Evaluation of Subgrid-scale Models in Large-eddy Simulations of Turbulent Flow in a Centrifugal Pump Impeller

The current research of large eddy simulation （LES） of turbulent flow in pumps mainly concentrates in applying conventional subgrid-scale （SGS） model to simulate turbulent flow, which aims at obtaining the flow field in pump. The selection of SGS model is usually not considered seriously, so the accuracy and efficiency of the simulation cannot be ensured. Three SGS models including Smagorinsky-Lilly model, dynamic Smagorinsky model and dynamic mixed model are comparably studied by using the commercial CFD code Fluent combined with its user define function. The simulations are performed for the turbulent flow in a centrifugal pump impeller. The simulation results indicate that the mean flows predicted by the three SGS models agree well with the experimental data obtained from the test that detailed measurements of the flow inside the rotating passages of a six-bladed shrouded centrifugal pump impeller performed using particle image velocimetry （PIV） and laser Doppler velocimetry （LDV）. The comparable results show that dynamic mixed model gives the most accurate results for mean flow in the centrifugal pump impeller. The SGS stress of dynamic mixed model is decompose into the scale similar part and the eddy viscous part. The scale similar part of SGS stress plays a significant role in high curvature regions, such as the leading edge and training edge of pump blade. It is also found that the dynamic mixed model is more adaptive to compute turbulence in the pump impeller. The research results presented is useful to improve the computational accuracy and efficiency of LES for centrifugal pumps, and provide important reference for carrying out simulation in similar fluid machineries.

Applying resolved-scale linearly forced isotropic turbulence in rational subgrid-scale modeling

In previous attempts of rational subgrid-scale (SGS) modeling by employing the Kolmogorov equation of filtered (KEF) quantities, it was necessary to assume that the resolved-scale second-order structure function is stationary. Forced isotropic turbulence is often used as a framework for establishing and validating such SGS models based on stationary restrictions, for it generates statistical stationary samples. However, traditional forcing method at low wavenumbers cannot provide an analytic form of forcing term for a complete KEF in physical space, which has been illustrated to be essential in the modeling of such SGS models. Thus, an alternative forcing method giving an analytic forcing term in physical space is needed for rational SGS modeling. Giving an analytic linear driving term in physical space, linearly forced isotropic turbulence should be considered an ideal theoretical framework for rational SGS modeling. In this paper, we demonstrate the feasibility of establishing a rational SGS model with stationary restriction based on linearly forced isotropic turbulence. The performance of this rational SGS model is validated. We, therefore, propose the use of linearly forced isotropic turbulence as a complement to free-decaying isotropic turbulence and low-wavenumber forced isotropic turbulence for SGS model validations.

Mathematical Constraints in Multiscale Subgrid-Scale Modeling of Nonlinear Systems

To shed light on the subgrid-seale （SGS） modeling methodology of nonlinear systems such as the Navier-Stokes turbulence, we define the concepts of assumption and restriction in the modeling procedure, which are shown by generalized derivation of three general mathematical constraints for different combinations of restrictions. These constraints are verified numerically in a one-dimensional nonlinear advection equation. This study is expected to inspire future research on the SGS modeling methodology of nonlinear systems.

Stochastic modeling of subgrid-scale effects on particle motion in forced isotropic turbulence

The subgrid-scale effects on particle motion were investigated in forced isotropic turbulence by DNS and priorLES methods.In the DNS field,the importance of Kolmogorov scaling to preferential accumulation was validated by comparing the radial distribution functions under various particle Stokes numbers.The prior-LES fields were generated by filtering the DNS data.The subgrid-scale Stokes number(StSGS)is a useful tool for determining the effects of subgrid-scale eddies on particle motion.The subgrid-scale eddies tend to accumulate particles with StSGSb 1 and disperse particles with 1 b StSGSb 10.For particles with StSGS?1,the effects of subgrid-scale eddies on particle motion can be neglected.In order to restore the subgrid-scale effects,the Langevin-type stochastic model with optimized parameters was adopted in this study.This model is effective for the particles with StSGS N 1 while has an adverse impact on the particles with StSGSb 1.The results show that the Langevin-type stochastic model tends to smooth the particle distribution in the isotropic turbulence.

A SECOND-ORDER DYNAMIC SUBGRID-SCALE STRESS MODEL

A second-order dynamic model based on the general relation between the subgrid-scale stress and the velocity gradient tensors was proposed. A priori test of the second-order model was made using moderate resolution direct numerical simulation date at high Reynolds number ( Taylor microscale Reynolds number R-lambda = 102 similar to 216) for homogeneous, isotropic forced flow, decaying flow, and homogeneous rotating flow. Numerical testing shows that the second-order dynamic model significantly improves the correlation coefficient when compared to the first-order dynamic models.

A new mixed subgrid-scale model for large eddy simulation of turbulent drag-reducing flows of viscoelastic fluids

A mixed subgrid-scale（SGS） model based on coherent structures and temporal approximate deconvolution（MCT） is proposed for turbulent drag-reducing flows of viscoelastic fluids. The main idea of the MCT SGS model is to perform spatial filtering for the momentum equation and temporal filtering for the conformation tensor transport equation of turbulent flow of viscoelastic fluid, respectively. The MCT model is suitable for large eddy simulation（LES） of turbulent dragreducing flows of viscoelastic fluids in engineering applications since the model parameters can be easily obtained. The LES of forced homogeneous isotropic turbulence（FHIT） with polymer additives and turbulent channel flow with surfactant additives based on MCT SGS model shows excellent agreements with direct numerical simulation（DNS） results. Compared with the LES results using the temporal approximate deconvolution model（TADM） for FHIT with polymer additives, this mixed SGS model MCT behaves better, regarding the enhancement of calculating parameters such as the Reynolds number.For scientific and engineering research, turbulent flows at high Reynolds numbers are expected, so the MCT model can be a more suitable model for the LES of turbulent drag-reducing flows of viscoelastic fluid with polymer or surfactant additives.

Rational subgrid-scale modelling: a short survey

We review the previous attempts of rational subgrid-scale （SGS） modelling by employing theKolmogorov equation of filtered quantities. Aiming at explaining and solving the underlyingproblems in these models, we also introduce the recent methodological investigations for therational SGS modelling technique by defining the terms of assumption and restriction. Thesemethodological works are expected to provide instructive criterions for not only the rational SGSmodelling, but also other types of SGS modelling practices.

A new dynamic subgrid-scale model using artificial neural network for compressible flow

The subgrid-scale(SGS)kinetic energy has been used to predict the SGS stress in compressible flow and it was resolved through the SGS kinetic energy transport equation in past studies.In this paper,a new SGS eddy-viscosity model is proposed using artificial neural network to obtain the SGS kinetic energy precisely,instead of using the SGS kinetic energy equation.Using the infinite series expansion and reserving the first term of the expanded term,we obtain an approximated SGS kinetic energy,which has a high correlation with the real SGS kinetic energy.Then,the coefficient of the modelled SGS kinetic energy is resolved by the artificial neural network and the modelled SGS kinetic energy is more accurate through this method compared to the SGS kinetic energy obtained from the SGS kinetic energy equation.The coefficients of the SGS stress and SGS heat flux terms are determined by the dynamic procedure.The new model is tested in the compressible turbulent channel flow.From the a posterior tests,we know that the new model can precisely predict the mean velocity,the Reynolds stress,the mean temperature and turbulence intensities,etc.

Assessment of subgrid-scale models in wall-modeled large-eddy simulations of turbulent channel flows

Considering the demanding of grid requirements for high-Reynolds-number wall-bounded flow,the wall-modeled large-eddy simulation(WMLES)is an attractive method to deal with near wall turbulence.However,the effect of subgrid-scale(SGS)models for wall-bounded turbulent flow in combination with wall stress models is still unclear.In this paper,turbulent channel flow at Reτ=1000 are numerically simulated by WMLES in conjunction with four different SGS models,i.e.,the wall-adapting local eddy-viscosity model,the dynamic Smagorinsky model,the dynamic SGS kinetic energy model and the dynamic Lagrangian model.The mean velocity profiles are compared with the law of the wall,and the velocity fluctuations are compared with direct numerical simulation data.The energy spectrum of velocity and wall pressure fluctuations are presented and the role of SGS models on predicting turbulent channel flow with WMLES is discussed.

An improved dynamic subgrid-scale model and its application to large eddy simulation of rotating channel flows

A new dynamic subgrid-scale (SGS) model, which is proved to satisfy the principle of asymptotic material frame indifference (AMFI) for rotating turbulence, is proposed based on physical and mathematical analysis. Comparison with direct numerical simulation (DNS) results verifies that the new SGS model is effective for large eddy simulation (LES) on rotating turbulent flow. The SGS model is then applied to the LES of the spanwise rotating turbulent channel flow to investigate the rotation effect on turbulence characteristics, budget terms in the transport equations of resolved Reynolds stresses, and flow structures near the wall regions of the rotating channel.

Recent understanding on the subgrid-scale modeling of large-eddy simulation in physical space

In the last 50 years,the methodology of large-eddy simulation(LES)has been greatly developed,while lots of different subgridscale(SGS)models have appeared.However,the understanding of the procedure of SGS modeling is still not clear.The present contribution aims at reviewing the recent SGS models and,more importantly,expressing our recent understanding on the SGS modeling of LES in physical space.Taking the Kolmogorov equation for filtered quantities(KEF)as an example,it is argued that the KEF alone is not enough to be a closure method.Three physical laws are then introduced to complete this closure procedure and are expected to inspire the future researches of SGS modeling.

Subgrid-scale model based on the vorticity gradient tensor for rotating turbulent flows

A new subgrid-scale(SGS)stress model is proposed for rotating turbulent flows,and the new model is based on the traceless symmetric part of the square of the velocity gradient tensor and the symmetric part of the vorticity gradient tensor(or the so-called vorticity strain rate tensor).The new subgrid-scale stress model is taken into account the effect of the vortex motions in turbulence,which is reflected on the anti-symmetric part of the velocity gradient tensor.In addition,the eddy viscosity of the new model reproduces the proper scaling as O(y^3)near the wall.Then,the new SGS model is applied in large-eddy simulation of the spanwise rotating turbulent channel flow.Different simulating cases are selected to test the new model.The results demonstrate that the present model can well predict the mean velocity profiles,the turbulence intensities,and the rotating turbulence structures.In addition,it needs no a second filter,and is convenient to be used in the engineering rotational flows.

An improved dynamic subgrid-scale model and its application to large eddy simulation of stratified channel flows

In the present paper, a new dynamic subgrid-scale (SGS) model of turbulent stress and heat flux for stratified shear flow is proposed. Based on our calculated results of stratified channel flow, the dynamic subgrid-scale model developed in this paper is shown to be effective for large eddy simulation (LES) of stratified turbulent shear flows. The new SGS model is then applied to the LES of the stratified turbulent channel flow to investigate the coupled shear and buoyancy effects on the behavior of turbulent statistics, turbulent heat transfer and flow structures at different Richardson numbers.

A dynamic subgrid-scale model for the large eddy simulation of stratified flow

A new dynamic subgrid-scale (SGS) model, including subgrid turbulent stress and heat flux models for stratified shear flow is proposed by using Yoshizawa’ s eddy viscosity model as a base model. Based on our calculated results, the dynamic subgrid-scale model developed here is effective for the large eddy simulation (LES) of stratified turbulent channel flows. The new SGS model is then applied to the large eddy simulation of stratified turbulent channel flow under gravity to investigate the coupled shear and buoyancy effects on the near-wall turbulent statistics and the turbulent heat transfer at different Richardson numbers. The critical Richardson number predicted by the present calculation is in good agreement with the value of theoretical analysis.

风琴管喷嘴空化水射流流场的大涡模拟

针对空化水射流流场空化云演化采用RANS方法模拟不准确的问题,采用大涡模拟(Large-eddy simulation,LES)对风琴管空化喷嘴流场进行数值模拟。基于Mixture多相流模型和Zwart-Gerber-Belamri(ZGB)空化模型,分别采用SM模型(Smagorinsky-Lilly model,SM)、壁面适应的局部涡黏模型(Wall-adapted local eddy-viscous,WALE)和代数壁面模型(Wall-modeled large eddy simulation,WMLES)3种不同的亚格子模型,对风琴管喷嘴空化水射流流场进行数值模拟,分析空化水射流特性、空化云演化规律及脱落频率。结果表明:WALE模型可以较为准确模拟空化云演化周期,与高速摄像拍摄结果吻合较好;在喷嘴内圆柱段以及扩散段壁面附近产生涡环,加快了空化泡析出,WALE模型较好的模拟了涡环结构;涡量分析表明涡流在喷嘴出口附近出现,射流的末端空化泡破碎产生扰动,大尺度涡演化为小尺度涡,WALE模型模拟出涡的破碎范围较SM模型更大,WMLES模型未捕捉到小尺度涡。

基于LES的射流火焰湍流辐射交互作用研究

在大涡模拟中,研究了亚格子湍流辐射交互作用(SGS-TRI)对Sandia Flame D和Flame 4D(Scaled Sandia flame D)辐射源项的影响。模拟中,采用概率密度函数输运方程(TPDF)湍流燃烧模型模拟湍流燃烧过程,球谐函数(P1近似)法、灰气体加权和模型(WSGGM)模拟辐射换热。采用湍流涡团光学薄脉动近似(OTFA)处理滤波吸收项,分别采用考虑和忽略SGS-TRI的方法求解滤波发射项。结果显示,SGS-TRI仅在辐射源项本身数值较小的区域对时均辐射源项有相对较大的影响(最大为25%);考虑和忽略SGS-TRI计算得到的时均温度以及CO_(2)浓度等径向分布基本重合(相对差别小于3%)。因此,亚格子湍流辐射交互作用对无碳烟湍流射流火焰(Flame D和Flame 4D)的影响较小。

WRF模式中参数化方案对新疆北部不同地形地区风场模拟的影响

WRF(weather research and forecasting)模式中参数化方案的选择与近地面风场的仿真模拟结果关系密切。为解决新疆北部不同地形地区风场模拟准确性的问题,采用WRF中尺度气象模式,探究4类参数化方案(边界层、微物理、陆面过程、近地面层)以及次网格地形方案对新疆北部不同地形地区风场模拟结果的影响。结果表明:每组试验均能模拟出风速的变化趋势;陆面过程RUC(rapid update cycle)方案和微物理Lin(Purdue Lin)方案对平原地区模拟结果较好,陆面过程Noah方案和微物理WSM6(WRF single moment 6 class)方案对山区地形模拟结果较好,且对于平原和山谷地形,次网格地形方案对模拟地区均能起到较好的修正作用。

基于WRF-LES的门头沟地区近地面风场模拟与边界层方案敏感性研究

近地面风场与人类生活密切相关,精细准确的风场模拟可以为气象实况产品提供数据支撑。目前,中尺度模式的时空分辨率无法满足局地精细化模拟的需求,因此本研究基于中尺度模式(WRF)内层嵌套大涡模拟(LES),采用五层双向嵌套在门头沟东部区域展开百米近地面风场模拟。结合地面观测,设计边界层和亚网格模型敏感性试验,对比1 km分辨率下YSU和LES边界层方案对里层嵌套的影响,检验和评估不同亚网格模型在真实大气中的适用性。结果表明,在中微尺度过渡分辨率1 km开启YSU边界层较LES边界方案模拟效果更佳,在保持捕捉湍流细节的能力下,拥有较低的误差。不同的亚网格敏感性试验中,1.5TKE,SMAG,1.5TKE_NBA和SMAG_NBA四种亚网格模型差异较小,统计指标上,SMAG的均方根误差最低,相关系数最高。同时,四种亚网格模型依旧存在模式本身造成的高值低估,低值高估的现象,SMAG在风矢量分布和风速风向概率密度分布中与观测最为接近。在风场的瞬时空间分布上,SMAG_NBA表现出对细小的湍流捕捉能力的优势。

面向电大多尺度模型的大规模并行对称半正定亚网格FDTD算法

针对计算电磁学算法应用于实际工程问题时面临的计算模型电大尺寸多尺度瓶颈,基于自研的高性能并行计算框架JASMIN,通过提出融合h-自适应时间积分算法流程的对称半正定亚网格FDTD算法流程,实现了具备大规模并行能力的亚网格时域有限差分(FDTD)算法.该算法保留了FDTD算法高并行可扩展性优势,同时可局部使用细化网格剖分电大尺寸模型精细结构部位,支持粗网格区域和细网格区域分别采用大时间步长和小时间步长进行迭代计算.算例表明,该方法在解决电大多尺度电磁模拟问题时,具有较强的并行可扩展性,同时能够保证数值计算结果的精度和稳定性,与传统的FDTD算法相比,可显著降低内存需求,提高计算效率.

大涡模拟及其在湍流燃烧中的应用

大涡模拟作为一种研究湍流流动和湍流燃烧的有效手段，在国际上已经得到广泛应用．本文在回顾了大涡模拟（ＬＥＳ）的基本思想及其实施方法的基础上着重介绍了前人在大涡模拟的亚格子湍流模式和亚格子燃烧模式中的研究成果，同时给出了采用不同亚格子模式的大涡模拟在湍流燃烧中的应用实例，指出了大涡模拟在湍流燃烧中的重要作用，为大涡模拟的进一步发展和应用提供参考。