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。      

基于人工神经网络的湍流大涡模拟方法

大涡模拟方法(LES)是研究复杂湍流问题的重要工具,在航空航天、湍流燃烧、气动声学、大气边界层等众多工程领域中具有广泛的应用前景.大涡模拟方法采用粗网格计算大尺度上的湍流结构,并用亚格子(SGS)模型近似表达滤波尺度以下的流动结构对大尺度流场的作用.传统的亚格子模型由于只利用了单点流场信息和简单的函数关系,在先验验证中相对误差较大, 在后验验证中耗散过强. 近几年来,机器学习方法在湍流建模问题中得到了越来越多的应用.本文介绍了基于人工神经网络(ANN)的湍流亚格子模型的最新进展.详细地讨论了人工神经网络混合模型、空间人工神经网络模型和反卷积人工神经网络模型的构造方法.借助于人工神经网络强大的数据插值能力,新的亚格子模型的先验精度和后验精度均有显著提升. 在先验验证中,新模型所预测的亚格子应力的相关系数超过了0.99,在预测精度上远高于传统的大涡模拟模型. 在后验验证中,新模型对各类湍流统计量和瞬态流动结构的预测都优于隐式大涡模拟方法、动态Smagorinsky模型、动态混合模型等传统模型.因此, 人工神经网络方法在发展复杂湍流的先进大涡模拟模型中具有很大的潜力.      

基于人工神经网络的湍流大涡模拟方法

大涡模拟方法 (LES)是研究复杂湍流问题的重要工具,在航空航天、湍流燃烧、气动声学、大气边界层等众多工程领域中具有广泛的应用前景.大涡模拟方法采用粗网格计算大尺度上的湍流结构,并用亚格子(SGS)模型近似表达滤波尺度以下的流动结构对大尺度流场的作用.传统的亚格子模型由于只利用了单点流场信息和简单的函数关系,在先验验证中相对误差较大,在后验验证中耗散过强.近几年来,机器学习方法在湍流建模问题中得到了越来越多的应用.本文介绍了基于人工神经网络(ANN)的湍流亚格子模型的最新进展.详细地讨论了人工神经网络混合模型、空间人工神经网络模型和反卷积人工神经网络模型的构造方法.借助于人工神经网络强大的数据插值能力,新的亚格子模型的先验精度和后验精度均有显著提升.在先验验证中,新模型所预测的亚格子应力的相关系数超过了0.99,在预测精度上远高于传统的大涡模拟模型.在后验验证中,新模型对各类湍流统计量和瞬态流动结构的预测都优于隐式大涡模拟方法、动态Smagorinsky模型、动态混合模型等传统模型.因此,人工神经网络方法在发展复杂湍流的先进大涡模拟模型中具有很大的潜力.     

湍流燃烧数值模拟研究的综述

对湍流燃烧数值模拟的研究进行了综合评述,其中涉及直接数值模拟（ＤＮＳ）、大涡模拟（ＬＥＳ）、随机涡模拟、概率密度函数输运方程模拟、条件矩模型、简化概率密度函数模型、关联矩模型以及基于简单物理概念的一些唯象模型等几个重要方面．对全面了解湍流燃烧数值模拟的研究现状及前景提出了看法．     

流对波边界层湍流运动的影响

采用大涡模拟方法和Smagorinsky亚格子模型，求解三维Navier-Stokes方程，研究了波流边界层中的湍流特性．将大涡模拟结果与相应的直接数值模拟结果和实验数据进行比较，吻合较好．获得了不同波雷诺数，不同波流比情况下的大涡模拟数据库，并由此分析了波流边界层中各种湍流统计量，如速度廓线、剪应力、湍流强度等的变化规律．     

燃烧室中悬跨圆柱的火焰绕流

利用高速阴影照相系统,对方形管道内火焰流经悬吊圆柱时所产生的变化及其诱导的流场进行了实验研究,并利用高精度PPM格式对上述现象进行了数值模拟,其中,湍流采用大涡模拟(LES)方法,燃烧采用旋涡破碎(EBU)模型,圆柱边界采用沉浸边界法(IBM).实验结果与计算结果比较吻合,在一定程度上揭示了火焰阵面变形和加速的规律. 根据研究结果,对火焰与悬吊圆柱相互作用过程中火焰的三维形状、流场中涡量和湍流强度的分布、火焰与流场的相互影响以及火焰传播轨迹等进行了讨论.      

土体腐殖酸组分对水泥土强度影响效果试验

利用高速阴影照相系统,对方形管道内火焰流经悬吊圆柱时所产生的变化及其诱导的 流场进行了实验研究,并利用高精度PPM格式对上述现象进行了数值模拟,其中,湍流采用 大涡模拟(LES)方法,燃烧采用旋涡破碎(EBU)模型,圆柱边界采用沉浸边界法(IBM). 实验结果与计算结果比较吻合,在一定程度上揭示了火焰阵面变形和加速的规律. 根据研究 结果,对火焰与悬吊圆柱相互作用过程中火焰的三维形状、流场中涡量和湍流强度的分布、 火焰与流场的相互影响以及火焰传播轨迹等进行了讨论.     

大涡模拟的壁模型及其应用

大涡模拟是研究湍流的非定常特性的重要方法. 但解析壁面层的大涡模拟所需的计算量与直接数值模拟相当,是大涡模拟在高雷诺数壁湍流数值模拟中所面临的主要困难. 解析壁面层所需的网格尺度与壁面黏性长度同量级,是引起壁湍流大涡模拟计算量增加的主要原因. 壁模型通过模化近壁流动避免了完全解析壁面层,可以显著地降低壁湍流大涡模拟的计算量,是克服上述困难的有效方法. 本文介绍了大涡模拟壁模型的主要类型;详细讨论了常用的壁面应力模型,特别是平衡层模型和双层模型的构建思路和特点;基于近壁流动的特征讨论了应力边界条件的必要性和适用性;指出了壁面应力模型的局限性以及考虑非平衡效应修正的各种方法;讨论了壁面应力模型的研究历史、最新进展和发展趋势,给出了常用的壁面应力模型的分支与发展关系图;并基于Werner-Wengle模型实现了周期山状流的大涡模拟.      

燃烧室两相流场亚网格燃烧模型的研究

在三维任意曲线坐标系下采用不同的亚网格燃烧模型对环形燃烧室火焰筒气液两相湍流瞬态反应流进行大涡模拟．计算中所采用的数学度模型有：k方程亚网格尺度模型估算亚网格湍流黏性；热通量辐射模型估算辐射换热，分别采用亚网格EBU燃烧模型（E-A model）、亚网格二阶矩输运方程模型（SOM）和亚网格二阶矩代数模型（SOM-A）估算化学反应速率．并在非交错网格系统下气相采用SIMPLE算法和混合差分格式求解，液相采用Lagrange处理，并用PSIC算法对其进行求解．通过实验结果和计算结果的比较，表明在三维任意曲线坐标系下对燃烧室火焰简两相湍流油雾燃烧流场进行大涡模拟，3种不同的亚网格燃烧模型都能真实反映两相湍流化学反应流流动及实际燃烧过程，而采用亚网格二阶矩输运方程模型稍优于其他两种亚网格燃烧模型．     

大涡模拟的壁模型及其应用

大涡模拟是研究湍流的非定常特性的重要方法.但解析壁面层的大涡模拟所需的计算量与直接数值模拟相当,是大涡模拟在高雷诺数壁湍流数值模拟中所面临的主要困难.解析壁面层所需的网格尺度与壁面黏性长度同量级,是引起壁湍流大涡模拟计算量增加的主要原因.壁模型通过模化近壁流动避免了完全解析壁面层,可以显著地降低壁湍流大涡模拟的计算量,是克服上述困难的有效方法.本文介绍了大涡模拟壁模型的主要类型;详细讨论了常用的壁面应力模型,特别是平衡层模型和双层模型的构建思路和特点;基于近壁流动的特征讨论了应力边界条件的必要性和适用性;指出了壁面应力模型的局限性以及考虑非平衡效应修正的各种方法;讨论了壁面应力模型的研究历史、最新进展和发展趋势,给出了常用的壁面应力模型的分支与发展关系图;并基于Werner-Wengle模型实现了周期山状流的大涡模拟.     

Large eddy simulation of fuel injection and mixing process in a diesel engine

The large eddy simulation(LES) approach implemented in the KIVA-3V code and based on one-equation sub-grid turbulent kinetic energy model are employed for numerical computation of diesel sprays in a constant volume vessel and in a Caterpillar 3400 series diesel engine.Computational results are compared with those obtained by an RANS(RNG k-ε) model as well as with experimental data.The sensitivity of the LES results to mesh resolution is also discussed.The results show that LES generally provides flow and spray characteristics in better agreement with experimental data than RANS;and that small-scale random vortical structures of the in-cylinder turbulent spray field can be captured by LES.Furthermore,the penetrations of fuel droplets and vapors calculated by LES are larger than the RANS result,and the sub-grid turbulent kinetic energy and sub-grid turbulent viscosity provided by the LES model are evidently less than those calculated by the RANS model.Finally,it is found that the initial swirl significantly affects the spray penetration and the distribution of fuel vapor within the combustion chamber.     

Joint RANS/LES Approach to Premixed Flame Modelling in the Context of the TFC Combustion Model

We present an original timesaving joint RANS/LES approach to simulate turbulent premixed combustion. It is intended mainly for industrial applications where LES may not be practical. It is based on successive RANS/LES numerical modelling, where turbulent characteristics determined from RANS simulations are used in LES equations for estimation of the subgrid chemical source and viscosity. This approach has been developed using our TFC premixed combustion model, which is based on a generalization of the Kolmogorov’s ideas. We assume existence of small-scale statistically equilibrium structures not only of turbulence but also of the reaction zones. At the same time, non-equilibrium large-scale structures of reaction sheets and turbulent eddies are described statistically by model combustion and turbulence equations in RANS simulations or follow directly without modelling in LES. Assumption of small-scale equilibrium gives an opportunity to express the mean combustion rate (controlled by small-scale coupling of turbulence and chemistry) in the RANS and LES sub-problems in terms of integral or subgrid parameters of turbulence and the chemical time, i.e. the definition of the reaction rate is similar to that of the mean dissipation rate in turbulence models where it is expressed in terms of integral or subgrid turbulent parameters. Our approach therefore renders compatible the combustion and turbulent parts of the RANS and LES sub-problems and yields reasonable agreement between the RANS and averaged LES results. Combining RANS simulations of averaged fields with LES method (and especially coupled and acoustic codes) for simulation of corresponding nonstationary process (and unsteady combustion regimes) is a promising strategy for industrial applications. In this work we present results of simulations carried out employing the joint RANS/LES approach for three examples: High velocity premixed combustion in a channel, combustion in the shear flow behind an obstacle and the impinging flame (a premixed flame attached to an obstacle).     

On the influences of key modelling constants of large eddy simulations for large-scale compartment fires predictions

An in-house large eddy simulation (LES) based fire field model has been developed for large-scale compartment fire simulations. The model incorporates four major components, including subgrid-scale turbulence, combustion, soot and radiation models which are fully coupled. It is designed to simulate the temporal and fluid dynamical effects of turbulent reaction flow for non-premixed diffusion flame. Parametric studies were performed based on a large-scale fire experiment carried out in a 39-m long test hall facility. Several turbulent Prandtl and Schmidt numbers ranging from 0.2 to 0.5, and Smagorinsky constants ranging from 0.18 to 0.23 were investigated. It was found that the temperature and flow field predictions were most accurate with turbulent Prandtl and Schmidt numbers of 0.3, respectively, and a Smagorinsky constant of 0.2 applied. In addition, by utilising a set of numerically verified key modelling parameters, the smoke filling process was successfully captured by the present LES model.     

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.     

On the Evolution of the Flow Field in a Spark Ignition Engine

The development of the turbulent flow field inside a spark ignition engine is examined by large-eddy simulation (LES), from the intake flow to the tumble break-down. Ten consecutive cold flow engine cycles on a coarse and twenty cycles on a fine grid are simulated and compared to experiments of the same engine. The turbulent subgrid scales are modeled by the standard Smagorinsky and by the recently developed Sigma model. A comparison of the intake flow is made against Particle Image Velocimetry (PIV) measurements along horizontal and vertical lines and to an LES simulation performed by the Darmstadt group. Furthermore, we show the first LES comparison to Magnetic Resonance Velocimetry (MRV conducted by Freudenhammer et al.) measurements, which provided the 3D flow field inside a full scale dummy of the entire upper cylinder head including the valve seat region, at a time which mimics inflow conditions of the corresponding engine. Our LES is in good qualitative and quantitative agreement with the simulation and the experiments, with the notable exception of the measured in-cylinder pressure, which is discussed in detail and compared to 0D simulations and simulations from other groups. A criterion is proposed for estimating the number of cycles needed in a simulation, if experimental data is available. We put emphasis on the flow in the valve seat region, where turbulence is generated, and discuss the formation of the large scale tumble motion, including a comparison of the radial velocity fields on rolled-up planes around the valve seat. Here, spots of high velocities were found in the under flow region, which cannot been seen by the ensemble averaged MRV measurement. Within the compression stroke, a 2D vortex center identification algorithm is applied on slices inside the combustion chamber, yielding a 3D visualization of the tumble vortex, which is found to have a “croissant-like” shape. The tumble vortex trajectory is plotted on the symmetry plane and compared to measurements. Finally, we consider a modified definition of the (turbulent) integral length scale that provided further insight to the tumble break-down process.     

Subgrid Modeling of Turbulent Premixed Flames in the Flamelet Regime

Large eddy simulation (LES) models for flamelet combustion are analyzed by simulating premixed flames in turbulent stagnation zones. ALES approach based on subgrid implementation of the linear eddy model(LEM) is compared with a more conventional approach based on the estimation of the turbulent burning rate. The effects of subgrid turbulence are modeled within the subgrid domain in the LEM-LES approach and the advection (transport between LES cells) of scalars is modeled using a volume-of-fluid (VOF) Lagrangian front tracking scheme. The ability of the VOF scheme to track the flame as a thin front on the LES grid is demonstrated. The combined LEM-LES methodology is shown to be well suited for modeling premixed flamelet combustion. The geometric characteristics of the flame surfaces, their effects on resolved fluid motion and flame-turbulence interactions are well predicted by the LEM-LES approach. It is established here that local laminar propagation of the flamelets needs to be resolved in addition to the accurate estimation of the turbulent reaction rate. Some key differences between LEM-LES and the conventional approach(es) are also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Chemical kinetics modeling and LES combustion model effects on a perfectly premixed burner

Chemical kinetics modeling and coupling with turbulent combustion models for compressible Large Eddy Simulations (LES) is a critical issue. Accurate flow predictions can only be guaranteed if the coupling is well mastered. In a first attempt to qualify the effect of each model, the case of a lean premixed swirled combustor with comprehensive measures is targeted (species mass fractions and temperature fields). For the investigation, two turbulent combustion models are considered. The first model relies on a presumed PDF approach coupled to a look-up chemistry table obtained with a reduced chemical scheme. The second model makes use of the thickened flame approach using the same reduced chemical scheme but with reaction rates computed explicitly as the computation advances. Then, to estimate kinetic schemes reduction effects, the first model is compared to a third one, with the same PDF approach, but coupled to a look-up chemistry table obtained with a complete chemical scheme. All LES are very close to each other. The main difference between the different predictions relies on CO mass fractions. Although they are all able to return good outlet mass fractions, CO values inside the flame are different depending on the model used. To cite this article: G. Albouze et al., C. R. Mecanique 337 (2009).