用基于M-SST模型的DES数值模拟喷流流场

脱体涡数值模拟方法(dettached eddy simulation,DES)是把雷诺平均Navier-Stokes方程(RANS)方法及大涡模拟方法(LES)结合起来模拟有脱体涡的湍流流场的数值模拟方法,其主要思想是在物面附近解雷诺平均Navier-Stokes方程、在其他区域采用Smagorinski大涡模拟方法。本文在剪切应力传输(SST)湍流模型的基础上用DES及混合非结构网格数值模拟具有横向喷流的湍流流场,算法采用Osher逆风格式,利用该套程序(包括网格生成及算法),对导弹在不同马赫数下的喷流流场进行了数值模拟,并与同时开展的实验研究的结果进行了对比,结果表明用该方法处理这类问题是较准确的。     

LES/RANS方法混合函数特性研究

将两方程k-ω SST湍流模型和Sagaut的混合尺度亚格子模型通过一个混合函数相结合, 构造出一种混合大涡/雷诺平均N-S方程模拟方法(hybird large eddy simulation/reynolds-averaged navier-stokes, Hybrid LES/RANS), 采用这种混合模拟方法结合5阶WENO格式对Ma=2.8平板湍流边界层进行了数值模拟, 并在计算区域上游入口处采用“回收/调节”方法生成湍流脉动边界条件, 通过考查RANS区域向LES区域的过渡参数及网格分辨率对这种混合模拟方法进行了评价. 计算结果表明: 该文采用的混合模拟方法可以捕捉到湍流边界层中的大尺度结构且入口边界层平均参数不会发生漂移, 混合函数应当将RANS区域和LES区域的过渡点设置在对数律层和尾迹律层的交界处, 而过渡应当迅速以获得正确的雷诺剪切应力分布, 在该文采用的模型及数值方法的条件下, 流向及展向的网格小至与Escudier混合长相当时, 能够获得可以接受的脉动速度的单点-二阶统计值.     

超声速多喷流干扰流场特性研究

研究了旋成体上超声速来流与超声速横向多喷流相撞产生的层流干扰流场特性. 数 值方法针对三维可压缩Navier-Stokes方程按二阶精度Roe格式进行离散,采用基于多区对 接网格技术的有限体积法. 数值模拟结果描述了多喷流干扰流场的空间结构以及激波/边界层 干扰引起的分离范围,探讨了沿流向等间距排列的喷口个数对表面和空间流场结构以及压力 分布的影响规律. 结果表明,第一喷口对多喷流干扰流场主要结构和喷口上游表面分离范围 起主导作用. 其中三喷流流场数值模拟的对称面激波结构与实验纹影结果进行对比,符合较 好.     

方柱绕流的数值模拟

采用有限差分法,对雷诺数为2.2×10~4的方柱绕流进行了大涡模拟(简称LES)。运用时间分裂控制(Split-Operator)法,将N-S方程分为对流步、扩散步和传播步。对Smagorinsky假设在近壁区的发散问题用两层模型进行处理。对流项用迎风—中心差分格式模拟,压力方程用SOR法迭代求解。计算得到的沿对称线的时均顺流向速度与文献上的实验结果进行了比较,结果吻合较好,同时还对绕方柱流的流场结构进行了分析研究。     

激励射流在超声速流场中的混合特性研究

将Hartmann-Sprenger(H-S)管集成到传统射流装置中形成激励射流,研究其在超声速流场中的混合特性.设计了3种频率的CO_2激励射流,将其横向注入Ma=2.5的均匀超声速来流当中.采用平面激光散射技术对其进行瞬态可视化成像,利用组分采样进行时均分析.结果表明:H-S激励射流可以有效提高射流穿透深度,形成较多大尺度结构,从而获得较好的混合效果.     

高亚声速湍流喷流气动噪声数值分析

为适应航空噪声管制规定要求,发动机喷流噪声控制成为目前气动声学研究中的重要课题,预测分析喷流噪声辐射并揭示其产生机理将为噪声控制奠定基础.采用高精度并行LES（large eddy simulation）方法计算分析马赫数0.9高亚声速喷流的湍流演化和气动噪声现象.首先,仔细验证喷流LES湍流场计算保真性,并分析流场中不同尺度涡结构的演化形态.其次,利用可穿透面FW-H（Ffowcs Williams and Hawkings）方法外推喷流近场声源数据获得精确声辐射远场,进而分析声场主导声模态特性.最后,通过分析声源机制、分离声模态等方法研究势流核末端大尺度拟序涡运动演化形成的低波数波包在噪声主导声模态产生中的重要作用.数值结果表明LES结合可穿透面FW-H方法可精确预测高亚声速喷流的流场及声场特征,且数值分析揭示涡环对并形成的大尺度拟序结构在喷流中心线上沿径向融合,产生了在远场低方位角占优的主导声模态,并构成强指向性声场,噪声峰值方位角约为30°.      

非定常俯抑振荡下的横向喷流数值模拟

采用高精度格式数值求解RANS方程,研究了定常状态下横向喷流流场,压力分布计算结果与实验结果基本吻合,并捕捉到喷流干扰流场中多种流动结构.在非定常计算过程中,飞行器的振动引起了法向力和俯仰力矩系数的相位滞后,推力放大因子随俯仰角周期变化.飞行器振动过程中,喷流流场的动态气动特性与稳态喷流有明显的区别,因此在利用横向喷流对飞行器进行姿态控制时,应该考虑由于飞行器姿态的变化对横向喷流所产生的非定常影响问题.     

激励射流在超声速流场中的混合的特性研究

Hartmann-Sprenger（H-S）管处于吞吐模式下可以产生高频高幅度的气流振荡。本文将H-S管集成到传统射流装置中形成激励射流,研究其在超声速流场中的混合特性。本文设计了三种频率的CO2激励射流,将其横向注入马赫数2.5的均匀超声速来流当中。采用平面激光散射技术对其进行瞬态可视化成像,利用组分采样、测量总压进行时均分析。结果表明:H-S激励射流可以有效提高射流穿透深度,形成较多大尺度结构,从而获得较好的混合效果。关键词: 激励射流 超声速混合 穿透深度 大尺度结构      

高速流场气动光学RANS/DSMC混合数值模拟算法研究

提出了适用于高速流场气动光学数值模拟的RANS/DSMC混合算法.通过RANS对全局时均流场进行数值模拟,再对其中局部流场的脉动量采用DSMC进行数值模拟,以Maxwellian速度分布实现RANS宏观量信息向DSMC微观量信息的传递.采用超声速环境下尖劈模型对该混合算法进行校验,对比试验结果论证了算法的正确性.在计算...     

超声速平面混合层小激波的形成与演变

为了揭示超声速混合层中小激波形成机理及其与涡相互作用的演变过程,本文基于大涡模拟（LES）方法,结合五阶精度混合TCD／WENO格式,对超声速平面混合层在对流马赫数为Mc=0．65条件下的流场结构进行了数值模拟,数值结果详细描述了超声速混合层中小激波的形成过程。研究了小激波形成后,随涡运动而产生的变形、脱落及发展过程。同时,对混合层双涡合并过程中,小激波与相邻涡相互作用所产生的变形与演变过程进行了讨论。     

Large eddy simulation (2D) of spatially developing mixing layer using vortex‐in‐cell for flow field and filtered probability density function for scalar field

A large eddy simulation based on filtered vorticity transport equation has been coupled with filtered probability density function transport equation for scalar field, to predict the velocity and passive scalar fields. The filtered vorticity transport has been formulated using diffusion‐velocity method and then solved using the vortex method. The methodology has been tested on a spatially growing mixing layer using the two‐dimensional vortex‐in‐cell method in conjunction with both Smagorinsky and dynamic eddy viscosity subgrid scale models for an anisotropic flow. The transport equation for filtered probability density function is solved using the Lagrangian Monte‐Carlo method. The unresolved subgrid scale convective term in filtered density function transport is modelled using the gradient diffusion model. The unresolved subgrid scale mixing term is modelled using the modified Curl model. The effects of subgrid scale models on the vorticity contours, mean streamwise velocity profiles, root‐mean‐square velocity and vorticity fluctuations profiles and negative cross‐stream correlations are discussed. Also the characteristics of the passive scalar, i.e. mean concentration profiles, root‐mean‐square concentration fluctuations profiles and filtered probability density function are presented and compared with previous experimental and numerical works. The sensitivity of the results to the Schmidt number, constant in mixing frequency and inflow boundary conditions are discussed. Copyright © 2005 John Wiley & Sons, Ltd.     

On the influence of numerical schemes and subgrid–stress models on large eddy simulation of turbulent flow past a square cylinder

Influence of finite difference schemes and subgrid‐stress models on the large eddy simulation calculation of turbulent flow around a bluff body of square cylinder at a laboratory Reynolds number, has been examined. It is found that the type and the order of accuracy of finite‐difference schemes and the subgrid‐stress model for satisfactory results are dependent on each other, and the grid resolution and the Reynolds number. Using computational grids manageable by workstation‐level computers, with which the near‐wall region of the separating boundary layer cannot be resolved, central‐difference schemes of realistic orders of accuracy, either fully conservative or non‐conservative, suffer stability problems. The upwind‐biased schemes of third order and the Smagorinsky eddy‐viscosity subgrid model can give reasonable results resolving much of the energy‐containing turbulent eddies in the boundary layers and in the wake and representing the subgrid stresses in most parts of the flow. Noticeable improvements can be obtained by either using higher order difference schemes, increasing the grid resolution and/or by implementing a dynamic subgrid stress model, but each at a cost of increased computational time. For further improvements, the very small‐scale eddies near the upstream corners and in the laminar sublayers need to be resolved but would require a substantially larger number of grid points that are out of the range of easily accessible computers. Copyright © 2002 John Wiley & Sons, Ltd.     

Investigation of Convective Heat and Mass Transfer in Northern Reservoirs using an Eddy-Resolving Model

This paper presents a mathematical formulation of the problem of thermodynamic interaction between a reservoir with supercooled bottom and ice cover on the free surface. Turbulent flows with coherent structures was simulated using the equations of thermohydrodynamics in the Boussinesq approximation in which the average (uniform along the horizontal) and convective (large eddies) components are distinguished. Motions of subgrid scales are described using a one-parameter model for the kinetic energy of turbulence. The applicability of models of various classes for calculating vertical heat transfer in deep-water reservoirs above permafrost zones are discussed.     

Extension du modèle d'échelles mixtes à la diffusivité de sous-maille

In the large-eddy simulation frame for non-isothermal turbulent flow, the Mixed Scale Model is extended to the subgrid diffusivity, in order to dissociate the computation of subgrid viscosity and diffusivity. The identification of the subgrid thermal dissipation term in the subgrid flux transport equation leads to an algebraic expression of the subgrid diffusivity. This diffusive model, as the Smagorinsky one, is weighted by a model based on scale similarity. This expression leads to satisfactory results when applied to a buoyant turbulent flow in a differentially heated cavity.     

Subgrid scale formulation of optical flow for the study of turbulent flow

We propose a new formulation of optical flow in the case of passive scalar or solid particles spreading in turbulent flows. The flow equation is defined from the scalar transport equation and a decomposition of our physical quantities (velocity and concentration fields) into two contributions, large and small scales, to account for the lack of spatial resolution in processed images. A subgrid scale model is introduced to model the small scale contribution. Comparisons are made with existing optical flow methods and Particle Image Velocimetry on synthetic and real sequences. The improvement of the estimation of velocity field by the proposed formulation is discussed in the case of a scalar turbulent propagation.     

模型燃烧室内瞬态紊流的大涡模拟

本文采用三种不同亚网格尺度模型对带有V型稳定器的模型燃烧室二维瞬态紊流流动进行了大涡模拟。并在交错网格系下用SIMPLE算法和混合差分格式求解离散方程。数值研究拟不同型式入口速度分布和不同亚网格尺度模型下模型燃烧室二维瞬态紊流流场。计算结果表明不同入口速度分布和不同亚网格尺度模型对瞬态流场和出口速度分布有一定的影响。本文通过数值模拟,揭示了V型稳定器后旋涡的产生和脱落过程。通过计算结果及实验数据的比较可知,本文采用的亚网格尺度模型可以用来模拟模型燃烧室紊流流场及稳定器后面回流区的流动情况。     

Large-eddy structures of turbulent swirling flows and methane-air swirling diffusion combustion

Turbulent swirling flows and methane-air swirling diffusion combustion are studied by large-eddy simulation (LES) using a Smagorinsky-Lilly subgrid scale turbulence model and a second-order moment (SOM) SGS combustion model, and also by RANS modeling using the Reynolds Stress equation model with the IPCM+wall and IPCM pressure-strain models and SOM combustion model. The LES statistical results for swirling flows give good agreement with the experimental results, indicating that the adopted subgrid-scale turbulence model is suitable for swirling flows. The LES instantaneous results show the complex vortex shedding pattern in swirling flows. The initially formed large vortex structures soon break up in swirling flows. The LES statistical results of combustion modeling are near the experimental results and are as good as the RANS-SOM modeling results. The LES results show that the size and range of large vortex structures in swirling combustion are different from those of isothermal swirling flows, and the chemical reaction is intensified by the large-eddy vortex structures. The project supported by the Special Funds for Major State Basic Research (G-1999-0222-07). The English text was polished by Keren Wang.     

颗粒在大涡结构中的弥散

气相采用大涡模拟方法,颗粒相采用轨道模型研究了三维后台阶气粒两相湍流流动的大尺度涡结构的瞬时演变过程以及颗粒的瞬时弥散规律.比较了不同入流速度的颗粒在大涡结构中的瞬时弥散特性,尤其研究了高速释放大颗粒的弥散特性.三维流动中大尺度涡结构具有明显的脱离、发展、合并和破碎过程.小颗粒的分布受大涡结构的控制,其空间的弥散过程与流体 大涡结构的空间发展相一致,但是由于三维流动中大涡边缘和中心区的压力差,颗粒在大尺度 涡的边缘出现密集.而大颗粒在流场中的分布受其惯性控制,对气相的涡结构不敏感.高速释放到流场中的大颗粒受惯性影响最大,保持在其原有动量方向上运动.     

Multi-scale analysis of subgrid stress and energy dissipation in turbulent channel flow

In present study, the subgrid scale （SGS） stress and dissipation for multiscale formulation of large eddy simulation are analyzed using the data of turbulent channel flow at Ret = 180 obtained by direct numerical simulation. It is found that the small scale SGS stress is much smaller than the large scale SGS stress for all the stress components. The dominant contributor to large scale SGS stress is the cross stress between small scale and subgrid scale motions, while the cross stress between large scale and subgrid scale motions make major contributions to small scale SGS stress. The energy transfer from resolved large scales to subgrid scales is mainly caused by SGS Reynolds stress, while that between resolved small scales and subgrid scales are mainly due to the cross stress. The multiscale formulation of SGS models are evaluated a priori, and it is found that the small- small model is superior to other variants in terms of SGS dissipation.     

Subgrid Scale Modeling in Large-Eddy Simulation of Turbulent Combustion Using Premixed Flamelet Chemistry

Large-eddy simulation (LES) of turbulent combustion with premixed flamelets is investigated in this paper. The approach solves the filtered Navier–Stokes equations supplemented with two transport equations, one for the mixture fraction and another for a progress variable. The LES premixed flamelet approach is tested for two flows: a premixed preheated Bunsen flame and a partially premixed diffusion flame (Sandia Flame D). In the first case, we compare the LES with a direct numerical simulation (DNS). Four non-trivial models for the chemical source term are considered for the Bunsen flame: the standard presumed beta-pdf model, and three new propositions (simpler than the beta-pdf model): the filtered flamelet model, the shift-filter model and the shift-inversion model. A priori and a posteriori tests are performed for these subgrid reaction models. In the present preheated Bunsen flame, the filtered flamelet model gives the best results in a priori tests. The LES tests for the Bunsen flame are limited to a case in which the filter width is only slightly larger than the flame thickness. According to the a posteriori tests the three models (beta-pdf, filtered flamelet and shift-inversion) show more or less the same results as the trivial model, in which subgrid reaction effects are ignored, while the shift-filter model leads to worse results. Since LES needs to resolve the large turbulent eddies, the LES filter width is bounded by a maximum. For the present Bunsen flame this means that the filter width should be of the order of the flame thickness or smaller. In this regime, the effects of subgrid reaction and subgrid flame wrinkling turn out to be quite modest. The LES-results of the second case (Sandia Flame D) are compared to experimental data. Satisfactory agreement is obtained for the main species. Comparison is made between different eddy-viscosity models for the subgrid turbulence, and the Smagorinsky eddy-viscosity is found to give worse results than eddy-viscosities that are not dominated by the mean shear. Paper presented on the Eccomas Thematic Conference Computational Combustion 2007, submitted for a special issue of Flow, Turbulence and Combustion.