments were applied for validations. In the result, the computational time is greatly reduced (over 90%) compared with wall-resolved LES. The improved wall-modeled LES shows a better prediction of wall friction (drag) with acceptable mean velocity profiles. Moreover, the self-adaptive model captures the acceleration of the mean streamwise velocity near the corner but not for the conventional Smagorinsky approach. The methodology developed in this study provides a fast and accurate wall-modeled LES approach for the simulation of high-Reynolds-numbers ventilation duct flows.

Large Eddy Simulation of airflow in a test ventilated room

The goal of the study is the validation of Wall-Modeled Large Eddy Simulation (WMLES) of ventilation in an isothermal room at conditions of experiments by Nielsen et al. Two values of the inlet slot width were considered. Calculations were carried out using the ANSYS Fluent 16.2 software with an algebraic WMLES subgrid-scale model. The uniform computational meshes up to 58 million cells were used. The focus in the results discussion is on the accuracy of mean and rms velocity fields prediction both in the jet zone and the low-velocity occupied zone. It is concluded that to predict the jet spread the meshes used are sufficient. However, there is visible difference between the computed and measured data in the occupied zone especially in the essentially 3D case with narrow inlet slot.

Preliminary estimate of the impact of support structures on the aerodynamic performance of very large wind farms

An extended theoretical model, which is based on a two-scale coupled momentum conservation argument, is proposed to estimate aerodynamic effects of support structures on the performance of ideal very large wind farms. A key implication of this extended model is that the parameter (As/A) ∙ where A and As are the rotor swept area and support-structure frontal projected area, respectively, and is an effective support-structure drag coefficient, plays an important role in the design of very large wind farms. In particular, the optimal farm density tends to decrease as the normalised support-structure drag increases. To validate this extended model, Wall-Modelled Large-Eddy Simulations (WMLES) of a periodic array of actuator discs with and without support structures are conducted; results agree qualitatively with the model.

Wall-modeled large-eddy simulation of the flow past a rod-airfoil tandem by the Lattice Boltzmann method

PurposeThe Lattice Boltzmann (LB) method offers an alternative to conventional computational fluid dynamics (CFD) methods. However, its practical use for complex turbulent flows of engineering interest is still at an early stage. This paper aims to outline an LB wall-modeled large-eddy simulation (WMLES) solver.Design/methodology/approachThe solver is dedicated to complex high-Reynolds flows in the context of WMLES. It relies on an improved LB scheme and can handle complex geometries on multi-resolution block structured grids.FindingsDynamic and acoustic characteristics of a turbulent airflow past a rod-airfoil tandem are examined to test the capabilities of this solver. Detailed direct comparisons are made with both experimental and numerical reference data.Originality/valueThis study allows assessing the potential of an LB approach for industrial CFD applications.

Wall-Modeled Large-Eddy Simulation of Transonic Airfoil Buffet at High Reynolds Number

In this study, wall-modeled large-eddy simulation of transonic buffet phenomena over the OAT15A supercritical airfoil at two different Mach number conditions (nonbuffet and buffet conditions) at high Reynolds number is conducted to investigate the prediction accuracy of the wall-modeled large-eddy simulation for the buffet phenomena. Thanks to the wall model, at the present Reynolds number, the grid spacing and the time-step size are increased by one order of magnitude compared with the traditional wall-resolved large-eddy simulation, and the high-fidelity large-eddy simulation can be applied to the buffet phenomena, which include the very low-frequency shock-wave oscillation. The results demonstrate the capability of the present wall-modeled large-eddy simulation for predicting the buffet phenomena. The present wall-modeled large-eddy simulation successfully predicts the buffet onset in the Mach number sweep, the buffet frequency, and turbulence statistics without the use of any ad hoc corrections. Instantaneous flow structures show that the present wall-modeled large-eddy simulation properly resolves the key flow features in the unsteady dynamics of the transonic airfoil buffet phenomena such as various scales of turbulent structures, generated acoustic waves, detailed shock-wave/boundary-layer interaction, and shock-induced flow separation.

A rapid and low noise switch from RANS to WMLES on curvilinear grids with compressible flow solvers

A turbulent inflow for a rapid and low noise switch from RANS to Wall-Modelled LES on curvilinear grids with compressible flow solvers is presented. It can be embedded within the computational domain in practical applications with WMLES grids around three-dimensional geometries in a flexible zonal hybrid RANS/LES modelling context. It relies on a physics-motivated combination of Zonal Detached Eddy Simulation (ZDES) as the WMLES technique together with a Dynamic Forcing method processing the fluctuations caused by a Zonal Immersed Boundary Condition describing roughness elements. The performance in generating a physically-sound turbulent flow field with the proper mean skin friction and turbulent profiles after a short relaxation length is equivalent to more common inflow methods thanks to the generation of large-scale streamwise vorticity by the roughness elements. Comparisons in a low Mach-number zero-pressure-gradient flat-plate turbulent boundary layer up to Reθ=6100 reveal that the pressure field is dominated by the spurious noise caused by the synthetic turbulence methods (Synthetic Eddy Method and White Noise injection), contrary to the new low-noise approach which may be used to obtain the low-frequency component of wall pressure and reproduce its intermittent nature. The robustness of the method is tested in the flow around a three-element airfoil with WMLES in the upper boundary layer near the trailing edge of the main element. In spite of the very short relaxation distance allowed, self-sustainable resolved turbulence is generated in the outer layer with significantly less spurious noise than with the approach involving White Noise. The ZDES grid count for this latter test case is more than two orders of magnitude lower than the Wall-Resolved LES requirement and a unique mesh is involved, which is much simpler than some multiple-mesh strategies devised for WMLES or turbulent inflow.

Flow over a realistic car model: Wall modeled large eddy simulations assessment and unsteady effects

Numerical simulations are carried out on the flow over a realistic generic car geometry, the DrivAer-fastback car model. Pure large eddy simulations (LES) and wall-modeled large eddy simulations (WMLES) are used and compared to numerical and experimental results to assess the validity of these approaches when solving the flow field around complex automotive geometries. Results show a 70% CPU time reduction when using the wall model. Drag coefficient results show the influence of the wall model on coarser meshes is positive, reducing the difference on those obtained using finer meshes. Pressure profiles exhibit mixed results. The wall model used works well in adverse pressure gradients and smooth geometry changes. Results worsen in sections where the flow detaches and experiences large pressure drops. Flow structures and unsteady effects around the car are also analyzed, obtaining several characteristic frequencies for the different flow structures encountered. It should be noted that the present investigation shows how WMLES helps to reduce the computing cost and response vs pure LES, while providing high-quality unsteady data, although computational cost remains high. Results show potential in the introduction of this tool as a competitive simulation strategy for complex geometries.

Modeling boundary-layer transition in direct and large-eddy simulations using parabolized stability equations.

We examine the potential of the nonlinear parabolized stability equations (PSE) to provide an accurate yet computationally efficient treatment of the growth of disturbances in H-type transition to turbulence. The PSE capture the nonlinear interactions that eventually induce breakdown to turbulence and can as such identify the onset of transition without relying on empirical correlations. Since the local PSE solution at the onset of transition is a close approximation of the Navier-Stokes equations, it provides a natural inflow condition for direct numerical simulations (DNS) and large-eddy simulations (LES) by avoiding nonphysical transients. We show that a combined PSE-DNS approach, where the pretransitional region is modeled by the PSE, can reproduce the skin-friction distribution and downstream turbulent statistics from a DNS of the full domain. When the PSE are used in conjunction with wall-resolved and wall-modeled LES, the computational cost in both the laminar and turbulent regions is reduced by several orders of magnitude compared to DNS.

Turbulence intensities in large-eddy simulation of wall-bounded flows.

A persistent problem in wall-bounded large-eddy simulations (LES) with Dirichlet no-slip boundary conditions is that the near-wall streamwise velocity fluctuations are overpredicted, while those in the wall-normal and spanwise directions are underpredicted. The problem may become particularly pronounced when the near-wall region is underresolved. The prediction of the fluctuations is known to improve for wall-modeled LES, where the no-slip boundary condition at the wall is typically replaced by Neumann and no-transpiration conditions for the wall-parallel and wall-normal velocities, respectively. However, the turbulence intensity peaks are sensitive to the grid resolution and the prediction may degrade when the grid is refined. In the present study, a physical explanation of this phenomena is offered in terms of the behavior of the near-wall streaks. We also show that further improvements are achieved by introducing a Robin (slip) boundary condition with transpiration instead of the Neumann condition. By using a slip condition, the inner energy production peak is damped, and the blocking effect of the wall is relaxed such that the splatting of eddies at the wall is mitigated. As a consequence, the slip boundary condition provides an accurate and consistent prediction of the turbulence intensities regardless of the near-wall resolution.

Wall-Modeled Large-Eddy Simulation for Complex Turbulent Flows.

Large-eddy simulation (LES) has proven to be a computationally tractable approach to simulate unsteady turbulent flows. However, prohibitive resolution requirements induced by near-wall eddies in high-Reynolds number boundary layers necessitate the use of wall models or approximate wall boundary conditions. We review recent investigations in wall-modeled LES, including the development of novel approximate boundary conditions and the application of wall models to complex flows (e.g., boundary-layer separation, shock/boundary-layer interactions, transition). We also assess the validity of underlying assumptions in wall-model derivations to elucidate the accuracy of these investigations, and offer suggestions for future studies.

Flow and Heat Transfer Analysis in a Single Row Narrow Impingement Channel: Comparison of Particle Image Velocimetry, Large Eddy Simulation, and RANS to Identify RANS Limitations

The present study aims to understand the flow, turbulence, and heat transfer in a single row narrow impingement channel for gas turbine heat transfer applications. Since the advent of several advanced manufacturing techniques, narrow wall cooling schemes have become more practical. In this study, the Reynolds number based on jet diameter was ≃15,000, with the jet plate having fixed jet hole diameters and hole spacing. The height of the channel is three times the impingement jet diameter. The channel width is four times the jet diameter of the impingement hole. The dynamics of flow and heat transfer in a single row narrow impingement channel are experimentally and numerically investigated. Particle image velocimetry (PIV) was used to reveal the detailed information of flow phenomena. PIV measurements were taken at a plane normal to the target wall along the jet centerline. The mean velocity field and the turbulent statistics generated from the mean flow field were analyzed. The experimental data from the PIV reveal that the flow is highly anisotropic in a narrow impingement channel. To support experimental data, wall-modeled large eddy simulation (LES) and Reynolds-averaged Navier–Stokes (RANS) simulations (shear stress transport k–ω, ν2−f, and Reynolds stress model (RSM)) were performed in the same channel geometry. Mean velocities calculated from the RANS and LES were compared with the PIV data. Turbulent kinetic energy budgets were calculated from the experiment, and were compared with the LES and RSM model, highlighting the major shortcomings of RANS models to predict correct heat transfer behavior for the impingement problem. Temperature-sensitive paint (TSP) was also used to experimentally obtain a local heat transfer distribution at the target and the side walls. An attempt was made to connect the complex aerodynamic flow behavior with the results obtained from heat transfer, indicating heat transfer is a manifestation of flow phenomena. The accuracy of LES in predicting the mean flow field, turbulent statistics, and heat transfer is shown in the current work as it is validated against the experimental data through PIV and TSP.

Blast Wave-Induced Mixing in a Laser Ignited Hypersonic Flow

A laser ignition system suitable for a hypersonic scramjet engine is considered. Wall-modeled large eddy simulation (LES) is used to study a scramjet-like geometry with a single hydrogen injector on the inlet, at a Mach 8 flight condition with a total enthalpy of 2.5 MJ. Detailed chemical kinetics and high fidelity turbulence modeling are used. The laser forms a kernel of high temperature plasma inside the fuel plume that briefly ignites the flow and leads to massive disruption of the flow structures around the jet, due to the expanding plasma kernel driving a blast wave that collides with the surrounding flow. The blast wave produces vorticity as it passes through the fuel–air interface, but comparably less than that produced by the jetting of the hot gas affected by the laser as it expands outward into the crossflow. The remnant of the plasma rolls up into a powerful vortex ring and noticeably increases the fuel plume area and the volume of well mixed reactants present in the simulation. These results indicate that the laser ignition system does more than just supply the energy to ignite the flow; it also substantially alters the flow structure and the mixing process.

Aerodynamic Heating in Wall-Modeled Large-Eddy Simulation of High-Speed Flows

Aerospace vehicles flying at supersonic and hypersonic speeds are subject to increased wall heating rates caused by viscous friction with the gas environment. This extra heat is commonly referred to as convective aerodynamic heating. In wall-modeled large-eddy simulations, the near-wall region of the flow is not resolved by the computational grid. As a result, the effects of aerodynamic heating need to be modeled using a large-eddy simulation wall model. In this investigation, wall-modeled large-eddy simulations of turbulent high-speed flows are performed to address this issue. In particular, an equilibrium wall model is employed in high-speed turbulent Couette flows subject to different combinations of thermal boundary conditions and grid sizes as well as in transitional hypersonic boundary layers interacting with incident shock waves. Specifically, the wall-modeled large-eddy simulations of the Couette flow configuration demonstrate that the shear-stress and heat-flux predictions made by the wall model show only a small sensitivity to the grid resolution even in the most adverse case where aerodynamic heating prevails near the wall and generates a sharp temperature peak there. Additionally, the simulations indicate that the wall model predicts shear stresses and heat fluxes that are mostly proportional to the near-wall velocity in a manner that resembles an approximate power law. In the wall-modeled large-eddy simulation of hypersonic boundary-layer/shock-wave interaction, the model is tested against direct numerical simulations and experiments. It is shown to correctly capture aerodynamic heating and the overall heat transfer rate around the shock-impingement zone, despite the fact that the adverse pressure gradients in that region may involve nonequilibrium effects.

Application of Hybrid RANS/LES turbulence models in rotor-stator fluid machinery: a comparative study

Purpose This paper aims to improve performance prediction and to acquire more detailed flow structures so as to analyze the turbulence in complex rotor-stator flow. Design/methodology/approach Hydraulic retarder as typical fluid machinery was numerically investigated by using hybrid Reynolds-averaged Navier–Stokes (RANS)/large eddy simulation (LES) models CIDDES Algebraic Wall-Modeled Large Eddy Simulation (LES) (WMLES) S-Ω and dynamic hybrid RANS/LES (DHRL). The prediction results were compared and analyzed with a RANS model shear stress transport (SST) k-omega which was a recommended choice in engineering. Findings The numerical results were verified by experiment and indicated that the predicted values for three hybrid turbulence models were more accurate. Then, the transient flow field was further analyzed visually in terms of turbulence statistics, Reynolds number, pressure-streamline, vortex structure and eddy viscosity ratio. The results indicated that HRL approaches could capture unsteady flow phenomena. Practical implications This study achieves both in performance prediction improvement and better flow mechanism understanding. The computational fluid dynamics (CFD) could be used instead of flow visualization to a certain extent. The improved CFD method, the fine computational grid and the reasonable simulation settings jointly enhance the application of CFD in the rotor-stator flow. Originality/value The improvement was quite encouraging compared with the reported literatures, contributing to the CFD playing a more important role in the flow machinery. DHRL provided the detailed explanation of flow transport between rotor and stator, which was not reported before. Through it, the flow mechanism can be better understood.