Numerical studies towards practical large-eddy simulation

Large-eddy simulation developments and validations are presented for an improved simulation of turbulent inter-nal flows.Numerical methods are proposed according to two competing criteria:numerical qualities(precisionand spectral characteristics),and adaptability to complex configurations.First,methods are tested on academictest-cases,in order to abridge with fundamental studies.Consistent results are obtained using adaptable finitevolume method,with higher order advection fluxes,implicit grid filtering and"low-cost"shear-improved Sma-gorinsky model.This analysis particularly focuses on mean flow,fluctuations,two-point correlations and spectra.Moreover,it is shown that exponential averaging is a promising tool for LES implementation in complex geome-try with deterministic unsteadiness.Finally,adaptability of the method is demonstrated by application to a con-figuration representative of blade-tip clearance flow in a turbomachine.     

Large eddy simulation of turbulent flow in porous media

An LES (large eddy simulation) study was conducted using one of standard numerical models for a porous medium, namely, a flow through a periodic array of square cylinders. The LES results were processed to extract macroscopic results such as the macroscopic turbulent kinetic energy and the macroscopic pressure gradient. These macroscopic results are compared against those obtained using conventional models of turbulent kinetic energy and its dissipation rate, so as to examine the validity of extending the conventional two equation models of turbulence to the flow in porous media. The spectrum of turbulence was also examined to appreciate the onset of turbulence.     

Numerical simulation of flatback airfoil aerodynamic noise

The present paper presents a possible path for developing a large eddy simulation (LES) applicable to high Reynolds-number complex turbulent flows and the performance of the coupling of LES with statistical turbulence models around the flow over a blunt trailing edge configuration. The turbulent fluctuations in the boundary layers at the inflow region of the LES domain are generated by a synthesized turbulence method. The hybrid RANS-LES model showed considerable improvement in prediction accuracy even at a moderate grid resolution. The aerodynamic comparison with experimental data shows like results for the pressure distributions surrounding a flatback airfoil. To predict accurately the noise radiation from the blunt trailing edge and to save computational costs, the near-field region is computed by embedded LES while the surrounding region is simultaneously computed by RANS. The Brooks, Pope, and Marcolini (BPM) semi-empirical model is used for noise comparison with the hybrid RANS-LES result and experimental data. The present hybrid RANS-LES method is found to be adequate for predicting aerodynamic noise generation by vortical flow in the vicinity of a blunt trailing edge airfoil over a range of frequencies.     

LES of a Turbulent Slot Impinging Jet to Predict Fluid Flow and Heat Transfer

In this article, large eddy simulation (LES) is performed for a turbulent slot jet impingement heat transfer at a Reynolds number of 13,500 and a nozzle to plate spacing of 10. Various aspects of predicting a turbulent jet impinging flow in an optimum domain size and grid resolution for LES have been assessed. Two inflow conditions, one without any fluctuations and the other with fluctuations generated by the spectral synthesizer, were tested and comparisons of various mean flow, turbulence, and heat transfer data showed that LES without any inflow fluctuations provides good agreement with the corresponding experimental and numerical results reported in the literature. Further, various important dynamical flow structures have been visualized from the instantaneous computed data. Finally, mean flow and turbulence statistics have been presented in the wall jet region close to the stagnation point, which could be useful as data for validation of RANS-based turbulence models.     

A filtered-laminar-flame PDF sub-grid scale closure for LES of premixed turbulent flames. Part I: Formalism and application to a bluff-body burner with differential diffusion

A sub-grid scale closure for Large Eddy Simulation (LES) of turbulent combustion based on physical-space filtering of laminar flames is discussed. Applied to an unstructured grid, the combustion LES filter size is not fixed in this novel approach devoted to LES with refined meshes, but calibrated depending on the local level of unresolved scalar fluctuations. The context is premixed or stratified flames, the derived model relies on four balance equations for mixture fraction and its variance, and a progress variable and its variance. The proposed formalism is based on a presumed probability density function (PDF) derived from the filtered flames. Closures for the terms of the equations that are unresolved over LES grids are achieved through the PDF. The method uses flamelet tabulated detailed chemistry and is first applied to the simulation of laminar flames (1D and 2D) over various grids for validation, before simulating a turbulent burner studied experimentally by Sweeney et al. (2012). Since this burner also features differential diffusion effects, the numerical model is modified to account for accumulation of carbon in the recirculation zone behind the bluff-body. A differential diffusion number based on the gradient of residence times is proposed, in an attempt to globally quantify differential diffusion effects in burners.     

Large-eddy simulation of melt turbulence in a 300-mm Cz–Si crystal growth

We built a curvilinear dynamic Smagorinsky subgrid-scale (SGS) model based on filtering the covariant physical velocity components in the computational space. We implemented our proposed SGS model in large-eddy simulations (LES) of turbulent flows in complex configurations. Our model was validated when compared with direct numerical simulation (DNS) data of the melt turbulent flow in an idealized cylindrical crucible in a Cz–Si crystal growth. Then, we carried out LES computations for the melt turbulence in a real ellipsoidal crucible in a 300 mm Cz–Si crystal growth. We studied instantaneous behaviors and statistical features of the melt turbulence. Spectral analyses of the temperature fluctuations show that the melt flow is in a soft turbulence state of Rayleigh–Bénard convection under the rotating crystal. A cluster of big vortices is formed in the time-averaged bulk flow due to the complex interaction among the thermal buoyancy, surface tension and crucible/crystal rotations. Heat transport in the melt flow is turbulence-dominated with notable fluctuations. The maximal temperature fluctuation in the crystallization zone is close to the crystal edge with a value of 1.8 K. The flow instability mainly attributes to the thermal buoyancy in the melt.     

4气门直喷式汽油机缸内湍流场多周期循环变动的大涡模拟

应用大涡模拟方法对一台4气门直喷式汽油机(DISI)缸内冷态湍流流场进行了三维瞬态数值分析.通过连续13个工作循环的模拟计算,探索缸内湍流流动的循环变动特征与规律,并与PIV流场测试结果进行了对比.模拟相平均值及循环变动的均方值和试验值在总体上吻合得较好.计算结果表明:在进气过程前期缸内流场湍流脉动和循环变动都很强烈,两者强度为同一量级;但在后续过程中,湍流脉动不断衰减,其与循环变动的比值小于15%.大涡模拟方法不仅可以真实地反映内燃机循环过程中缸内气体流动的细节和规律,而且非常适合于研究内燃机的循环变动特性.     

Evaluation and modification of gas-particle covariance models by Large Eddy Simulation of a particle-laden turbulent flows over a backward-facing step

Three-dimensional numerical investigation of a low speed particle-laden turbulent flow over a backward-facing step has been carried out. An assumption of incompressibility of the flow is used due to low Mach number of the flow. The gas phase is performed by Large Eddy Simulation (LES) and the particle phase is solved by a Lagrangian particle tracking model. The simulation results such as mean streamwise velocities and fluctuation velocities for the both phase are validated by experimental results performed by Fessler and Eaton (1999) [1]. Reynolds number of the gas phase over the backward-facing step with an expansion ratio of 5:3 is 18,400, based on the maximum inlet velocity and step height. The flow is considered as dilute. Hence a one-way coupling method is applied, in which we only consider the effect of fluid on the particle. Particle–particle collisions are also neglected. The success of simulation in predicting a particle-laden turbulent flow using LES and Lagrangian trajectory model provides a numerical basis for revisiting the gas-particle correlations models. Four second-order closure models for gas-particles covariance are evaluated in the present study. A modified better gas-particle covariance model is proposed in this paper.     

Large-eddy simulation of two-phase spray combustion for gas turbine combustors

In three-dimensional arbitrary curvilinear coordinates, an Eulerian–Lagrangian formulation is applied to large-eddy simulation (LES) of instantaneous gas–liquid two-phase turbulent combustion flows in gas turbine combustors. Three dimensional block-structured grids are generated by zone method and solving a system of elliptic partial differential equations. The k-equation sub-grid scale model is used to simulate the sub-grid eddy viscosity and the EBU combustion sub-grid scale model is employed to predict the chemical reaction rate. The gas-phase governing equations are solved with SIMPLE algorithm and hybrid scheme in non-staggered grid system. A stochastic separated flow formulation is used to track the droplet trajectories velocities, size and temperature history by Lagrangian equations of motion and thermal balance. Multi-zone coupling method is employed to transport data between interfaces. The influences of two different primary hole positions and three different fuel–air ratios on turbulent two-phase reacting flows are calculated. Predictions are in reasonable agreement with the measured velocity using PIV system and temperature, species concentration measurements at the exit. It is shown that the present approach may be used to study spray combustion flow fields for guiding the design of advanced gas turbine combustors.     

Dynamic stall analysis using harmonic balance and correlation‐based γ– transition models for wind turbine applications

In this paper, we use the harmonic balance method to study an oscillating S809 airfoil in dynamic stall. The periodic behavior of this problem makes it well suited for the harmonic balance method, which is able to model unsteady aerodynamics at greatly reduced computational costs when compared with time‐accurate unsteady‐flow solvers. A finite‐volume technique based on the lower–upper symmetric Gauss–Seidel scheme with Roe fluxes is used to solve the Reynolds‐averaged Navier–Stokes equations. The turbulent viscosity is computed with the one‐equation Spalart–Allmaras turbulence model. In addition, the laminar–turbulent transition is modeled using a correlation‐based approach originally developed by Langtry and Menter. Comparisons with experimental data for steady flows with the S809 airfoil highlight the necessity of the transition model to accurately predict the onset of static stall. For unsteady cases, the transition model provides improved agreement with experimental data, predicting dynamic stall when the fully turbulent model cannot. Copyright © 2014 John Wiley & Sons, Ltd.     

Large eddy simulation of turbulent channel flow with mass transfer at high-Schmidt numbers

Large eddy simulation (LES) of turbulent channel flow with mass transfer has been performed to investigate the effect of the Schmidt number on the turbulence behaviors. The three-dimensional filtered Navier-Stokes equations and the concentration equation are numerically solved using a fractional-step method. Dynamic subgrid-scale (SGS) models for the turbulent SGS stresses and mass fluxes are employed to closure the governing equations. The objectives of this study are to examine the reliability of the LES technique for predicting the turbulent mass transfer at high-Schmidt numbers and to analyze the behavior of turbulent mass diffusion from a solid boundary to the adjacent shear flow at different Schmidt numbers. Fully developed turbulent channel flows with constant difference of concentrations imposed on different walls are calculated for a wide range of the Schmidt number from 0.1 up to 200 and the Reynolds number 13 800 based on the centerline mean velocity and the half-width of the channel. To show the effects of Schmidt number on the turbulent mass transfer, mean and fluctuating resolved concentrations, mass transfer coefficient, turbulent mass fluxes, and some instantaneous flow and concentration fields are exhibited and analyzed.     

Large-eddy simulation of turbulent heat convection in a spanwise rotating channel flow

In this paper, we investigate the effects of the Coriolis force in a heated plane channel flow subjected to spanwise rotation using the method of large-eddy simulation. We present both the general and simplified transport equations for the resolved turbulent stresses, which are essential for understanding the unique pattern of turbulent kinetic energy production in a rotating system. Numerical simulations are performed using primarily two dynamic subgrid-scale stress models and one dynamic subgrid-scale heat flux model; namely, the conventional dynamic model (DM) and a novel dynamic nonlinear model (DNM) for closure of the filtered momentum equation, and an advanced dynamic full linear tensor thermal diffusivity model (DFLTDM) for closure of the filtered thermal energy equation. The turbulent flow field studied herein is characterized by a Reynolds number Re_τ = 150 and various rotation numbers Ro_τ ranging from 0 to 7.5. In order to validate the LES approach, turbulent statistics obtained from the simulations are thoroughly compared with the available experimental results and direct numerical simulation (DNS) data. A detailed comparative study has been conducted in order to evaluate the performance of the DM and DNM in terms of their prediction of characteristic features of the velocity and temperature fields and their capability of reflecting both forward and backward scatter of kinetic energy between the filtered and subgrid scales.     

网格密度对内燃机冷态流场大涡模拟的影响

通过修改发动机多维CFD计算程序KIVA-3V,建立了内燃机压缩过程冷态流场的大涡模拟(LES)计算模型.利用此模型对内燃机压缩过程中缸内流场的水平速度及湍流动能进行分析,同时,分析了网格密度对内燃机缸内流场大涡模拟的影响.结果表明,当采用k-ε模型计算时,网格的精细程度对流场结构影响不大;在相同的计算网格下,与采用k-ε模型计算相比,采用LES计算显示了更为复杂的湍流结构,而且LES所能捕捉到的涡团结构范围要大于k-ε模型,计算得到的湍流动能也要低于k-ε模型;同时,网格越精细,这种效应越明显.     

内燃机压缩过程冷态流场的大涡模拟

通过修改发动机多维CFD计算程序KIVA-3V,建立了内燃机压缩过程冷态流场的大涡模拟（LES）计算模型。利用此模型对内燃机压缩过程中缸内流场的水平速度及湍流动能进行了详细分析,并与k-ε模型进行了比较。结果表明与采用k-ε模型计算时相比,采用LES计算时显示了更为复杂的湍流结构,而且LES所能捕捉到的涡团结构范围要大于k-ε模型。同时,采用LES计算时得到的湍流动能要远远低于k-ε模型。     

A priori and a posteriori analyses of algebraic flame surface density modeling in the context of Large Eddy Simulation of turbulent premixed combustion

The flame surface density (FSD) based reaction rate closure is one of the most important methodologies of turbulent premixed flame modeling in the context of Large Eddy Simulations (LES). The transport equation for the Favre-filtered reaction progress variable needs closure of the filtered reaction rate and the subgrid scalar flux (SGSF). The SGSF in premixed turbulent flames has both gradient and countergradient components, where the former is typically modeled using eddy diffusivity and the latter can be modeled either on its own or in combination with the filtered reaction rate term using an appropriate wrinkling factor. The scope of the present work is to identify an explicit SGSF closure for the optimum performance in combination with an already established LES FSD model. The performance of different SGSF models for premixed turbulent combustion has been assessed recently by the authors using a Direct Numerical Simulation (DNS) database of freely propagating turbulent premixed flames with a range of different values of turbulent Reynolds number. The two most promising models have been implemented in the LES code. The modeling methodology identified based on a priori DNS analysis is assessed further a posteriori by comparing the LES simulation results of turbulent methane Bunsen flames with the well-documented experimental data. A significant change of the overall flame speed is not observed for different SGSF models. However, the flame shape and thickness respond to the modeling of SGSF. Considering the fact that the SGSF models have very different characteristics, the overall effect on the LES results in this work is smaller than expected. An extension of a previous a priori DNS analysis provides detailed explanations for the observed behavior.     

Frequency response of an electrochemical probe to the wall shear stress fluctuations of a turbulent channel flow

We analyzed numerically the frequency response of the mass transfer rates produced at the surface of an electrochemical probe to the fluctuations of the wall shear stress obtained from a direct numerical simulation of a turbulent plane channel flow at Re_τ = 150. At low and high frequencies the response to the turbulent wall shear stress is in accordance with the existing relations between the amplitudes of the wall shear stress and the mass transfer rates obtained for harmonic perturbations. At intermediate frequencies the turbulent wall shear stress fluctuations produce a larger damping in comparison with the harmonic perturbations. The phase angle of the response of mass transfer rates with respect to the wall shear stress shows important deviations in comparison with the phase angle obtained with harmonic perturbations, specially at high frequencies.     

Analysis of the interaction between turbulent combustion and thermal radiation using unsteady coupled LES/DOM simulations

Radiation exchanges must be taken into account to improve the prediction of heat fluxes in turbulent combustion. The strong interaction with turbulence and its role on the formation of polluting species require the study of unsteady coupled calculations using Large Eddy Simulations (LESs) of the turbulent combustion process. Radiation is solved using the Discrete Ordinate Method (DOM) and a global spectral model. A detailed study of the coupling between radiative heat transfer and LES simulation involving a real laboratory flame configuration is presented. First the impact of radiation on the flame structure is discussed: when radiation is taken into account, temperature levels increase in the fresh gas and decrease in the burnt gas, with variations ranging from 100 K to 150 K thus impacting the density of the gas. Coupling DOM and LES allows to analyze radiation effects on flame stability: temperature fluctuations are increased, and a wavelet frequency analysis shows changes in the flow characteristic frequencies. The second part of the study focuses on the Turbulence Radiation Interaction (TRI) using the instantaneous radiative fields on the whole computational domain. TRI correlations are calculated and are discussed along four levels of approximation. The LES study shows that all the TRI correlations are significant and must be taken into account. These correlations are also useful to calculate the TRI correlations in the Reynolds Averaged Navier–Stokes (RANS) approach.     

Large-eddy simulation of thermal striping in unsteady non-isothermal triple jet

Unsteady temperature fluctuations of non-isothermal turbulent jets are encountered in many engineering applications including liquid metal cooled fast reactors (LMFR), and can cause thermal stresses on solid boundaries. An accurate prediction of the temperature fluctuations is important to assess potential thermal fatigue damage to components, and traditionally this has been done by RANS turbulence modelling calculations with limited success. In this study, a large eddy simulation (LES) technique was applied to predict the temperature fluctuations of thermal striping observed in a triple jet. The triple jet model was used as a mock-up of the outlet of fuel subassemblies in a nuclear fast reactor. The results show that LES predicted the highly oscillatory nature of unsteady thermal mixing of the triple jet. The LES results were in good agreement with the available experimental data in terms of mean, RMS, skewness and kurtosis. The large amplitude of the temperature fluctuations associated with the thermal striping was captured correctly, demonstrating that LES can be used to analyse unsteady characteristics of thermal striping. Instantaneous and time mean thermal fields were further analysed to assess the capability and accuracy of LES in the thermal striping study. The Spalart–Allmaras and realisable k ? ε turbulence models were also considered along with LES. It is found that these turbulence models produced a very small amplitude of fluctuations, and failed to predict the correct magnitude of unsteady thermal fluctuations, highlighting the limitations of the RANS approach in unsteady heat transfer simulations.     

基于大涡模拟的发动机缸内湍流流动及拟序结构

应用大涡模拟方法对发动机缸内湍流流场进行了三维瞬态数值分析.主要从湍流脉动、湍动能和缸内拟序结构演变等方面考察了发动机缸内流场特性.计算结果表明:相比雷诺平均模型,大涡模拟方法可以更真实地反映发动机循环过程中缸内气体流动的细节和规律.利用大涡模拟结合Q准则判别法可以较好地识别缸内大尺度湍流拟序结构;拟序结构对于缸内大尺度动能的产生及湍流的维持具有关键的作用.RANS类模型则不具备充分捕获大尺度拟序结构的能力.湍流脉动与活塞平均运行速度接近于成正比.