Wall-adapting Local Eddy-viscosity Model Research Articles

Artificial-neural-network-based nonlinear algebraic models for large-eddy simulation of compressible wall-bounded turbulence

In this paper, we propose artificial-neural-network-based (ANN-based) nonlinear algebraic models for the large-eddy simulation (LES) of compressible wall-bounded turbulence. An innovative modification is applied to the invariants and the tensor bases of the nonlinear algebraic models through using the local grid widths along each direction to normalise the corresponding gradients of the flow variables. Furthermore, the dimensionless model coefficients are determined by the ANN method. The modified ANN-based nonlinear algebraic model (MANA model) has much higher correlation coefficients and much lower relative errors than the dynamic Smagorinsky model (DSM), Vreman model and wall-adapting local eddy-viscosity model in the a priori test. The significantly more accurate estimations of the mean subgrid-scale (SGS) fluxes of the kinetic energy and temperature variance are also obtained by the MANA models in the a priori test. Furthermore, in the a posteriori test, the MANA model can give much more accurate predictions of the flow statistics and the mean SGS fluxes of the kinetic energy and the temperature variance than other traditional eddy-viscosity models in compressible turbulent channel flows with untrained Reynolds numbers, Mach numbers and grid resolutions. The MANA model has a better performance in predicting the flow statistics in supersonic turbulent boundary layer. The MANA model can well predict both direct and inverse transfer of the kinetic energy and temperature variance, which overcomes the inherent shortcoming that the traditional eddy-viscosity models cannot predict the inverse energy transfer. Moreover, the MANA model is computationally more efficient than the DSM.

Anisotropic minimum dissipation subgrid-scale model in hybrid aeroacoustic simulations of human phonation.

This article deals with large-eddy simulations of three-dimensional incompressible laryngeal flow followed by acoustic simulations of human phonation of five cardinal English vowels, /ɑ, æ, i, o, u/. The flow and aeroacoustic simulations were performed in OpenFOAM and in-house code openCFS, respectively. Given the large variety of scales in the flow and acoustics, the simulation is separated into two steps: (1) computing the flow in the larynx using the finite volume method on a fine moving grid with 2.2 million elements, followed by (2) computing the sound sources separately and wave propagation to the radiation zone around the mouth using the finite element method on a coarse static grid with 33 000 elements. The numerical results showed that the anisotropic minimum dissipation model, which is not well known since it is not available in common CFD software, predicted stronger sound pressure levels at higher harmonics, and especially at first two formants, than the wall-adapting local eddy-viscosity model. The model on turbulent flow in the larynx was employed and a positive impact on the quality of simulated vowels was found.

Strongly Heated Turbulent Flow in a Channel with Pin Fins

Large-eddy simulations (LES) were performed to study the turbulent flow in a channel of height H with a staggered array of pin fins with diameter D = H/2 as a function of heating loads that are relevant to the cooling of turbine blades and vanes. The following three heating loads were investigated—wall-to-coolant temperatures of Tw/Tc = 1.01, 2.0, and 4.0—where the Reynolds number at the channel inlet was 10,000 and the back pressure at the channel outlet was 1 bar. For the LES, two different subgrid-scale models—the dynamic kinetic energy model (DKEM) and the wall-adapting local eddy-viscosity model (WALE)—were examined and compared. This study was validated by comparing with data from direct numerical simulation and experimental measurements. The results obtained show high heating loads to create wall jets next to all heated surfaces that significantly alter the structure of the turbulent flow. Results generated on effects of heat loads on the mean and fluctuating components of velocity and temperature, turbulent kinetic energy, the anisotropy of the Reynolds stresses, and velocity-temperature correlations can be used to improve existing RANS models.

Large eddy simulation of ship airflow control with steady Coanda effect

This paper numerically studies the steady Coanda effect for drag reduction and airwake manipulations on the Chalmers ship model (CSM) using large eddy simulation with wall-adapting local-eddy viscosity model. Numerical methods are validated by experimental data acquired from the baseline CSM. In creating the flow control model, the hanger base of the baseline CSM is modified with Coanda surfaces and injection slots along its roof edge and two side edges. Four representative cases are studied: a no-jet case and three cases with the same momentum coefficient of the jet flow activated at different locations (roof, sides, and combined). The results show that the four cases have various performances in drag reduction and vortex structures on the deck. They are also different in mean and turbulent quantities as well as POD (proper orthogonal decomposition) modes in their airwake. It is found that the roof-jet has a stronger Coanda effect and is more vectored toward the low-speed area (LSA) on the deck than the side-jets that detach earlier from the Coanda surface. The energization process is, therefore, different where the roof-jet is more effective that directly brings high momentum to LSA and side-jets manipulate shear layers for mixing enhancement. The cases with roof-jet achieve better mitigation of flow re-circulation and higher recovery of streamwise velocity with lower turbulent fluctuation in the airwake. POD analysis suggests that the roof-jet can stabilize the wake.

Analysis of numerical factors affecting large eddy simulation of pollutant diffusion around buildings

In recent years, Large Eddy Simulation (LES) has been often used for predicting the pollutant diffusion around buildings. However, due to the numerical discretization error and the sub-grid scale modeling error, some empirical factors related to LES have not yet formed with completion for the best practice. This study focused on LES simulation of the airflow and pollutant diffusion around building with a synthetic inflow turbulence method and Wall-Adapting Local Eddy-Viscosity (WALE) model, discuss LES empirical factors, such as effects of grid resolution, sub-grid scale (SGS) WALE model with coefficient Cw and the sub-grid scale Schmidt number ScSGS in details. The results indicate that the numerical wind tunnel created by LES can better reproduce the characteristics of the airflow and concentration field around buildings while keeping predicted physical fields are in line with the evaluation index. In addition, the results also indicate that the solution with the finest grid resolution may not have the best agreement with the experimental data, the flow field is more sensitive to the change of SGS model coefficient Cw when the grid resolution is lower, which shows a trend that the larger coefficient Cw has better simulation in the windward sides of the building, while the value of the coefficient Cw shows an opposite trend in the recycling area at the rear of the building.

A modified wall-adapting local eddy-viscosity model for large-eddy simulation of compressible wall-bounded flow

The wall-adapting local eddy-viscosity (WALE) model in large-eddy simulation can well predict wall-bounded flows but it is also well known for excessive dissipation. In this study, we apply the minimum-dissipation model to constrain the WALE model in compressible flows and obtain the coefficient of the WALE model. Through this process, the dissipation of WALE model can be lower while it still maintains strong stability. In the modified WALE model, the isotropic part of the subgrid-scale (SGS) stress is also reconstructed. In the filtered total energy equation, all of the extra SGS unclosed terms (besides SGS stress and SGS heat flux) are modeled instead of neglecting some SGS terms, such as the SGS viscous diffusion. The modified WALE model is tested in a compressible turbulent channel flow and a supersonic turbulent boundary layer over a compression corner. The new model can well predict the mean velocity, the mean temperature, the Reynolds stress, and the separation bubble.

Extended wall-adapting local eddy-viscosity model for the large-eddy simulations of multiscale flows

We report on progress made in improving the performance of the well-established wall-adapting local eddy-viscosity model in an important class of multiscale flows, namely, that of unsteady, massively separated flows at high Reynolds number. While this model succeeds in capturing the asymptotic near-wall behavior of the eddy viscosity that enters the formulation of the model for the sub-grid scale correlations, its performance in these flows has proved to be unsatisfactory due to high levels of dissipation leading to poor predictions in the separated wakes. We have sought to improve the performance of this model by combining it with another, the regularized variational multiscale model, which better represents the interactions that occur in multiscale flows. This combination, which was implemented in OpenFOAM, was validated against experimental data for the challenging case of vortex shedding from circular cylinders at the high Reynolds number. Distinct improvements over the original model were obtained.

An efficient numerical approach for simulating airflows around an isolated building

The efficiency and precision of numerical simulation are very important to study the airflow around buildings. A new simulating approach for predicting airflow around a building efficiently is proposed. The modeled turbulence kinetic energy is converted into resolved energy using the vortex method at the interface away from the inlet. This treatment helps to converge. A novel hybrid turbulence model, namely production-limited eddy simulation (PLES), works in the whole computational region. Then, the PLES model needs not treat the LES-RANS (large eddy-simulation-Reynolds-averaged Navier-Stokes) interface, making the computation running more easily. For testing the accuracy and efficiency of the PLES model, the WALE (wall-adapting local eddy-viscosity) model, an LES model, is applied to simulate the airflow around an isolated building on the same grid. And the effects of the underlying RANS models on the performance of the PLES models are investigated. The underlying RANS models are the BSL (baseline) k-ω and the SST (shear stress transport) k-ω models. The BSL PLES model obtains good predictions of the time-averaged streamwise velocity, but over-predicts the resolved turbulence kinetic energy (TKE) in comparison with the WALE model. The performance of the SST PLES model in predicting the time-averaged streamwise velocity is comparable to the WALE and the BSL PLES models. Furthermore, the SST PLES model gives better resolved TKE comparing with two other models. Meanwhile, the computing time needed in the new approach is 35% less than that needed in the LES computation.

Effect of Local Turbulent Structures on the Hot Jet in Transitional Flow

Turbulent parts localized in flow direction may arise in a pipe with transitional regime of the stable laminar Poiseuille flow. A key condition for occurrence of such structures is a pipe with rather long length relative to its diameter. Our paper presents numerical modelling of the hot air jet flowing from the long pipe into the cold open volume at Re=2426. The modelling was performed in OpenFOAM software on the basis of the large eddy simulation (LES) method. The WALE (Wall-adapting local eddy-viscosity) model was used for closure of Navier-Stokes equations on subgrid scales. We demonstrated that local turbulent structures have a weak effect on the hot jet at flowing into the cold open volume.

Estimating the Reattachment Length by Realizing a Comparison between URANS k-Omega SST and LES WALE Models on a Symmetric Geometry

In this study, a water reattachment length was calculated by adopting two different models. The first was based on Unsteady Reynolds-Averaged Navier–Stokes (URANS) k-omega with Shear Stress Transport (SST); the second was a Large Eddy Simulation (LES) with Wall-Adapting Local Eddy-Viscosity (WALE). Both models used the same mesh and were checked with Taylor length-scale analysis. After the analysis, the mesh had 11,040,000 hexahedral cells. The geometry was a symmetrical expansion–contraction tube with a 4.28 expansion ratio that created mechanical energy losses, which were taken into account. Moreover, the reattachment length was estimated by analyzing the speed values; the change of speed value from negative to positive was used as the criterion to recognize the reattachment point.

Large-Eddy Simulation Study of Flow and Heat Transfer in Swirling and Non-Swirling Impinging Jets on a Semi-Cylinder Concave Target

Swirling impinging jet (SIJ) is considered as an effective means to achieve uniform cooling at high heat transfer rates, and the complex flow structure and its mechanism of enhancing heat transfer have attracted much attention in recent years. The large eddy simulation (LES) technique is employed to analyze the flow fields of swirling and non-swirling impinging jet emanating from a hole with four spiral and straight grooves, respectively, at a relatively high Reynolds number (Re) of 16,000 and a small jet spacing of H/D = 2 on a concave surface with uniform heat flux. Firstly, this work analyzes two different sub-grid stress models, and LES with the wall-adapting local eddy-viscosity model (WALEM) is established for accurately predicting flow and heat transfer performance of SIJ on a flat surface. The complex flow field structures, spectral characteristics, time-averaged flow characteristics and heat transfer on the target surface for the swirling and non-swirling impinging jets are compared in detail using the established method. The results show that small-scale recirculation vortices near the wall change the nearby flow into an unstable microwave state, resulting in small-scale fluctuation of the local Nusselt number (Nu) of the wall. There is a stable recirculation vortex at the stagnation point of the target surface, and the axial and radial fluctuating speeds are consistent with the fluctuating wall temperature. With the increase in the radial radius away from the stagnation point, the main frequency of the fluctuation of wall temperature coincides with the main frequency of the fluctuation of radial fluctuating velocity at x/D = 0.5. Compared with 0° straight hole, 45° spiral hole has a larger fluctuating speed because of speed deflection, resulting in a larger turbulence intensity and a stronger air transport capacity. The heat transfer intensity of the 45° spiral hole on the target surface is slightly improved within 5–10%.

CFD simulation of natural ventilation of a generic building in various incident wind directions: Comparison of turbulence modelling, evaluation methods, and ventilation mechanisms

Single-sided and cross- ventilation of a cube-shaped building at various incident wind directions were stimulated using computational fluid dynamics (CFD). The simulation used either the Reynolds-averaged Navier-Stokes (RANS) equations in conjunction with a 2-equation turbulence model (Standard k-ε model (SKT), Realizable k-ε model (RLZ), or Renormalization group k-ε model (RNG)), or Large Eddy Simulation (LES) with the Wall-Adapting Local Eddy-viscosity model (WALE). LES showed good agreement with wind tunnel data when modeling indoor and outdoor wind fields; RLZ had slightly better results than SKT and RNG. Two evaluation methods were examined: the integration of opening velocities method under-estimated single-sided ventilation rates but over-estimated cross-ventilation rates; the tracer-gas decay method was more computationally demanding of the two. When the tracer-gas method and LES were used in combination, their estimations were most accurate especially if the ventilation was driven by wind flow fluctuations. RANS and LES predicted steady decreases of 92.5% and 81.8% (single-sided ventilation), and 52.6% and 37.2% (cross-ventilation) in ventilation rate as incident wind direction varies from 0° to 90°. Discrepancies between the RANS and LES predictions of ventilation rates were mainly attributable to their respective turbulence-modeling methods.

Assessment of the wall-adapting local eddy-viscosity model in transitional boundary layer

The main idea of this manuscript is to assess the wall-adapting local eddy-viscosity (WALE) model, which is designed for large-eddy simulation (LES) of turbulent boundary layer, in transitional flow. In contrast to many other sub-grid-scale (SGS) models, the WALE model demonstrates the asymptotic decay of the eddy viscosity in the vicinity of a solid wall in turbulent boundary layer without relying on a dynamic procedure on the SGS model coefficient. Furthermore, the WALE model yields zero eddy viscosity in pure shear flow. Such features are attractive for LES of laminar-to-turbulent transition, yet the WALE model has not been thoroughly investigated in transitional boundary layer flow. Well-resolved LES is conducted for canonical boundary layer transition triggered by sub-harmonic resonance. The model formulation is thoroughly analyzed in the transition process. The asymptotic behavior of the two major tensors, i.e., the strain rate tensor and the traceless tensor of the velocity gradient squared, are confirmed in the current well-resolved LES. The cubic decay of the eddy viscosity in the wall distance is confirmed from the asymptotic analysis on the transitional flow. The presence of the strain rate in the model formulation generates practically zero eddy viscosity in the pre-transition region, allowing interactions of small-amplitude instabilities. The traceless tensor of the velocity gradient squared escalates if small-scale eddies appear, leading to sizable eddy viscosity particularly in the log-law layer in turbulent flow. The response of the WALE model to grid resolution is also discussed. The performance of the WALE model is compared to two other SGS models, Smagorinsky and Vreman models with a constant coefficient, in the current transitional wall-bounded flow.

Large eddy simulation of polydispersed inertial particles using two-way coupled PDF-PBE

This paper presents large eddy simulation (LES) of polydispersed turbulent recirculating flows using a two-way coupled probability density function of the population balance equation (PDF-PBE). A stochastic Monte Carlo method is adopted to solve the PDF-PBE on an ensemble of notional Lagrangian particles and the method of Stokes binning, that was recently developed by Salehi et al. (2017) is employed to explicitly treat effects of inertia. The PDF-PBE is applied to the experiment of Boŕee et al. (2001) which studied dispersion of polysized inertial particles in a bluff body configuration. The particle mass loading is 22% where the dispersed elements affect the carrier phase velocity. The simulations are performed using both the standard Smagorinsky and the Wall-Adapting Local Eddy-viscosity (WALE) subgird turbulent models. It is found that the subgrid model has a significant impact on the results, particularly on the carrier velocity. The WALE model shows a better agreement with the measurements. Different boundary conditions are tested for injection of notional particles. It is demonstrated both dispersed and carrier phase velocities are initially sensitive to the particle boundary condition whereas the difference between tested conditions becomes marginal further downstream. The best results are obtained for the particle boundary condition that accounts for effects of inertia at the inlet. Finally, the sensitivity of the PDF-PBE simulations to the number of Stokes bins is studied. It is found that eight Stokes bins are enough to accurately model the polysized particles dispersion in a bluff body configuration. The results indicate that particle dispersion is notably sensitive to the number of Stokes bins but the particle velocity predictions are much less sensitive with small variation in velocity results over the range of Stokes bins that were tested.

Numerical study on turbulent mixed convection in a vertical plane channel using hybrid RANS/LES and LES models

Abstract Two Delayed-Detached Eddy Simulation (DDES) models, and a Large-Eddy Simulation (LES) model are used to investigate the turbulent flows and mixed convection between a hot plate and a cold plate via the software FLUENT. The two DDES models include Production-limited DDES (PL-DDES) and Improved DDES (IDDES) models. The Wall-Adapting Local Eddy-Viscosity (WALE) model is the used LES model. The numerical computations are performed at Reynolds number Reb = 4494 and different Richardson numbers Ri = 0.025, 0.048, 0.1. The comparing data is from the Direct Numerical Simulation (DNS) at Reb = 4494 and Ri = 0.048. The comparison reveals that the two DDES models have better performance in predicting time-averaged parameters than the WALE model in the aiding flow. The best predicted time-averaged results are obtained by the PL-DDES model in the opposing flow. Furthermore, the results of different Ri obtained by the PL-DDES model agree well with the DNS data.

Large eddy simulation of pollutant dispersion in a naturally cross-ventilated model building: Comparison between sub-grid scale models

In the present article, the flow field and passive gaseous pollutant dispersion in a naturally cross-ventilated isolated single-zone model building have been investigated. The large eddy simulation (LES) approach has been applied together with three different sub-grid scale (SGS) models, namely, wall-adapting local eddy-viscosity (WALE), dynamic Smagorinsky-Lilly model (DSLM), and standard Smagorinsky-Lilly model (SSLM). The flow and passive scalar concentration fields have been compared with available experimental data. It is demonstrated that the three SGS models predict similar flow field. Also, it can be observed that the mean concentration field results obtained from the WALE sub-grid scale (SGS) model are in better agreement with the experimental data than those of DSLM and SSLM by about 8% and 5%, respectively. Moreover, the CPU time required for the computations by the WALE and SSLM models was almost 20% less than that of DSLM, making WALE a more suitable SGS model than the other two models for the prediction of flow and concentration fields in an isolated building. Furthermore, contributions of convective flux and turbulent diffusion flux to the pollutant transportation have been studied. It has been shown that although the convective flux is the main mechanism, the two fluxes have a significant influence on pollutant transportation and distribution.

Numerical investigation of aerodynamic and acoustic characteristics of bionic airfoils inspired by bird wing

Airfoil is the basic element of fluid machinery and aircraft, and the noise generated from that is an important research aspect. Aiming to reduce the aerodynamic noise around the airfoil, this study proposes an airfoil inspired by the long-eared owl wing and another airfoil coupled with the bionic airfoil profile, leading edge waves, and trailing edge serrations. Numerical simulations dependent on the large eddy simulation method coupled with the wall-adapting local eddy-viscosity model and the Ffowcs Williams and Hawkings equation are conducted to compare the aerodynamic and acoustic characteristics of two types of bionic airfoils at low Reynolds number condition. The simulations reveal the dipole characteristic of acoustic source and sound pressure level distribution at various frequencies. Two types of bionic airfoils show lower noise compared with the conventional NACA 0012 airfoil with a similar relative thickness of 12%. Compared with the bionic airfoil, the average value of sound pressure level at the monitoring points around the bionic coupling airfoil is decreased by 9.94 dB, meanwhile the lift-to-drag ratio also keep higher. The bionic coupling airfoil exerts a suppression of sound pressure fluctuation on the airfoil surfaces, which result from that the range and size of separation vortices are reduced and the distance between vortices and airfoil surface are increased. The tube-shaped vortices in the wake of airfoil are effectively restrained and split into small scale vortices, which are important to cause less aerodynamic noise around the bionic coupling airfoil. Consequently, a novel bionic coupling airfoil is developed with the excellent aerodynamic and acoustic performance.

OpenFOAM based LES of slot jet impingement heat transfer at low nozzle to plate spacing using four SGS models

The objective of the present investigation is to assess the effectiveness of large eddy simulation (LES) in turbulent slot jet impingement heat transfer at low nozzle to plate spacing. Four different sub-grid stress (SGS) models, namely, Smagorinsky, WALE (wall adapting local eddy-viscosity), k-equation and dynamic k-equation, were considered for Reynolds number of 20,000. Computations were performed using OpenFOAM, an open source finite-volume based CFD code. Time and span-wise averaged mean streamwise velocity and root mean square (r.m.s.) velocity fluctuations in the stagnation and wall jet regions are presented. Nusselt number distributions on the impingement wall are also presented. The computed LES results are compared with the reported experimental data. A secondary peak in the Nusselt number was observed using the four SGS models as in the experimental data. LES of slot jet impingement heat transfer using four SGS models, including WALE, has been investigated for the first time in the present paper. It is observed that the WALE and dynamic k-equation SGS models perform well in complex flow regions of turbulent slot jet impingement heat transfer.

Jet noise computation based on enhanced DES formulations accelerating the RANS-to-LES transition in free shear layers

The paper presents jet noise computations performed using the original and some recently proposed modified formulations of detached-eddy simulation, based on alternative definitions of the subgrid length-scale and/or a modified version of the subgrid Spalart-Allmaras model, which involves the Wall-Adapting Local Eddy-viscosity model. The modifications are aimed at the elimination or, at least, a significant weakening of a known flaw of detached-eddy simulation, namely, the severe delay of the Kelvin–Helmholtz instability and transition from fully modeled to mostly resolved turbulence in the free and separated shear layers. This flaw makes the original detached-eddy simulation formulation, in fact, non-applicable to the prediction of the noise of high Reynolds number jet flows when typical grids are used, and has led to the use of Implicit Large-Eddy Simulation instead. Based on examples of application of these modifications to such flows, already available in the literature, they were found capable of resolving the issue and providing quite accurate predictions of the aerodynamic characteristics and turbulent statistics of the high-Re jets. The study performed in the present work allows us to draw a similar conclusion regarding the accuracy of the enhanced detached-eddy simulation formulations in terms of jet-noise prediction. The new formulation gives noise results very close to those of ILES except at the highest frequencies, while being more general and less sensitive to grid resolution. In addition, it removes a (1–3) dB underestimation of the noise spectral maximums for the peak radiation direction, which is typical of ILES on fine grids.

Flow characteristics prediction in pump mode of a pump turbine using large eddy simulation

To obtain more accurate flow characteristics of pump turbines, the method of large eddy simulation with wall-adapting local eddy viscosity model is applied in simulating several operating points in the pump mode. Firstly, based on the experimental validation, the method of large eddy simulation could better predict the external performance and internal flow characteristics in a pump turbine in the pump mode compared with the method of Reynolds-averaged Navier–Stokes with two-equation turbulence model shear stress transport k–ω. Then, flow characteristics under 1.00 QBEP (best efficiency point), 0.91 QBEP, 0.88 QBEP, and 0.85 QBEP operating points are investigated to find out the causes of the head drop in the energy-discharge curve through large eddy simulation. The detailed analysis reveals that the head drop at the point 0.85 QBEP is related to the recirculation flow at the runner inlet. Finally, unsteady studies confirm that vortex movement at the runner inlet lead to the variation of the amplitudes and directions of the velocity, which generates the rotation of the separation vortices in the runner and stay vane channels.