混合层流动拟序结构的大涡模拟

采用大涡模拟方法对空间发展的二维平面混合层进行了数值模拟 ,动量方程采用分步投影法求解 ,亚格子项采用标准Smagorinsky亚格子模式模拟 ,压力泊松方程采用修正的循环消去法快速求解 ,同时求解了标志物输运方程以实现数值流场显示。模拟结果给出了混合层流动的瞬态发展过程以及流动中拟序结构的发展演变过程 ,成功地模拟了混合层发展中的各种瞬态细节过程 ,如涡的卷起、增长 ,涡与涡之间的配对、合并过程 ,以及大涡破碎为小涡的级联过程 ,为各种以混合层流动为原型流动的射流、尾流等工业流动的控制和优化提供了理论基础。     

Developments in Formulation and Application of the Filtered Density Function

An overview is presented of the state of progress in large eddy simulation of turbulent combustion via the filtered density function (FDF). This includes the formulations based on both the joint velocity-scalar FDF, and the marginal scalar FDF. In the former, the most up-to-date and comprehensive form of the model is presented along with its applications for LES of some relatively simple flows. In the latter, results are presented of the most recent LES of a complex turbulent flame. Both of the models are described in the context of a variable density flow via consideration of the filtered mass density function (FMDF).Presented at the Workshop on Quality Assessment of Unsteady Methods for Turbulent Combustion, Darmstadt, Germany, June 16–17, 2005.     

Non-Equilibrium Mixing of Turbulence Scales Using a Continuous Hybrid RANS/LES Approach: Application to the Shearless Mixing Layer

A new subgrid-scale (SGS) model based on partially integrated transport method (PITM) is applied to the case of a turbulent spectral non-equilibrium flow created by the mixing of two turbulence fields of differing scales: the shearless mixing layer. The method can be viewed as a continuous hybrid RANS/LES approach. In this model the SGS length scale is no longer given by the size of the discretization step, but is dynamically estimated using an additional transport equation for the dissipation rate. The results are compared to those corresponding to the classical model of Smagorinsky and to the experimental data of Veeravalli and Warhaft. A method for creating an anisotropic analytical pseudo-random field for inflow conditions is also proposed. This approach based on subgrid-scale transport modelling combined with anisotropic inlet conditions gives better results for the prediction of the shearless mixing layer.     

Vortex-In-Cell and Probability Density Function Approach for a Passive Scalar Field in a Mixing Layer

A Reynolds-averaged simulation based on the vortex-in-cell (VIC) and the transport equation for the probability density function (PDF) of a scalar has been developed to predict the passive scalar field in a two-dimensional spatially growing mixing layer. The VIC computes the instantaneous velocity and vorticity fields. Then the mean-flow properties, i.e. the mean velocity, the root-mean-square (rms) longitudinal and lateral velocity fluctuations, the Reynolds shear stress, and the rms vorticity fluctuations are computed and used as input to the PDF equation. The PDF transport equation is solved using the Monte Carlo technique. The convection term uses the mean velocities from the VIC. The turbulent diffusion term is modeled using the gradient transport model, in which the eddy diffusivity, computed via the Boussinesq's postulate, uses the Reynolds shear stress and gradients of mean velocities from the VIC. The molecular mixing term is closed by the modified Curl model.

The computational results were compared with two-dimensional experimental results for passive scalar. The predicted turbulent flow characteristics, i.e. mean velocity and rms longitudinal fluctuations in the self-preserving region, show good agreement with the experimental measurements. The profiles of the mean scalar and the rms scalar fluctuations are also in reasonable agreement with the experimental measurements. Comparison between the mean scalar and the mean velocity profiles shows that the scalar mixing region extends further into the free stream than does the momentum mixing region, indicating enhanced transport of scalar over momentum. The rms scalar profiles exhibit an asymmetry relative to the concentration centerline, and indicate that the fluid on the high-speed side mixes at a faster rate than the fluid on the low-speed side. The asymmetry is due to the asymmetry in the mixing frequency cross-stream profiles. Also, the PDFs have peaks biased toward the high-speed side.     

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.     

Experimental 3D Analysis of the Large Scale Behaviour of a Plane Turbulent Mixing Layer

A new structure education method is proposed for representing the three dimensional large scale structures from hot wires measurement data. Due to the limited number of hot wire probes that can be used simultaneously, we firstly have to optimise the spatial location of sensors in order to detect large scale coherent structures. For such a purpose a detailed analysis of the 3D space-time flow organisation is given with particular emphasis on the full two point correlation tensor of the velocity field. Secondly, several procedures are developed in order to reconstruct instantaneous 3D velocity field on any finer and larger mesh than the experimental one from the limited available experimental information. The particular case of a spatially developing turbulent plane mixing layer is investigated. We then show that Stochastic Estimation and Proper Orthogonal Decomposition lead to a satisfactory reconstruction preserving the 3D coherent character of the flow. Finally, this reconstructed velocity field provides a 3D space-time representation of the large scale structures in a streamwise section of the flow and can then be used as realistic turbulent inflow condition for Large Eddy Simulations.     

后台阶流动再附着过程的大涡模拟研究

应用自主开发的大涡模拟程序数值模拟研究了后台阶流动中再附着过程的演变。在流动几何参数不变情况下，给出了再附长度随雷诺数的变化规律，并与实验进行了比较，二者相符得比较好。在此基础上，给出了三种典型雷诺数下，后台阶流动的回流区特征。在湍流情况下，研究了突扩比对再附长度的影响，与实验结果吻合的比较好。详细讨论了湍流情况下大涡拟序结构的瞬时再附着过程。 这些研究结果对具有再附着现象的流动结构的工程应用具有指导意义。     

Study of a Filtered Flamelet Formulation for Large Eddy Simulation of Premixed Turbulent Flames

Despite significant advances in the understanding and modelling of turbulent combustion, no general model has been proposed for simulating flames in industrial combustion devices. Recently, the increase in computational possibilities has raised the hope of directly solving the large turbulent scales using large eddy simulation (LES) and capturing the important time-dependant phenomena. However, the chemical reactions involved in combustion occur at very small scales and the modelling of turbulent combustion processes is still required within the LES framework. In the present paper, a recently presented model for the LES of turbulent premixed flames is presented, analysed and discussed. The flamelet hypothesis is used to derive a filtered source term for the filtered progress variable equation. The model ensures proper flame propagation. The effect of subgrid scale (SGS) turbulence on the flame is modelled through the flame-wrinkling factor. The present modelling of the source term is successfully tested against filtered direct numerical simulation (DNS) data of a V-shape flame. Further, a premixed turbulent flame, stabilised behind an expansion, is simulated. The predictions agree well with the available experimental data, showing the capabilities of the model for performing accurate simulations of unsteady premixed flames.     

后台阶分离流动中大涡结构演变的数值模拟

本文对后台阶分离流动中涡结构的演变进行了大涡模拟，研究了流场结构的变化规律。详细讨论了随着雷诺数的增加流场结构的典型特征的变化规律，指出流场中的涡结构随着雷诺数的增大变得十分复杂和丰富，回流区的数目、大小及其出现的位置也显著地不同。这些结果与已有的一些实验值和流场显示结果是吻合的。在此基础上，进一步研究了高雷诺数时流场中大尺度涡结构的瞬时发展和演化过程，展示了其中大涡的产生、追随、吸引、合并和破碎等过程。对于高雷诺数情况，对大涡模拟得到的数值结果进行了统计，得到的时均速度分布以及台阶后方的回流区长度与现有的其他实验结果符合得很好。本研究是针对后台阶分离流动深入开展湍流控制以及两相流动研究的基础。     

带整流罩隔水管流场特性的大涡模拟研究

为了深入揭示整流罩抑制涡激振动的机理,采用CFD软件FLUENT,结合大涡模拟方法,分别对附加整流罩前后隔水管模型的绕流流场进行模拟,将附加整流罩前后隔水管周围的流场特性、升力系数、阻力系数及其频谱特性等进行对比分析.结果表明,整流罩能有效抑制隔水管尾流区的漩涡脱落,隔水管背部的压力升高,升力系数和阻力系数显著降低,从而抑制了隔水管的涡激振动.     

超声速来流横向狭缝喷流标量输运的混合RANS/LES模拟

采用基于时均湍流模型的混合RANS/LES模拟方法对超声速来流狭缝横向喷流标量输运过程进行大涡模拟,控制方程对流项采用五阶精度的WENO格式求解,过滤后的组分方程中的亚格子组分对流通量采用梯度扩散模化.模拟得到了超声速来流狭缝横向喷流大尺度涡和组分的演化过程,研究表明喷流具有明显的非定常周期性特点,并且组分随时间的空间分布特征决定于流动的大尺度结构.模拟得到的压力的统计时均结果和实验结果定性一致,但喷流前后的分离区较实验值过大,这表明混合RANS/LES需要进一步改进.     

Large Eddy Simulation of Turbulent Reacting Shear Layers Including Finite-Rate Chemistry and Detailed Diffusion Processes

Large eddy simulations (LES) of turbulent temporal shear layers with hydrogen chemistry are performed. In these simulations, approximate deconvolution is applied as an implicit subgrid-scale modeling approach to a reacting flow in combination with a steady flamelet model for the filtered heat release term. No additional heuristical or physical subgrid models are used. The formulation of the flamelet equations in physical space does not only allow to consider a detailed reaction scheme and the extinguished phase but also to take into account detailed diffusion mechanisms (Soret and Dufour effects, multicomponent diffusion coefficients). Two different levels of diffusion approximations are investigated in this work, the aim of which is twofold: Firstly, to verify approximate deconvolution as a tool for convective transport of mass, momentum and energy in gas flow, by comparing the LES results with those of a direct numerical simulation and secondly, to investigate the influence of detailed diffusion on the laminar flamelets and the LES results.     

Large Eddy Simulation of a plane impinging jet

Large Eddy Simulations of a plane turbulent impinging jet have been carried out using the dynamic Smagorinsky model. The statistical results are first validated with the measurements from the literature: mean and turbulent quantities along the jet axis and at different vertical locations are presented. This study is completed by the analysis of the wall shear stress at the impingement wall. The effect of the jet Reynolds number (3000Re13500) on the kinematic development of the jet is also discussed. To cite this article: F. Beaubert, S. Viazzo, C. R. Mecanique 330 (2002) 803–810.     

Large eddy simulation (2D) using diffusion–velocity method and vortex‐in‐cell

A large eddy Simulation based on the diffusion‐velocity method and the discrete vortex method is presented. The vorticity‐based and eddy viscosity type subgrid scale model simulating the enstrophy transfer between the large and small scale appears as a convective term in the diffusion‐velocity formulation. The methodology has been tested on a spatially growing mixing layer using the two‐dimensional vortex‐in‐cell method and the Smagorinsky subgrid scale model. The effects on the vorticity contours, momemtum thickness, mean streamwise velocity profiles, root‐mean‐square velocity and vorticity fluctuations and negative cross‐stream correlation are discussed. Comparison is made with experiment and numerical work where diffusion is simulated using random walk. Copyright © 2004 John Wiley & Sons, Ltd.     

Numerical prediction of isothermally reacting mixing layer using vortex‐in‐cell and filtered density function

A large eddy simulation based on the filtered vorticity transport equation and the filtered density function (FDF) transport equation developed in an earlier study is extended to predict a chemically reacting flow with no heat release. The filtered vorticity transport equation is solved using the vortex‐in‐cell scheme in conjunction with the dynamic eddy viscosity subgrid‐scale models. The transport equation for FDF is solved using the Lagrangian Monte‐Carlo method. The methodology is tested on a chemically reacting spatially growing mixing layer with no heat release. The effects of Damköhler number (Da) on the concentration structure of the reacting mixing layer, the mean reactant and product concentrations and on the reactant FDF are investigated. It is shown that mixing has a greater effect on scalar field within the vortex structure as compared with the braid regions. Also for high Da, the reaction zones are mainly limited to the thin reacting interfacial zones, i.e. the contact zone between the reactants, whereas for low Da, the reacting zones are spread as reacting pockets within the vortex structure. The effects of Da on mean reactant and product concentrations, root‐mean‐square concentration fluctuations and probability density are discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

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 Simulation of Scalar Mixing in a Coaxial Confined Jet

The highly turbulent flow occurring inside gas-turbine combustors requires accurate simulation of scalar mixing if CFD methods are to be used with confidence in design. This has motivated the present paper, which describes the implementation of a passive scalar transport equation into an LES code, including assessment/testing of alternative discretisation schemes to avoid over/undershoots and excessive smoothing. Both second order accurate TVD and higher order accurate DRP schemes are assessed. The best performance is displayed by a DRP method, but this is only true on fine meshes; it produces similar (or larger) errors to a TVD scheme on coarser meshes, and the TVD approach has been retained for LES applications. The unsteady scalar mixing performance of the LES code is validated against published DNS data for a slightly heated channel flow. Excellent agreement between the current LES predictions and DNS data is obtained, for both velocity and scalar statistics. Finally, the developed methodology is applied to scalar transport in a confined co-axial jet mixing flow, for which experimental data are available. Agreement with statistically averaged fields for both velocity and scalar, is demonstrated to be very good, and a considerable improvement over the standard eddy viscosity RANS approach. Illustrations are presented of predicted time-resolved information e.g. time histories, and scalar pdf predictions. The LES results are shown, even using a simple Smagorinsky SGS model, to predict (correctly) lower values of the turbulent Prandtl number in the free shear regions of the flow, compared to higher values in the wall-affected regions. The ability to predict turbulent Prandtl number variations (rather than input these as in combustor RANS CFD models) is an important and promising feature of the LES approach for combustor flow simulation since it is known to be important in determining combustor exit temperature traverse.     

On the Effect of Vortex Grid Density in the Vortex-in-cell Simulation of Mixing Layers

The two-dimensional vortex-in-cell method is used in the simulation of a spatially growing mixing layer at a high Reynolds number. Criteria, showing that the vortex grid density plays a role in obtaining a converged solution, are presented. Numerical experiments are conducted to test the criteria by varying the number of vortices and the grid sizes. The effects on the momentum thickness and on the peak values of the root-mean-square velocity fluctuations, and negative cross-stream correlation are discussed in terms of the vortex grid density.     

Large Eddy Simulation of a Planar Jet Flow with Nanoparticle Coagulation

Coagulation and growth of nanoparticles subject to large coherent structures in a planar jet has been explored by using large eddy simulation. The particle field is obtained by employing a moment method to approximate the nanoparticle general dynamic equation. An incompressible fluid containing particles of 1 nm in diameter is projected into a particle-free ambient. The results show that the coherent structures dominate the evolution of the nanoparticle number intensity diameter and polydispersity distributions as the jet develops. In addition, the coherent structures act to increase the diffusion of particles, and the vortex rolling-up makes the particles distributing more irregularly while the vortex pairing causes particle distributions to become uniform. As the jet travels downstream, the time-averaged particle number concentration becomes lower in the jet core and higher in the outskirts, whereas the time-averaged particle mass over the entire flow field maintains unaltered, and the time-averaged particle diameter and geometric standard deviations grow and reach their maximum on the interface of the jet region and the ambient.The project was supported by the National Natural Science Foundation of China (10372090) and the Doctoral Program of Higher Education of China (20030335001).The English text was polished by Yunming Chen.