溢流坝及水跃强紊流区流速场数值模拟

采用修正Ｋ－ε模型封闭Ｒｅｙｎｏｌｄｓ方程作为紊流控制方程，引入通度概念处理曲面不规则边界、用ＶＯＦ法追踪自由表面，对溢流坝及水跃强紊流区流速场进行数值模拟；并用毕托管测水跃流速，将数值模拟结果与实测结果进行了比较。     

半封闭狭缝湍流冲击射流的数值模拟

将Yakhot和Orszag提出的RNGk-ε模型推广应用于半封闭狭缝冲击射流场的数值模拟，以评价该模型对这种复杂湍流的预测能力。将计算得到的流场平均速度分布、湍流强度分布和流函数分布与标准k—ε模型的预测结果以及相应的实验数据进行了比较，结果表明：RNGk—ε模型的预测结果总体上要好于标准k—ε模型，但与实验值相比，所有预测结果都还存在不同程度的误差，尤其是近壁区和滞止点较远下游处的湍流强度分布。说明RNG模型虽然已在某些湍流的预测中取得了一定的成功，但要定量准确地预测冲击射流场，还必须针对其流动特征对模型加以改进。     

湍流加速火焰的三维数值模拟

火焰在设有障碍物的管内传播时会自身加速，并可能导致爆炸。本文基于湍流κ-ε模型和改进的EBU—Arrhenius反应模型，对该现象进行了三维空间的数值模拟。计算结果反映了障碍物、湍流和火焰之间相互作用的正反馈机理，描绘了火焰在管内加速传播的三维图像。     

薄板坯连铸结晶器中钢液三维流动的数值模拟

采用高 Re数的紊流 K- ε两方程模型 ,且结合壁面函数法对薄板坯连铸结晶器中钢液的紊流流动进行了有限元数值模拟。在数值模拟中 ,考虑了凝固壳厚度分布对流场的影响。在掌握了结晶器内紊流粘性系数分布规律的基础上 ,将流动域在空间进行了分区 ,采用分区选定有效粘性系数的当量层流模型数值模拟了三维紊流流动 ,结果表明 ,当量层流模型可有效的获得与原紊流时均场基本一致的结果 ,但计算效率显著提高 ,为今后工程上计算薄板坯结晶器中的三维流动和考虑流动影响的凝固过程分析提供了实用的方法 ,计算结果也有助于人们对薄板坯连铸结晶器中钢液三维紊流流动特点和与流动有关的板坯质量等问题的理解     

天然弯曲河流的三维数值模拟

对Cartesian坐标系下的RANS方程进行三维$\xi$-$\eta$-$\zeta$坐标变换，建 立了非正交三维曲线坐标下弯曲河流的标准$k$-$\varepsilon$湍流模型. 对自由水面 的模拟采用``改进的刚盖假定'，河床和岸壁阻力的模拟采用壁面函数方法. 模型通 过具有实验数据的实验室连续弯曲水槽进行验证，模拟的流速值与实验数 据吻合良好，将模型应用于天然连续弯曲河流的流场计算，给出了表层和底层流速 矢量场和11个断面二次环流矢量图，显示该模型具有模拟天然弯曲河流的能力.     

多级轴流压气机叶栅内三维紊流流场的数值模拟

对多级轴流压气机叶栅内三维紊流流场进行数值模拟,采用高精度高分辨率的三阶ENN格式以保证对激波的捕捉和对紊流特征的正确模拟,利用LU-SGS隐式解法提高了计算速度,从而构成了一种既准确又高效的多级跨声速轴流压气机紊流流动数值求解系统。重点研究了动静网格的交接方法及相应的动静交接面处理模型。计算了某带进口导叶的三级轴流压气机,计算结果与实验数据吻合较好。     

钝锥三维粘性绕流背风面分离的数值模拟

本文将作者在文献[1]中提出的方法推广应用于求解三维可压缩 N-S 方程和简化 N-S 方程,并对近似因式分解法应用于三维问题的稳定性进行了分析。指出,对二维问题原无条件稳定的格式,经近似因式分解后仍是无条件稳定的;对于三维问题,原无条件稳定的格式经普通近似因式分解后所得到的格式可能是不稳定的或条件稳定的。利用系数矩阵分裂法所得到的近似因式分解格式可仍是无条件稳定的,只要适当加大分裂后的系数反差即可。 文中给出了钝锥超音速三维粘性绕流结果。得到了背风面分离的流动图像,物面压力值与实验值吻合。     

进口形式对平流式沉淀池水力特性影响的数值模拟

数值求解气液两相流模型,辅以Realizable k-ε湍流模型,模拟了不同的进口形式对平流式沉淀池水力特性的影响.采用有限体积法离散流体运动的控制方程,采用压力隐式算子分裂(PISO)算法求解离散方程;利用VOF法追踪自由水面的位置.经过对模拟结果的分析发现:在平流式沉淀池的进口设计中,当进口布置在进水墙竖直方向中间...     

铸造充型流动过程的三维数值模拟

本文以ＳＯＬＡ－ＶＯＦ方法为基础,结合作者提出的三维自由表面边界速度确定方法,实现了铸造充型流动过程的三维数值模拟,模拟结果准确地提供了液体金属在充型过程中的流动形态、充型时间、流量等实用的参数．这表明,通过数值计算的手段,对用实验方法难以观测的铸造充型流动过程进行分析研究是有意义可行的,所得到的参数有助于铸造工艺的设计及优化     

射流元件控制通道流动的三维数值模拟

针对一种射流元件控制通道的复杂结构 ,采用分块对接技术和网格“融合”技术生成计算网格 ,并运用五步显式格式的 Runge-Kutta法和多重网格法对含全 N-S方程、RNG k-ε湍流模型和两层分区壁面模型的流动模型进行数值求解。通过对控制通道内部流动的数值模拟和流场特性分析 ,提出了改进方案     

Numerical Study of Turbulent Channel Flow over Surfaces with Variable Spanwise Heterogeneities: Topographically-driven Secondary Flows Affect Outer-layer Similarity of Turbulent Length Scales

Wall-bounded turbulent flows over surfaces with spanwise heterogeneous surface roughness – that is, spanwise-adjacent patches of relatively high and low roughness – exhibit mean flow phenomena entirely different to what would otherwise exist in the absence of spanwise heterogeneity. In the outer layer, mean counter-rotating rolls occupy the depth of the flow, and are positioned such that “upwelling” and “downwelling” occurs above the low and high roughness, respectively. It has been comprehensively shown that these secondary flows are Prandtl’s secondary flow of the second kind (Anderson et al., J. Fluid Mech. 768, 316–347 2015). This behaviour indicates that spanwise spacing, s _y , between adjacent patches of high and low roughness is, itself, a problem parameter; in this study, we have systematically assessed how s _y affects turbulence structure in high Reynolds number channel flows via two-point correlations. “High roughness” is imposed with streamwise-aligned pyramid elements with height, h, selected to be ≈ 5% of the channel half height, H. For \(s_{y}/H \gtrsim 1\), we find that the aforementioned domain-scale mean circulations exist and the surface may be regarded as a topography. For s _y /H ? 0.2, turbulence statistics show characteristics very similar to a homogeneous roughness and thus the surface may be regarded as a roughness. For 0.2 ? s _y /H ? 2, the spatial extent of the counter-rotating rolls is controlled by proximity to adjacent rows, and we define such surfaces as being intermediate. We refer to such surfaces as intermediate state.     

Numerical Simulations of High-Speed Turbulent Cavity Flows

Detached-Eddy Simulations (DES) of flows over clean and controlled cavities with and without doors are presented in this paper. Mach and Reynolds numbers (based on cavity length) were 0.85 and one million respectively. Spectral analyses showed that the DES computations were able to correctly predict the frequencies of the Rossiter modes for both uncontrolled and controlled cases. Flow visualisations revealed that the impact of the shear layer formed along the cavity on a slanted aft wall no longer creates a large source of acoustic noise. Therefore little acoustic propagation was seen up the cavity. This was confirmed by the analysis of the cavity wall forces, which showed that the oscillations of the shear layer were reduced when the wall was slanted. This aided in reducing the overall Sound Pressure Levels throughout the cavity and far-field. Comparisons of the flow-fields suggested that the addition of the doors also aided in stabilising the shear layer, which was also shown in the analysis of the wall forces. As a result, the addition of the doors was found to affect the clean cavity configuration significantly more than the controlled one.     

Shape Optimization for 3D Turbulent Flows Using Automatic Differentiation

Abstract

We present through this paper some new results with our approach for optimal shape design based on a CAD-free framework for shape and unstructured mesh deformations, automatic differentiation by program for the gradient computation and mesh adaption by metric control. Automatic differentiation allows for an easy, reliable and fast discrete adjoint computation. We managed to get the Jacobian of our Navier-Stokcs solver including a k-e turbulence model and wall-laws. The CAD-free framework is shown to be particularly convenient for optimization when the mesh connectivities and the shape discretization arc variable during optimization. Using these ingredients constrained optimization for turbulent transonic flows has been investigated in both 2 and 3D.     

Vorticities in a LES Model for 3D Periodic Turbulent Flows

This paper is devoted to the study of a LES model to simulate turbulent 3D periodic flow. We focus our attention on the vorticity equation derived from this LES model for small values of the numerical grid size δ. We obtain entropy inequalities for the sequence of corresponding vorticities and corresponding pressures independent of δ, provided the initial velocity u₀ is in L_x² while the initial vorticity ω₀ = ∇ × u₀ is in L_x¹. When δ tends to zero, we show convergence, in a distributional sense, of the corresponding equations for the vorticities to the classical 3D equation for the vorticity.     

An Adaptive Cartesian Mesh Based Method to Simulate Turbulent Flows of Multiple Rotating Surfaces

An efficient Cartesian cut-cell/level-set method based on a multiple grid approach to simulate turbulent turbomachinery flows is presented. The finite-volume approach in an unstructured hierarchical Cartesian setup with a sharp representation of the complex moving boundaries embedded into the computational domain, which are described by multiple level-sets, ensures a strict conservation of mass, momentum, and energy. Furthermore, an efficient kinematic motion level-set interface method for the rotation of embedded boundaries described by multiple level-set fields on a computational domain distributed over several processors is introduced. This method allows the simulation of multiple boundaries rotating relatively to each other in a fixed frame of reference. To demonstrate the efficiency of the numerical method and the quality of the computed findings the generic test problem of a rotating cylinder surrounded by a stationary hull and the flow over a ducted rotating axial fan with a stationary turbulence generating grid at the inflow are simulated. The computational results of the axial fan show a good agreement with the experimental data.     

Study of the Spectral Difference Numerical Dissipation for Turbulent Flows Using Unstructured Grids

In this paper, the numerical dissipation properties of the Spectral Difference (SD) method are studied in the context of vortex dominated flows and wall-bounded turbulence, using uniform and distorted grids. First, the validity of using the SD numerical dissipation as the only source of subgrid dissipation (the so-called Implicit-LES approach) is assessed on regular grids using various polynomial degrees (namely, p = 3, p = 4, p = 5) for the Taylor-Green vortex flow configuration at R e = 5 000. It is shown that the levels of numerical dissipation greatly depend on the order of accuracy chosen and, in turn, lead to an incorrect estimation of the viscous dissipation levels. The influence of grid distortion on the numerical dissipation is then assessed in the context of finite Reynolds number freely-decaying and wall-bounded turbulence. Tests involving different amplitudes of distortion show that highly skewed grids lead to the presence of small-scale, noisy structures, emphasizing the need of explicit subgrid modeling or regularization procedures when considering coarse, high-order SD computations on unstructured grids. Under-resolved, high-order computations of the turbulent channel flow at R e _τ = 1000 using highly-skewed grids are considered as well and present a qualitatively similar agreement to results obtained on a regular grid.     

RNGκ-ε模式在工程湍流数值计算中的应用

自Yakhot和Orszag提出RNG k-ε湍流模式以来,很多学者对其进行了修正和发展,并将其应用于某些实际湍流问题的数值模拟,取得了一些与实验近似一致的结果。本文主要从工程湍流计算的角度出发,结合作者的部分研究工作,对近年来RNG k-ε模式在湍流流动数值研究中的应用现状及进展情况进行了总结,指出了该模式存在的问题,并对其应用前景进行了展望。     

Numerical simulation of flows over 2D and 3D backward-facing inclined steps

The work deals with the numerical solution of incompressible turbulent flow in a channel with a backward-facing step having various inclination angles. Also, the inclination of upper wall is considered. The mathematical model is based on the Reynolds averaged Navier–Stokes equations. The governing equations are closed by the explicit algebraic Reynolds stress (EARSM) model according to Wallin and Johansson or by linear eddy viscosity models (SST, TNT k–ω). The numerical solution is carried out by the implicit finite-volume method based on the artificial compressibility and by the finite-element method amd both approaches compared. The numerical simulations use as reference the experimental data by Makiola and Driver and Seegmiller in large aspect ratio channels. In these cases, the results are obtained by 2D and 3D simulations. Further narrow channel PIV experimental data are used as reference for 3D simulations.     

Review of Coherent Structures in Turbulent Free Shear Flows and Their Possible Influence on Computational Methods

A review of selected experiments on coherent structures in turbulent shear flows is performed. Different experimental approaches (conditional averages, filtering techniques, wavelets, linear stochastic estimation and proper orthogonal decomposition, etc.) are illustrated and their links with computations (LES, DNS, SDM, etc.) is emphasized. It is particularly shown that some kind of universal behavior of the background turbulence can be retrieved from these various experimental methods. This revised version was published online in August 2006 with corrections to the Cover Date.     

Numerical Simulation of Coherent Structures over Plant Canopy

This paper reports large eddy simulations of the interaction between an atmospheric boundary layer and a canopy (representing a forest cover). The problem is studied for a homogeneous configuration representing the situation encountered above a continuous forest cover, as well as for a heterogeneous configuration representing the situation similar to an edge or a clearing in a forest. The numerical results reproduces correctly all the main characteristics of this flow as reported in the literature: the formation of a first generation of coherent structures aligned transversally with the wind flow direction, the reorganization and the deformation of these vortex tubes into horse-shoe structures. The results obtained when introducing a discontinuity in the canopy (reproducing a clearing or a fuel break in a forest), are compared with the experimental data collected in a wind tunnel; here, the results confirm the existence of a strong turbulence activity inside the canopy at a distance equal to 8 times the height of the canopy, referenced in the literature as the Enhance Gust Zone (EGZ) characterized by a local peak of the skewness factor.