Stress-Blended Eddy Simulation of the Turbulent Flow Around Circular Cylinder with Dynamic Hybrid RANS/LES Model

In order to verify the effectiveness and superiority of the dynamic hybrid RANS/LES(DHRL)model,the flow around a cylinder with sinusoidal fluctuating velocity at the inlet was used as the test case.The latest computational fluid dynamics(CFD)model can flexibly choose any existing large-eddy simulation(LES)method combined with RANS method to calculate the flow field.In addition,the DLES model and DDES model are selected as typical representatives of the turbulence model to compare the capture ability of the flow field mechanism.The internal flow field including the y+value,velocity distribution,turbulent kinetic energy and vortex structures is comprehensively analyzed.Finally,the results show that the new model has enough sensitivity to capture the information of the flow field and has more consistent velocity distribution with the experimental value,which shows its potential in practical engineering applications to some extent.

A hybrid RANS/LES model for simulating time-dependent cloud cavitating flow around a NACA66 hydrofoil

Cloud cavitating flow is highly turbulent and dominated by coherent large-scale anisotropic vortical structures. For the numer- ical investigation of such a class of flow, large eddy simulation （LES） is a reliable method but it is computationally extremely costly in engineering applications. An efficient approach to reduce the computational cost is to combine Reynolds-averaged Navier-Stokes （RANS） equations with LES used only in the parts of interest, such as massively separated flow regions. A new hybrid RANS/LES model, the modified filter-based method （FBM）, is proposed in the present study which can perform RANS or LES depending on the numerical resolution. Compared to the original FBM, the new method has three modifications： the state-of-the-art shear stress transport （SST） model replaces the k-c model as a baseline RANS model. A shielding function is introduced to obviate the switch from RANS to LES occurring inside the boundary layer. An appropriate threshold controlling the switch from RANS to LES is added to achieve an optimal predictive accuracy. The new model is assessed for its predictive capability of highly unsteady cavitating flows in a typical case of cloud cavitation around a NACA66 hydrofoil. The new mod- el results are compared with data obtained from the Smagorinsky LES and SST model based on the same homogeneous Zwart cavitation model. It is found that the modified FBM method has significant advantages over SST model in all aspects of pre- dicted instantaneous and mean flow field, and its predictive accuracy is comparable to the Smagorinsky LES model even using a much coarser grid in the simulations.

3-D hybrid LES-RANS model for simulation of open-channel T-diversion flows

The study of flow diversions in open channels plays an important practical role in the design and management of open-channel networks for irrigation or drainage. To accurately predict the mean flow and turbulence characteristics of open-channel dividing flows, a hybrid LES-RANS model, which combines the large eddy simulation （LES） model with the Reynolds-averaged Navier-Stokes （RANS） model, is proposed in the present study. The unsteady RANS model was used to simulate the upstream and downstream regions of a main channel, as well as the downstream region of a branch channel. The LES model was used to simulate the channel diversion region, where turbulent flow characteristics are complicated. Isotropic velocity fluctuations were added at the inflow interface of the LES region to trigger the generation of resolved turbulence. A method based on the virtual body force is proposed to impose Reynolds-averaged velocity fields near the outlet of the LES region in order to take downstream flow effects computed by the RANS model into account and dissipate the excessive turbulent fluctuations. This hybrid approach saves computational effort and makes it easier to properly specify inlet and outlet boundary conditions. Comparison between computational results and experimental data indicates that this relatively new modeling approach can accurately predict open-channel T-diversion flows.

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

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.

An hybrid RANS/LES model for simulation of complex turbulent flow

A nonqinear eddy viscosity model （NLEVM） and a scalable hybrid Reynolds averaged Navier-Stokes/large eddy simula- tion （RANS/LES） strategy are developed to improve the capability of the eddy viscosity model （EVM） to simulate complex flows featuring separations and unsteady motions. To study the performance of the NLEVM, numerical simulations around S809 airfoil are carried out and the results show that the NLEVM performs much better when a large separation occurs. Calculated results of the flow around a triangular cylinder show that the NLEVM can improve the precision of the flow fields to some extents, but the error is still considerable, and the small turbulence structures can not be clearly captured as the EVM. Whereas the scalable hybrid RANS/LES model based on the NLEVM is fairy effective on resolving the turbulent structures and can give more satisfactory predictions of the flow fields.

亚临界区圆柱绕流相干结构壁面模化混合RANS/LES模型

本文发展了一种具有壁面模化大涡模拟能力的雷诺平均纳维-斯托克斯(RANS)和大涡模拟(LES)方法的混合模型(简称WM-HRL模型),致力于对亚临界区雷诺数钝体绕流相干结构这类复杂流动现象进行高置信度的CFD解析模拟研究.该方法通过一个仅与当地网格空间分布尺寸有关的湍动能解析度指标参数rk即可实现从RANS到LES的无缝快速转换,并且RANS/LES混合转换区的边界位置及其各个分区(包括RANS区、LES区及RANS/LES混合转换区)对湍动能的解析能力均可通过两个指标参数nrk1-Q和nrk2-Q准则进行预先设定.通过对雷诺数Re=3900下圆柱绕流场的系列数值模拟研究,获得了能够高置信度解析并捕捉其绕流场中三维时空瞬态发展相干结构特性的湍动能解析度指标参数nrk1-Q和nrk2-Q准则的组合条件.研究表明,该WM-HRL模型不仅能够准确获取圆柱绕流场中剪切层小尺度K-H不稳定性结构的精细谱结构,而且在同一套网格系统下通过变化湍动能解析度指标参数nrk2-Q和nrk1-Q准则的组合条件,还可以精细解析圆柱绕流场中两类不同回流区的长度结构特征,及其对应的圆柱尾部近壁面处V和U形两个平均流向速度剖面的分支结构特性.

Prediction of separation flows around a 6:1 prolate spheroid using RANS/LES hybrid approaches

This paper presents hybrid Reynolds-averaged Navier-Stokes （RANS） and large-eddy-simulation （LES） methods for the separated flows at high angles of attack around a 6：1 prolate spheroid. The RANS/LES hybrid meth- ods studied in this work include the detached eddy simulation （DES） based on Spalart-Allmaras （S-A）, Menter＇s k-ω shear-stress-transport （SST） and k-o9 with weakly nonlinear eddy viscosity formulation （Wilcox-Durbin＋, WD＋） models and the zonalANS/LES methods based on the SST and WD＋ models. The switch from RANS near the wall to LES in the core flow region is smooth through the implementation of a flow-dependent blending function for the zonal hybrid method. All the hybrid methods are designed to have a RANS mode for the attached flows and have a LES behavior for the separated flows. The main objective of this paper is to apply the hybrid methods for the high Reynolds number separated flows around prolate spheroid at high-incidences. A fourth-order central scheme with fourth-order artificial viscosity is applied for spatial differencing. The fully implicit lower-upper symmetric-Gauss-Seidel with pseudo time sub-iteration is taken as the temporal differentiation. Comparisons with available measurements are carried out for pressure distribution, skin friction, and profiles of velocity, etc. Reasonable agreement with the experiments, accounting for the effect on grids and fundamental turbulence models, is obtained for the separation flows.

Joint RANS/LES Modeling of Flameless Combustion

We present our timesaving joint RANS/LES approach (we originally developed it for numerical simulations of turbulent premixed combustion) to simulate flameless combustion with separate injection of gas fuel and strong exhaust gas recirculation. It is based on successive RANS/LES numerical modeling where part of the information (stationary average fields) is achieved by RANS simulations and part (instantaneous nonstationary image of the process) by LES. The latter is performed using the RANS field of mean dissipation rate to model the sub-grid turbulent viscosity in the context of the Kolmogorov theory of small-scale turbulence. We analyze flameless combustion in the FLOX? combustor where we also simulate non-premixed flame combustion used for preliminary heating of the combustor. Different regimes take place using different systems of air injection. We applied for both regimes the simple assumption of “mixed is burnt”. The main results are the following: 1) RANS simulations demonstrate for used two injection systems respectively more compact flame and distributed flameless combustion. 2)There is agreement between RANS and corresponding LES results: RANS and averaged LES profiles of the velocity and temperature are in reasonable agreement. 3) LES modeling with Kolmogorov independent on time sub-grid viscosity reproduce instantaneous image of the process including the vortex structures. Probably due to using an annular injector system for air the instantaneous field of the temperature demonstrate significant irregularity in the beginning of the burner, which in an animation looks like moving coherent structures. 4) In the joint RANS/LES approach the computer time of the LES sub-problems is much shorter than classic LES modeling due to using time independent subgrid transport coefficients and avoiding long-continued simulations, which are necessary for averaging of instantaneous LES fields. Practically in our simulations time consuming of the LES sub-problem was only several times lager then the RANS one and it makes this approach suitable for industrial applications.

Hybrid RANS/LES Simulations of Two and Three Dimensional Supersonic Cavity Flows

Supersonic unsteady flows of two and three dimensional cavities, which have the same basic measures, length, depth and aft wall angle, are investigated numerically by using hybrid RANS/LES （Reynolds-Averaged Navier-Stokes/Large-Eddy Simulation） method. In particular, the mass exchange and oscillation characteristics of the cavities are analyzed and compared. It is shown that the two and three dimensional cavities have almost the same residence time except that the three dimensional one has a little larger mass decay at beginning, which may attribute to the influence of the streamwise vortices. The two dimensional cavity has three dominant frequencies while the three dimensional one has only one oscillation dominant frequency, that is, the three dimensional effects simplify the oscillation modes in the cavity. The distributions of oscillation energy are approximately universal in the transverse and spanwise directions. However, the oscillation energy in the streamwise direction shows a hybrid monotone/periodic distribution, which may be caused by the streamwise-propagating pressure waves.

采用RANS/LES混合方法研究分离流动

采用结合湍流模式和大涡模拟(LES)的混合方法数值模拟Aerospatial A-profile翼型以及细长椭球体大迎角分离流动。该类混合方法比较适于模拟分离流动:近壁区附体流动采用湍流模式,而远离物面分离区域内的大尺度运动则用LES方法。通过计算和实验结果的对比,混合方法切实可行并具有很大的发展潜力。

不同RANS/LES混合模型的汽车气动噪声分析

采用Fluent UDF构造Realizable k-ε/LES、SST k-ε/LES和Spalart-Allmaras/LES 3种RANS/LES混合模型,并对汽车的气动噪声进行数值模拟。在近壁面采用RANS湍流模型求解,而远离物面的分离区域内的大尺度运动则用LES求解,将3种混合模型的计算结果与全LES湍流模型和风洞试验结果进行了比较。结果表明,3种混合模型对气动噪声的计算精度高于LES湍流模型;Realizable k-ε/LES和SST k-ε/LES混合模型数值模拟结果与试验值的相对误差均小于5%,而其中Realizable k-ε/LES混合模型更为准确和高效,消耗的计算资源更少。

嵌入式RANS/LES混合方法在槽道流动数值模拟中的应用

与传统整体式RANS/LES混合方法相比,嵌入式RANS/LES混合方法不再依靠改进后的湍流模型本身或网格疏密自动划分流场中的RANS区域和LES区域,而是通过对流动现象的预估,人为将LES区域嵌入到RANS区域,从而使RANS区过渡到LES区的交界面尽可能与流向垂直,方便合理附加额外的湍流脉动信息。这样做不但确保了LES在某些复杂流动区域彻底支配地位,更重要的是能够避免RANS区对LES区解析湍流发展的污染和抑制。将槽道流场划分为上游RANS区和下游LES区,在经典的SST-DES模型基础上,采用合成涡方法生成交界面的湍流信息,取得了良好的模拟结果。

Fatigue reliability based on residual strength model with hybrid uncertain parameters

The aim of this paper is to evaluate the fatigue reliability with hybrid uncertain parameters based on a residual strength model. By solving the non-probabilistic setbased reliability problem and analyzing the reliability with randomness, the fatigue reliability with hybrid parameters can be obtained. The presented hybrid model can adequately consider all uncertainties affecting the fatigue reliability with hybrid uncertain parameters. A comparison among the presented hybrid model, non-probabilistic set-theoretic model and the conventional random model is made through two typical numerical examples. The results show that the presented hybrid model, which can ensure structural security, is effective and practical.

Hybrid LES/RANS Simulations of Compressible Flow in a Linear Cascade of Flat Blade Profiles

The paper reports on 3D numerical simulations of unsteady compressible airflow in a blade cascade consisting of flat profiles using a hybrid LES/RANS approach including a transition model.As a first step towards simulation of blade flutter in turbomachinery,various incidence angle offsets of the middle blade were modeled.All simulations were run for the flow regime characterized by outlet isentropic Mach number M_(is)=0.5and zero incidence.The results of the LES/RANS simulations(pressure and Mach number distributions)were compared to a baseline RANS model,and to experimental data measured in a high-speed wind tunnel.The numerical results show that both methods overpredict flow separation taking place at the leading edge.In this regard,the hybrid LES/RANS method does not provide superior results compared to the traditional RANS simulations.Nevertheless,the LES/RANS results also capture vortex shedding from the blunt trailing edge.The frequency of the trailing edge vortex shedding in CFD simulations matches perfectly the spectral peak recorded during wind tunnel measurements.

不同RANS/LES混合方法在汽车标模外流场非定常数值模拟中的对比

针对MIRA汽车标模外流场,采用RANS/LES混合方法开展非定常数值模拟,通过与风洞实验测力、测压结果进行详细对比分析,探究不同RANS/LES混合方法(DDES、IDDES、SBES、SDES)以及基于不同RANS模型(RKE、SA、SST k-ω、GEKO k-ω)的DDES方法在汽车外流场计算中的适用性。研究表明:对于气动力系数,不同混合方法的预测结果都偏高,其中DDES-GEKO模型的相对误差最小;对于表面压力系数,不同混合方法对垂直中截线的压力预测结果与实验值吻合程度较高,其中SBES-GEKO模型的结果更优;DDES模型内嵌不同RANS模型对后风窗的压力预测差别明显,其中SA模型较优;而不同混合方法对车底的压力预测偏差较大,对车底前部压力预测都小于实验值,其中对车底后部,SBES-GEKO模型的结果较优;此外,SBESGEKO模型能较好的识别出尾迹区的非定常流动结构。

Interaction mechanisms of shock waves with the boundary layer and wakes in a highly-loaded NGV using hybrid RANS/LES

Accurate predictions of Shock Waves and Boundary Layer Interaction(SWBLI)and strong Shock Waves and Wake Vortices Interaction(SWWVI)in a highly-loaded turbine propose challenges to the currently widely used Reynolds-Averaged Navier-Stokes(RANS)model.In this work,the SWBLI and the SWWVI in a highly-loaded Nozzle Guide Vane(NGV)are studied using a hybrid RANS/LES strategy.The Turbulence Kinetic Energy(TKE)budget and the Proper Orthogonal Decomposition(POD)method are used to analyze flow mechanisms.Results show that this hybrid RANS/LES method can obtain detailed flow structures for flow mechanisms analysis.Strong shock waves induce boundary layer separation,while the presence of a separation bubble can in turn lead to a Mach reflection phenomenon.The shock waves cause trailing-edge vortices to break clearly,and the wakes,in turn,can change the shocks intensity and direction.Furthermore,the Entropy Generation Rate(EGR)is used to analyze the irreversible loss.It turns out that the SWWVI can reduce the flow field loss.There are several weak shock waves in the NGV flow field,which can increase the irreversible loss.This work offers flow mechanisms analysis and presents the EGR distribution in SWBLI and SWWVI areas in a transonic turbine blade.

Numerical Investigation on Porous Media Quenching Behaviors of Premixed Deflagrating Flame using RANS/LES Model

To understand the mechanism of premixed flame quenching by porous media,a zonal hybrid RANS/LES model was employed,in which the LES flow solver was used to resolve the large turbulent structures within the non-porous region,while RANS was applied to porous media zone.The predicted results were compared with previous experimental data.And it was evident that the premixed flame propagation rates and structure as well as quenching behaviors were reproduced by this numerical approach with a better accuracy.Due to the inherently higher heat transfer coefficients between the solid and gas phases in porous media,the gas phase temperature has been decreased rapidly.However,upstream obstacles can cause the flame propagating faster and thus reduce the axial gas temperature gradients,resulting in the invalidity of the operation of premixed flame quenching.By comparison with the case without upstream obstacle,the values of reaction rate attained in the case with three pairs of obstacles are higher,which makes a positive impact on the flame passing through the porous medium.In addition,the porous media with greater pore density has an excellent flame quenching property mainly owing to both the larger volumetric heat transfer and higher quenching temperature.

RANS and hybrid LES/RANS simulations of flow over a square cylinder

Unsteady RANS(URANS),hybrid LES/RANS and IDDES simulations were conducted to numerically investigate the velocity field around,and pressures distribution and forces over a square cylinder immersed in a uniform,steady oncoming flow with Reynolds number Re=21,400.The vortex shedding responses in terms of Strouhal number,the pressure distribution,the velocity profile and the velocity fluctuations obtained by numerical simulations are compared with experimental data.Compared with 2D URANS simulation,3D simulations using hybrid LES/RANS and IDDES models provide more accurate prediction on the responses in the wake,including mean streamwise velocity profile and rms velocity fluctuations.This also results in more accurate prediction of time-averaged surface pressure coefficient on the rear surface obtained by 3D hybrid LES/RANS and IDDES simulations than by URANS simulation.When a hybrid LES/RANS model or IDDES model is used,a more accurate prediction for either pressure coefficient or velocity profile(especially in the far wake region)is not guaranteed by increasing the mesh resolution along the spanwise direction of the square cylinder.

Application of the hybrid RANS/LES method on the hydraulic dynamic performance of centrifugal pumps

The nonlinear hybrid RANS/LES method has been developed considering the simulation efficiency and accuracy to simulate the hydraulic dynamic characteristics of the centrifugal pump. It has been proved that the nonlinear hybrid RANS/LES method could effectively capture the unsteady flow structure and pressure pulsation in 2?D centrifugal pumps. To further investigate the hydraulic dynamic performance of a redesigned 3-D-gap drainage centrifugal pump, the nonlinear hybrid RANS/LES method is employed. Both numerical simulation and experimental results show that the hydraulic performance and pressure pulsation characteristics of the 3-D-gap drainage impeller centrifugal pump are significantly enhanced.

基于当地流动结构的RANS/LES混合模型

针对传统雷诺平均Navier-Stokes/大涡模拟(RANS/LES)混合模型固有的RANS和LES区域衔接难题,借鉴Vreman提出的亚格子(SGS)模型,提取其中描述流动状态的判断函数,利用不可压缩槽道湍流标定模型系数,构造了一个新的RANS/LES混合模型。评估算例包括不可压缩槽道湍流、稳态超声速平板边界层、NACA4412翼型绕流和亚临界雷诺数圆柱绕流。该模型作为壁面模化LES,解决了传统RANS/LES方法中速度型偏离对数律(LLM)的缺陷,且拓宽了原Vreman模型在粗糙网格上的适用性,具有较高精度。该模型同时解决了模化应力折损问题,且对大尺度分离流动的模拟精度相较传统的RANS/LES混合模型如脱体涡模拟(DES)有进一步提升。