A new two-scale model for large eddy simulation of wall-bounded flows

A new hybrid approach to model high Reynolds number wall-bounded turbulent flows is developed based on coupling a two-level simulation (TLS) approach (Kemenov and Menon, 2006 [1], 2007 [2] in the inner region with conventional large eddy simulation (LES) away from the wall. This new approach is significantly different from previous near-wall approaches for LES. In this hybrid TLS–LES approach, a very fine small-scale (SS) mesh is embedded inside the coarse LES mesh. The SS equations capture fine-scale temporal and spatial variations in all three Cartesian directions for all three velocity components near the wall. The TLS–LES equations are derived using a new scale separation operator that allows a smooth transition between the two regions, with the equations in the transition region obtained by blending the TLS large-scale and LES equations. New terms in the hybrid region are identified. The TLS–LES approach is used to study the near-wall features in canonical turbulent channel flows for a range of Reynolds number using relatively coarse large-scale (LS) grids. Results show that the TLS–LES approach is able to capture the effect of both the LS and SS features in the wall region consistently for the range of simulated Reynolds number.     

Computation of unsteady turbomachinery flows: Part 2—LES and hybrids

The choice of turbulence model can have a strong impact on results for many turbomachinery zones. Palliative corrections to them and also transition modeling can have a further profound solution impact. The spectral gaps necessary for theoretically valid URANS solutions are also lacking in certain turbomachinery zones. Large Eddy Simulation (LES) alleviates the serious area of turbulence modeling uncertainty but with an extreme increase in computational cost. However, there seems a lack of validation data to explore in depth the performance of LES and thus strategies to refine it. LES best practices are needed. Although LES is, obviously, much less model dependent than RANS, grids currently used for more practical simulations are clearly insufficiently fine for the LES model and numerical schemes not to be playing an excessively strong role. Very few turbomachinery simulations make use of properly constructed, correlated turbulence inflow. Even if this is attempted, most measurement sets are incomplete and lack an adequate basis for modeling this inflow. Gas turbines are highly complex coupled systems and hence inflow and outflow boundary condition specification needs to go beyond just synthesizing turbulent structures and preventing their reflection.Despite the strong limitations of the dissipative Smagorinsky model, it still sees the most wide spread use, generally, in excessively dissipative flow solvers. Monotone Integrated LES (MILES) related approaches, hybrid LES–RANS and more advanced LES models seem to have an equal but subservient frequency of use in turbomachinery applications. Clearly the introduction of a RANS layer can have a substantial accuracy penalty. However, it does allow LES to be rationally used, albeit in a diluted sense for industrial applications. The Reynolds numbers found in turbomachinery are substantial. However, in certain areas evidence suggests they will not be enough to ensure a long inertial subrange and hence the use of standard LES modeling practices.Despite the excessively coarse grids used in much of the LES work reviewed, with essentially RANS based codes, meaningful results are often gained. This can perhaps be attributed to the choice of cases, these being ones for which RANS modeling gives extremely poor performance. It is a concern that for practical turbomachinery LES studies grid densities used tend to have an Reynolds number scaling to a strong negative power.     

Fourier伪谱方法在不可压缩平板边界层研究中的应用

以三维不可压缩平板边界层为研究对象,扰动形式N-S方程为控制方程,从空间模式的角度,直接数值模拟了三维不稳定T-S波传播的过程.时间离散采用三阶精度混合显隐分裂格式,空间离散则结合Fourier伪谱方法及高精度紧致有限差分逼近,法向采用非等间距网格坐标变换,出口边界条件采用嵌边函数法,程序采用MPI(Message passing interface)并行方法编写.实例验证,该方法计算结果与流动稳定性分析的结果一致.      

Wall-layer models for large-eddy simulations

The numerical simulation of high Reynolds number flows is hampered by model accuracy if the Reynolds-averaged Navier–Stokes (RANS) equations are used, and by computational cost if direct or large-eddy simulations (LES) that resolve the near-wall layer are employed. The cost of a calculation scales like the Reynolds number to the power 3 for direct numerical simulations, or 2.4 for LES, making the resolution of the wall layer at high Reynolds number infeasible even with the most advanced computers. In LES, an attractive alternative to compute high-Re flows is the use of wall-layer models, in which only the outer layer is resolved, while the near-wall region is modeled. Three broad classes of approaches are presently used: bypassing this region altogether using wall functions, solving a separate set of equations in the near-wall region, weakly coupled to the outer flow, or simulating the near-wall region in a global, Reynolds-averaged, sense. These approaches are discussed and their ranges of applicability are highlighted. Various unresolved issues in wall-layer modeling are presented.     

更准确、更精确、更高效——高超声速流动数值模拟研究进展

从准度、精度和效率3方面回顾了近几十年来高超声速流动数值模拟研究的进展。在物理模型方面,介绍了高超声速数值模拟中高温气体效应、稀薄气体效应以及湍流效应的建模与模拟,基于雷诺平均Navier-Stokes(RANS)方程重点对现阶段较为关注的高超声速边界层转捩的模式理论研究进行了介绍。在空间离散算法方面,主要介绍了高超声速数值模拟中常用的二阶精度迎风格式以及高阶精度格式的发展及其应用。在时间推进方面,主要回顾了隐式时间推进方法的发展及其应用。在误差和不确定度估计方面,主要介绍了其概念、来源以及常用的分析方法,同时给出了迭代误差估计、Richardson外插法以及敏感性导数方法等初步研究结果。最后,讨论了高超声速流动数值模拟中下一步需关注的问题。     

湍流大涡数值模拟进展

本文简要陈述湍流大涡数值模拟的原理、优点,着重讨论湍流大涡数值模拟方法的关键问题及其可能解决的途径,包括脉动的过滤、亚格子模型、近壁模型和标量湍流的大涡数值模拟中的特殊问题.文章强调大涡数值模拟中亚格子应力的本质是可解尺度湍流和不可解尺度湍流动量间的输运,并以作者最近提出的新型亚格子模型说明发展亚格子模型的正确途径.文章最后提出湍流大涡数值模拟近期需要迫切解决的问题和其他具有挑战性的方向.     

Towards the use of large eddy simulation in engineering

This paper reviews some important aspects of Large Eddy Simulation (LES) as applied to engineering flows. We first summarize the present status of modeling in incompressible, compressible and reacting multi-phase flows, with a view towards the overall formalism instead of the intrinsic details of different subgrid models. On the basis of the assumed requirements on future LES, expected to handle full-scale flows and reacting flows with detailed chemistry, we discuss some potentially interesting LES methods for the future. These methods are exclusively based on multi-scale modeling, in which simplified equations are solved within each LES cell, instead of semi-empirical modeling based on the resolved flow scales only. After that we outline a few flows studied by LES at the Swedish Defense Research Agency—FOI, which form the basis for the subsequent discussion of validation and verification, and quality management, being of increasing importance for practical engineering flows. Next we summarize some practical aspects of LES of engineering applications, many of which being crucial to the successful use of LES, and being of increasing importance for engineering flows. Finally, a view of the future use of LES in engineering is presented, which is based primarily on the evolutionary use of LES during the last decade in the fields of hydrodynamics and combustion.     

基于人工神经网络的可压缩湍流大涡模拟模型

在国家数值风洞（NNW）工程项目的指导下，空间人工神经网络（SANN）模型被用于强可压缩湍流大涡模拟（LES）研究，其中流场的湍流马赫数分别为0.6、0.8、1.0。基于湍流的多尺度空间结构特性和人工神经网络方法发展的高精度空间神经网络（SANN）模型适用于不可压缩湍流和弱可压缩湍流。对于强可压缩湍流，流场中会出现激波结构，给大涡模拟带来了挑战。本文的研究结果表明：SANN模型适用于强可压缩湍流的大涡模拟。在先验分析中，SANN模型预测的亚格子应力和亚格子热流的相关系数超过0.995，远远高于梯度模型和近似反卷积模型等传统模型；传统模型的相对误差大于30%，而SANN模型在这方面有很大的改进，相对误差低于11%。在后验分析中，与隐式大涡模拟（ILES）、动态Smagorinsky模型（DSM）、动态混合模型（DMM）相比，SANN模型能更精确地预测能谱、各类湍流统计特性以及瞬态流动结构。因此，基于湍流多尺度空间结构特性的人工神经网络模型加深了人们对强可压缩湍流亚格子建模的认识，同时可以服务于NNW工程的流体力学模型构造。     

非均匀网格湍流大涡模拟高精度有限体积解法

为准确预测不可压复杂湍流,提出了一种可用于大涡模拟均匀或非均匀网格上的高精度有限体积法。该方法空间离散采用有限体四阶紧致格式,时间推进采用四阶Runge-Kutta法,压力-速度耦合应用四阶紧致格式的动量插值。通过直接求解顶盖驱动方腔流动证实了该方法具有近四阶的空间精度;并在此基础上,采用动态Smagor-insky亚格子应力模式,成功地实现了充分发展槽道湍流和后台阶湍流流动的大涡模拟计算,所得结果与直接数值模拟结果吻合良好,且采用非均匀网格可在比均匀网格数少的离散系统上得到同样满意的结果。结果表明,该方法是实现高精度湍流大涡数值模拟的一个有效途径。     

突扩燃烧室流场RANS与LES模拟

以中心突扩燃烧室为模拟对象,分别从旋涡运动、空间速度分布和时间速度脉动等方面对RANS模拟结果、LES模拟结果和试验结果进行了比较分析。表明RANS方法没有模拟出燃烧室中的旋涡运动,与试验结果相差甚远;而LES方法则比较好地模拟出了燃烧室中旋涡运动和速度脉动。得出LES方法是对突扩燃烧室内流场数值研究的有效方法。     

两种亚网格燃烧模型的旋流扩散燃烧大涡模拟

用二阶矩亚网格（SOM—SGS）燃烧模型和文献中的涡旋破碎亚网格（EBU—SGS）燃烧模型，对甲烷一空气旋流扩散燃烧进行了大涡模拟，将二者得到的LES统计平均温度分布和实验结果以及用二阶矩燃烧模型的统观模拟（RANS—SOM）结果比较，表明LES—SOM和RANS—SOM的模拟结果都和实验符合较好，而LES—EBU的模拟结果和实验不符合，在不同区域内高估或者低估了燃烧温度。其原因是由于EBU模型不能有效地考虑有限反应动力学的作用。LES—SOM模拟的瞬态结果显示了旋流扩散火焰的湍流大涡结构不同于射流火焰的特点。     

Identification of a 2D turbulent wind spectrum

Turbulent wind spectrum models usually involve nonrational terms. For such models the development of a Markovian time simulator relies on a rational approximation obtained by the way of an identification stage. This paper presents a general method to manage this identification stage, that provides us with a family of stable rational approximations which is proved to converge towards the true model as the dimension increases. We first give an exact but infinite dimensional state space representation of the spectrum. It is based on the use of a diffusive equation. For bi-dimensional (2D) signals it also uses decoupling by spatial Fourier transformation. Then the discretization of this exact model leads easily to stable finite dimensional approximations over a prescribed frequency range. This approach is applied to the identification of a theoretical 2D turbulent wind spectrum, and of a 1D turbulent wind spectrum estimated from in flight recorded data.     

Computation of unsteady turbomachinery flows: Part 1—Progress and challenges

There are numerous unsteady flow influences in turbomachinery. These can potentially make a substantial total impact on efficiency, and hence the environment and operating costs over the life of a gas turbine engine. These unsteadiness sources are reviewed. Also, the turbomachinery zones where unsteady modeling is mandatory for meaningful solutions is outlined. The various unsteady modeling hierarchies are reviewed. These range from linear harmonic to Direct Numerical Simulations (DNS). Unsteady reduced order modeling encompassing deterministic stresses and body forces are reviewed. Hierarchies are presented for different modeling lineages and fidelity levels. Mixed fidelity methods are proposed, where low and high fidelity treatments are combined. For example, Large Eddy Simulation (LES) and Unsteady Reynolds Averaged Simulations (URANS) being combined with body forces to provide appropriate system boundary conditions.A daunting array of modeling and numerical methods and strategies are found for the user to select. Each has their own theoretical limitations. Clearly a user must be aware of these. Reported performances of the different approaches are found to vary considerably between relatively similar applications. The reviewed work suggests that Computational Fluid Dynamics (CFD), as ever, is an activity that needs strong reviewing of processes, tools and overseeing of modeling practices. With regard to LES, grid densities used for typical complex geometry simulations currently appear to be too coarse. This reflects the lack of current computational performance and hence the need for reduced order models.     

Navier–Stokes analysis methods for turbulent jet flows with application to aircraft exhaust nozzles

This article presents the current status of computational fluid dynamics (CFD) methods as applied to the simulation of turbulent jet flowfields issuing from aircraft engine exhaust nozzles. For many years, Reynolds-averaged Navier–Stokes (RANS) methods have been used routinely to calculate such flows, including very complex nozzle configurations. RANS methods replace all turbulent fluid dynamic effects with a turbulence model. Such turbulence models have limitations for jets with significant three-dimensionality, compressibility, and high temperature streams. In contrast to the RANS approach, direct numerical simulation (DNS) methods calculate the entire turbulent energy spectrum by resolving all turbulent motion down to the Kolmogorov scale. Although this avoids the limitations associated with turbulence modeling, DNS methods will remain computationally impractical in the foreseeable future for all but the simplest configurations. Large-Eddy simulation (LES) methods, which directly calculate the large-scale turbulent structures and reserve modeling only for the smallest scales, have been pursued in recent years and may offer the best prospects for improving the fidelity of turbulent jet flow simulations. A related approach is the group of hybrid RANS/LES methods, where RANS is used to model the small-scale turbulence in wall boundary layers and LES is utilized in regions dominated by the large-scale jet mixing. The advantages, limitations, and applicability of each approach are discussed and recommendations for further research are presented.     

室火轰燃的大涡模拟

采用一套修正的适用于低马赫数浮力流的N-S方程组来描述火灾场,运用Smagorinsky亚格子模型的大涡模拟方法对室内池火的轰燃发展过程进行了数值模拟,把模拟与实验结果进行了对比和分析。结果表明,该方法能较好地预测轰燃。同时模拟结果显示火焰区向房间后部偏转稍大,近壁处温度预测的误差大一些,随着亚格子模型的不断完善和计算能力的提高,用该技术来预测轰燃现象会更加准确。     

一种改进的类DES湍流模拟方法

构造了一种基于一方程S-A(Spalart-Allmaras)模型和一方程Yoshizawa亚格子模型的混合RANS/LES(Reynolds-averaged Navier-Stokes/large eddy simulation)湍流模拟方法.在涡黏假设的基础上,将Yoshizawa亚格子湍动能方程转化为等效的亚格子湍流涡黏性输运方程,并采用混合函数将其与S-A模型方程进行混合,从而改进了DES(detached eddy simulation)模型的亚格子行为,同时克服了其依靠网格控制模型转换的缺点.模拟了超声速的带斜坡凹腔流动,并与相同网格下的LES及DES结果进行了比较,结果表明该混合RANS/LES方法在远离壁面的自由剪切流区域与LES行为一致,而在附着边界层区域表现优于LES和DES方法.      

Large eddy simulation of turbulent combustion processes in propulsion and power systems

Progress toward application of the large eddy simulation (LES) technique to turbulent multiphase combustion processes is presented with emphasis placed on propulsion and power systems. The primary objective is to provide a systematic analysis of the current state-of-the-art and assist in the development of technical performance metrics for model development and validation. Research is currently required to provide both improved multiphase combustion models and improved datasets for validation. Requirements for further model development must be established through detailed analyses of the space–time characteristics of small-scale flame structures and turbulence–chemistry interactions. Concurrently, a refined set of implementation requirements must be established. Steps taken towards these goals are described by first providing a generalized formulation of the filtered conservation equations using an arbitrary filter function that operates on both spatial and temporal scales, with no a priori assumptions made regarding the character of the multiphase fluid elements present in the system. The distinct requirements for LES are listed with a discussion that highlights current progress and unresolved issues. Two case studies are then presented that demonstrate the predictive capabilities of LES when implemented with the appropriate numerics and grid resolution, under highly controlled conditions, and with well-defined boundary conditions. The paper is concluded by highlighting recent findings associated with the International Workshop on Measurement and Computation of Turbulent Nonpremixed Flames.     

基于共轭方程法的跨声速机翼阻力优化设计

机翼的气动特性从根本上决定了飞机巡航状态下的气动特性,经过良好设计的机翼会明显地提升全机的气动特性。连续共轭方程方法由于其求解梯度的计算量与设计变量数目无关,可以对复杂构型进行有效的优化设计。本文采用连续共轭方程方法对粘性条件下的机翼气动外形进行了优化设计。采用有限体积法进行流动控制方程和共轭方程的数值空间离散,多步Runge-Kutta方法进行时间推进,对于某三维机翼进行了有效的气动优化设计。     

不同湍流预混燃烧模型在本生灯火焰中的比较

湍流预混燃烧的大涡模拟数值计算近来受到很大的关注,为了进一步发展湍流预混燃烧模型,利用正在发展的湍流预混燃烧模型等值面集合的G函数方法、层流预混火焰生成流型的化学热力学建表FGM方法、G函数/FGM耦合方法三种燃烧模型,对甲烷/空气湍流本生灯火焰进行了大涡模拟计算,并对上述燃烧模型进行了比较。大涡模拟结果表明,G函数方法能够较为准确捕捉火焰的传播特性,FGM方法能够捕捉到火焰和湍流的相互作用,但无法准确预测火焰的传播特性。而耦合的方法既可以捕捉火焰的传播特性,又能够捕捉到火焰和湍流的相互作用。