大涡模拟在内燃机中应用的研究进展

大涡模拟 能够以比较合理的计算成本, 提供更多详细的湍流信息, 故近年来已经广泛地应用于科学和工程领域, 并也成为内燃机缸内湍流流动与燃烧过程模拟计算的最有潜力的数值方法.综合现有研究成果, 对内燃机中大涡模拟的研究进展和模拟方法进行了比较全面的评述.介绍了大涡模拟的基本概念、方法、亚网格模型,着重讨论了内燃机缸内冷态流场、燃油喷雾过程以及两相液雾湍流燃烧大涡模拟的国内外研究进展.最后论述了大涡模拟在内燃机应用中当前需要解决的问题及其发展趋势.     

基于Navier-Stokes方程残差的隐式大涡模拟有限元模型

对应于湍流的大尺度与小尺度流场信息,本文在有限元的框架下,假设Navier-Stokes方程的解的形函数由大尺度和不可解尺度形函数叠加组成,引入对应的权函数,将Navier-Stokes方程的有限元变分形式分解为大尺度和不可解尺度系统.根据不可解尺度系统,构建基于Navier-Stokes大尺度方程残差的不可解尺度模型...     

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。     

飞机尾涡演变及快速预测的大涡模拟研究

多尺度旋涡现象广泛地存在于工程领域的湍流中. 研究湍流的重要方法是大涡模拟, 但大涡模拟中的过度耗散问题是曲面边界、多尺度旋涡模拟所面临的主要困难. 为了克服过度耗散的缺陷, 使大曲率曲面边界的多尺度涡流脉动特性预报更准确, 文章采用正则化变分多尺度模型(RVM)改进了壁面自适应涡黏模型(WALE)中的亚格子模型, 并采用C++语言在OpenFOAM开源平台实现了计算代码编译. 文章以经典圆柱黏性绕流作为曲面边界黏性绕流的代表, 讨论了改进的大涡模型(MWALE)中滤波算子对亚临界雷诺数($ Re = 4.0 times {10^4} $)圆柱绕流大规模、多尺度旋涡发放预测能力的影响. 数值计算结果表明: 改进的大涡模型对滤波算子阶数(n)敏感. 四阶滤波算子(n = 2)能准确地预报圆柱壁面的平均涡量及尾流瞬时涡量分布、再循环旋涡长度和速度剖面, 而二阶滤波算子(n = 1)无法准确地预测分离区和尾流区内脉动速度剖面, 因此在强压力梯度区四阶滤波算子预测的压力分布、脉动升阻力与实验结果吻合的更好, 能更准确地捕捉到多尺度旋涡结构.     

沙漠地貌演化过程的湍流大涡模拟研究

采用大涡模拟和混合粒径群体沙粒运动算法结合内置边界方法来模拟沙波纹与沙丘的演化和发展,在平坦沙区形成沙波纹的同时,在沙丘上同样也出现了沙波纹的结构,即沙波纹的发展出现了多尺度现象.此外,还分析了粒径对沙波纹与沙丘自身形态演化和发展的影响,以及沙丘对平坦沙区沙波纹形成发展的作用.     

旋转圆管湍流的大涡模拟数值研究

采用大涡模拟(LES)方法,并结合动力学亚格子尺度应力(SGS)模型,通过数值求解柱坐标系下的滤波Navier-Stokes方程,研究了绕管轴旋转圆管内的湍流流动特性. 为验证计算的可靠性,以及动力学SGS模型对于旋转湍流的适用性,将大涡模拟计算所得的结果,与相应的直接模拟(DNS)结果和实验数据进行了对比验证,吻合良好. 进一步对旋转圆管湍流的物理机理进行了探讨,研究了湍流特性随旋转速率的变化规律. 当旋转速率增加时,湍流流动有层流化的发展趋势. 基于湍动能变化的关系,分析了旋转效应对湍流脉动生成的抑制作用.     

大涡模拟的代数模型

本文提出了一种用于计算拟雷诺应力中各向同性及非均匀二部份的代数模型将这种代数模型用作大涡模拟中的湍流模型。该模型首先用于直槽道内的湍流运动的数值模拟。初步结果表明该模型是有希望的。     

低密度流体界面不稳定性大涡模拟

采用拉伸涡亚格子尺度应力模型对湍流输运中的亚格子作用项进行模式化处理,发展了适用于可压多介质黏性流动和湍流的大涡模拟方法和代码MVFT（multi-viscous flow and turbulence）。利用MVFT代码对低密度流体界面不稳定性及其诱发的湍流混合问题进行了数值模拟。详细分析了扰动界面的发展,流场中冲击波的传播、相互作用、湍流混合区边界的演化规律,以及流场瞬时密度和湍动能的分布和发展。数值模拟获得的界面演化图像和流场中波系结构与实验结果吻合较好。三维和二维模拟结果的比较显示,两者得到的扰动界面位置、波系及湍流混合区边界基本一致,只是后期的界面构型有所不同,这也正说明湍流具有强三维效应。     

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...     

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.      

Large eddy simulation of fully developed turbulent flow in a curved channel

In this paper large eddy simulation of the fully developed turbulent flow in a curved channel is carried out. The computational results are presented and compared with the experimental results of Eskinazi and Yeh^[1]. It is shown that the numerical results of the present LES are reliable and the influence of the curvature on the turbulence feature is correctly revealed.     

Discrete filters for large eddy simulation

This paper summarizes several results relative to discrete filters for subgrid‐scale (SGS) models based on a multi‐level filtering procedure. First, a theoretical study of discrete filters in physical space is performed. The analysis is done in the uniform one‐dimensional case, and is then extended to the general multi‐dimensional case for arbitrary structured and unstructured meshes. Some equivalence classes for the discrete filters are defined, based either on a differential approximation or the associated transfer function. Methods for the definition of discrete filters are proposed in the general case, including the approximation of continuous convolution filters. Second, the sensitivity of several SGS models with respect to the test filter is investigated. The selected models are: the dynamic Smagorinsky model, the mixed scale model (MSM), the selective MSM and the Liu–Meneveau–Katz (LMK) similarity model. Improved versions, which explicitly account for the spectral width of the test filter of the MSM and the LMK similarity model are proposed. The analysis, which reveals a significant influence of the test filter, is done through a priori testing on a 128³ field issued from the large eddy simulation (LES) of freely decaying homogeneous isotropic turbulence. Copyright © 1999 John Wiley & Sons, Ltd.     

THE EXAMINATION OF TURBULENCE MODELING WITH LES DATABASE

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Large eddy simulation of free surface shallow‐water flow

Shallow‐water flow with free surface frequently occurs in ambient water bodies, in which the horizontal scale of motion is generally two orders of magnitude greater than the water depth. To accurately predict this flow phenomenon in more detail, a three‐dimensional numerical model incorporating the method of large eddy simulation (LES) has been developed and assessed. The governing equations are split into three parts in the finite difference solution: advection, dispersion and propagation. The advection part is solved by the QUICKEST scheme. The dispersion part is solved by the central difference method and the propagation part is solved implicitly using the Gauss–Seidel iteration method. The model has been applied to free surface channel flow for which ample experimental data are available for verification. The inflow boundary condition for turbulence is generated by a spectral line processor. The computed results compare favourably with the experimental data and those results obtained by using a periodic boundary condition. The performance of the model is also assessed for the case in which anisotropic grids and filters with horizontal grid size of the order of the water depth are used for computational efficiency. The coarse horizontal grid was found to cause a significant reduction in the large‐scale turbulent motion generated by the bottom turbulence, and the turbulent motion is predominately described by the sub‐grid scale (SGS) terms. The use of the Smagorinsky model for SGS turbulence in this situation is found inappropriate. A parabolic mixing length model, which accounts for the filtered turbulence, is then proposed. The new model can reproduce more accurately the flow quantities. Copyright © 2000 John Wiley & Sons, Ltd.     

An immersed boundary method wall model for high‐Reynolds‐number channel flow over complex topography

High‐Reynolds‐number channel flows regularly encounter topographies composed of multiple length scales and that protrude into the boundary layer. Physically, the presence of immersed obstacles leads to increased velocity gradients, turbulence production, and manifestation of wakes. Considerable challenges are associated with numerically describing the presence of obstacles in channel flows. Common approaches include generation of a computational mesh that is uniquely designed for the flow and obstacle, the immersed boundary method, and terrain‐following coordinates. There are challenges and limitations associated with each of these techniques. Specification of boundary conditions representing the perimeter of solid obstacles is a primary challenge of the immersed boundary method. In this document, a simplistic canopy stress‐like wall model is used to impose boundary conditions. The model isolates aerodynamically relevant local frontal areas through evaluation of the gradient of the topographic height field. The gradient of the height field describes both the surface‐normal direction and the frontal area, making it ideal for detecting areas on which the flow impinges. The model is tested in numerical simulations of turbulent half‐channel flow over topographies with different obstacles affixed–right prisms, rectangular prisms, ellipsoidal mounds, and sinusoids. In all cases, the performance is strong relative to datasets presented in the literature. Results are finally presented for numerical simulation of flow over complex synthetic fractal‐like topography and a synthetic city. These results show interesting trends in how the turbulent multiscale flow field responds to multiscale topography. Copyright © 2012 John Wiley & Sons, Ltd.     

植被层湍流的大涡模拟

研究植被层湍流的大涡模拟,发展了一个TSF（transientstructurefunction）亚格于模式,尽可能真实地处理植被湍流这种既有强剪切,又有热对流的流动．我们建立了植被湍流数据库,并进行了较为详细的分析研究．湍流统计量如平均风速剖面、雷诺应力、湍流脉动能等等,与有关观测结果作了对比,符合较好．大涡模拟计算同样发现已由现场观测到的、在强对流情况时出现的温度场斜坡型有组织结构．     

Large eddy simulation of temporally developing juncture flows

Large eddy simulation (LES) results are reported for temporally developing solid–solid and solid–rigid-lid juncture flows. A MacCormack-type scheme that is second-order in time, and fourth-order in space for the convective terms and second-order in space for the viscous terms, is used. The simulations are obtained for a low subsonic Mach number. The subgrid-scale stresses (SGS) are modeled using the dynamic modeling procedure. The turbulent flow field generated on a flat-plate boundary layer is used to initialize the juncture flow simulations. The results of the flat-plate boundary layer simulations are validated with experimental and direct numerical simulations (DNS) data. In juncture flow simulations, the presence of an adjacent solid-wall/rigid-lid boundary altered the mean and the turbulent field, setting up gradients in the anisotropy of normal Reynolds stresses resulting in the formation of turbulence-induced secondary vortices. The relative size of these secondary vortices and the distribution of mean and turbulent quantities are in qualitative agreement with the experimental observations for the solid–solid juncture. The overall distribution of the mean and turbulence quantities showed close resemblance between the solid–solid and the solid–rigid-lid junctures; except for the absence of a second vortical region near the rigid-lid boundary. In agreement with the experimental observations, it was found that the normalized anisotropy term exhibited similarity when plotted against the distance from the boundary, regardless of the type of boundary, i.e. solid-wall or rigid-lid. The turbulent kinetic energy increased near the rigid-lid boundary. While the surface normal velocity fluctuations decreased to zero at the rigid-lid boundary, the other two velocity components showed an increase in their energy, which is also consistent with the experimental observations. © 1998 John Wiley & Sons, Ltd.     

适用于浸入边界法的大涡模拟紊流壁面模型

基于近壁定常剪切应力假设,提出了一种新的适用于浸入边界法的大涡模拟紊流壁面模型。通过引入壁面滑移速度,修正了线性速度剖面计算得到的壁面剪切应力,使之满足Werner-Wengle模型。将其应用于平板紊流和高Re数圆管紊流的数值模拟,对比采用和不采用壁面模型的结果得知,采用此模型的速度剖面与实验值吻合良好,验证了此模型的有效性。研究了不同欧拉/拉格朗日网格相对位置对结果的影响,证明了此模型具有较好的鲁棒性,以及可根据局部流动状态和网格精度自动开闭的特点。     

波浪破碎的一种混合湍流模拟模式

近岸波浪的变形与破碎,一方面影响水体标识码运输,另一方面对消波护岸具有指导意义.本文提出一种三维混合湍流模拟模式方法,将求解区域分为造波区、波浪传播区和消波区. 造波区采用层流模式,通过基于 Fluent 的二次开发的 UDF 方法在边界进行速度造波.这种方法在给定入口速度的条件标识码据已知波高进行 精准插值,从而控制水的体积分数. 在波浪传播区域,采用大涡标识码行模拟研究,在消波区,采用 RANS 模型并利用多孔介质消波法进行消波. 模标识码 VOF 方法捕捉波浪破碎过程中的自由面变化.本文对波高为 5.5cm 的规则波 (M1)、波高为 13.5cm 的 规则波 (M3)、有效标识码 7.75cm 的 TMA 谱单向不规则波 (U1) 和有效波高为 19cm 的 TMA 谱多向不规则波 (B5) 展开标识码研究,并与前人的相关标识码果作比较,各条件下模拟结果与实验结果吻合.模拟结果说明本文提出的模型能够准确模拟出波浪传播过程中的折射和标识码象,并且能够捕捉的波浪破碎过程中的自由面变化,为三维波浪的传播与破碎的数值模拟提供一种模拟方法.