湍流边界层内准流向发卡涡生成的数值模拟

采用根据共振三波理论模型建立的初始准二维流场和直接数值模拟方法,对湍流边界层近壁区流向涡的生成进行了研究．计算得到了准流向发卡涡和次生准流向涡结构的生成过程及其主要特征,讨论了它们的产生机理．作为相干结构的主要特征,利用共振三波理论模型对流向涡结构生成与演化过程的研究为湍流边界层内相干结构的研究与流动控制提供了新的途径和思路．     

用子波谱分析壁湍流多尺度结构的能量传递

测量了壁湍流不同法向位置的流向速度.用子波变换研究湍流能谱,表明子波全局谱是Fourier谱按子波尺度加权平均的结果,高阶消失矩的子波变换能够表征湍谱的衰减特性;子波局部谱显示边界层内涡的变形、破碎等现象与边界层法向位置有关,含能涡结构呈多尺度分布,随着测点远离壁面,含能涡的尺度增大;在缓冲区出现更高频的小尺度含能涡,在外区能量集中在低频大尺度涡结构中,含能涡的频带变窄.     

直接数值模拟／大涡模拟中数值误差影响的研究

通过比较湍流的能谱和总动能,对数值误差(包括混淆误差、离散截断误差)、亚格子模型以及它们之间相互作用对直接数值模拟和大涡模拟的影响进行了系统研究.算例采用了三维各向均匀同性湍流.为了研究复杂几何形状,数值格式采用了谱方法和Pade紧致格式.大涡模型采用了truncated Navior-Stokes(TNS)模型结合Pade离散滤波器.结果表明直接数值模拟中离散误差对结果有很大影响,低阶格式会导致计算发散.而大涡模拟中亚格子模型不仅能表征小尺度对大尺度的影响,而且还缓解了数值误差对计算结果的影响.因而低精度格式也可取得不错的结果.     

太阳风中小尺度湍动磁重联的模拟研究 *

分析了Helios高分辨率数据 ,找到太阳风湍动中可能发生磁重联过程的观测事例 ,并将空气动力学中的三阶精度迎风紧致差分格式引入可压缩的二维磁流体动力学 (MHD)中 ,数值模拟研究行星际太阳风中这种小尺度湍动磁重联现象 .主要新结果是 :行星际太阳风中湍动磁重联过程可以发生 ;湍动磁重联是高磁Reynolds数 (RM=2 0 0 0 ,1 0 0 0 0 )太阳风中磁重联的基本特征 ,磁重联多由单X射线重联向多X射线重联发展 ;磁岛的不断形成、合并与演化十分复杂 ,最终将向低能态状态演化 ;不同演化阶段、不同穿越路径 ,将有很不相同的磁场、流场信号记录 .小尺度湍动磁重联的数值实验结果可从原理上解释飞船观测结果 ,有助于揭示太阳风基本物理过程.     

湍流边界层近壁区的传热问题

主要研究了湍流边界层近壁区的传热问题 .利用流动稳定性理论中的共振三波模型来求湍流边界层近壁区的相干结构所引起的流场 ,而小尺度湍流则用一简单的模型来表示 .通过这种新的模型 ,求解了三维温度场 ,得到了近壁区的平均温度剖面及表征热通量的Nusselt数 ,后者与实验符合得很好 .瞬时温度场具有条纹结构 ,也说明了数值模拟中所发现的条纹结构产生机理 .     

分层剪切湍流大涡模拟的一种动力学亚格子尺度模型

基于Yoshizawa涡粘性模型提出了一种适用于热分层剪切湍流大涡模拟的动力学亚格子尺度 (SGS)模型 ,包括SGS湍流应力和湍流热通量模型 ,计算验证了该模型的正确性 .进一步采用大涡模拟方法和该动力学SGS模型研究了稳定分层和不稳定分层槽道湍流的湍流特性 ,以及湍流主要变量关联量的变化规律 .同时 ,根据计算结果 ,预测了热分层槽道湍流的临界Richardson数 ,与理论分析结果基本相符 .     

可压缩均匀各向同性湍流的直接数值模拟

采用8阶精度的中心差分格式及7阶精度的迎风偏斜格式对Re_l = 72~153, M_t= 0.2~0.7的均匀各向同性湍流进行了直接数值模拟, 建立了湍流数据库. 与他人的计算结果吻合十分理想, 说明方法的有效性. 数值结果表明, 采用适当的迎风型差分格式可以克服起动问题(start-up problem)对湍流Mach数的限制, 提高可计算的湍流Mach数, 是可压湍流直接数值模拟的有效方法. 分析了压缩性效应对湍流统计量的影响, 发现压缩性使得湍动能的衰减加快. 探讨了可压湍流中微激波产生的机理, 对流场进行了标度律分析. 发现在本文的Reynolds数和湍流Mach数条件下, 流场中扩展自相似性仍然成立, 同时发现压缩性对标度指数影响不大.     

不可压缩Navier-Stokes方程数值模拟

建立不可压缩Navier-Stokes方程的Crank-Nicolson有限差分方法,数值模拟了在初始正弦波下的二维水槽内流体受到倾斜激励时流场和涡的演变机制.数值结果表明,初始波为正弦波时,流场内出现一个单涡,单涡下沉变成了两个小涡,两个小涡消失后流场内部出现三条规则的流场带,最后这三条流场带演变成一个尖涡,尖涡在周围流体的作用下演变成一个单涡,最后单涡在自由面消失,当耗散系数和Reynolds数增大时,流场和涡演化的周期变小.     

一种改进的分层剪切湍流亚格子尺度模型及其应用

提出了一种新型的分层剪切湍流应力和湍流热通量的动力学亚格子尺度(SGS)模型,并就热分层槽道湍流问题,计算验证了该模型的正确性.采用大涡模拟方法和该动力学亚格子尺度模型,数值研究了在浮力和剪切双重作用下的稳定分层和不稳定分层槽道热剪切湍流,探讨了某些湍流特性随Richardson数的变化规律,并分析了相关的流动物理机制.     

二维气固两相混合层中固粒对流场影响的研究

采用双向耦合模型对含有固粒的二维气固两相混合层流场进行了研究。流场采用拟谱方法直接数值模拟,固粒采用颗粒-轨道模型,在考虑流场对固粒作用的同时,考虑固粒对流场的反作用。结果发现固粒的浓度和Stokes数对流场影响明显。固粒的作用使涡量扩散加快,并阻碍流场的变化,减弱了流场中拟序结构的强度,缩短涡的生存期;固粒在流场中的分布规律与单相耦合所得结果相似。     

Numerical Solution of Transonic Flow in SE1050 Cascade

The SE1050 cascade is an open test case (QNET network) of plane turbine cascade measured at the IT ASCR wind tunnel. The two regimes with subsonic and supersonic outletMach number were selected for numerical simulation. Several numerical methods have been developed and also several turbulence models have been implemented. Comparison of computed results and experimental data gives us opportunity to discuss main features of transonic flow field in well designed turbine cascade, possibilities of its numerical capturing (grid quality, numerical viscosity, turbulence model, boundary layer transition) and its influence on prediction of energy losses. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Free vibration analysis of spatial Bernoulli–Euler and Rayleigh curved beams using isogeometric approach

This paper deals with the linear free vibration analysis of Bernoulli–Euler and Rayleigh curved beams using isogeometric approach. The geometry of the beam as well as the displacement field are defined using the NURBS basis functions which present the basic concept of the isogeometric analysis. A novel approach based on the fundamental relations of the differential geometry and Cauchy continuum beam model is presented and applied to derive the stiffness and consistent mass matrices of the corresponding spatial curved beam element. In the Bernoulli–Euler beam element only translational and torsional inertia are taken into account, while the Rayleigh beam element takes all inertial terms into consideration. Due to their formulation, isogeometric beam elements can be used for the dynamic analysis of spatial curved beams. Several illustrative examples have been chosen in order to check the convergence and accuracy of the proposed method. The results have been compared with the available data from the literature as well as with the finite element solutions.     

Small Oscillations of a Soap Bubble

We analyze the spectrum of small oscillations of a soap bubble surrounded by incompressible inviscid air. The analysis is based on the linearization of the exact system for the dynamics of free fluid films. The underlying model takes into account variations in the film's shape and thickness. The resulting dispersion relationship shows the fundamental interplay among these variations. In the limit of vanishing soap bubble mass, the presented dispersion relationship agrees with Rayleigh's classical formula for an oscillating liquid droplet under the influence of surface tension.     

Temporally regularized direct numerical simulation

Experience with fluid-flow simulation suggests that, in some instances, under-resolved direct numerical simulation (DNS), without a residual-stress model per se but with artificial damping of small scales to account for energy lost in the cascade from resolved to unresolved scales, may be as reliable as simulations based on more complex models of turbulence. One efficient and versatile manner to selectively damp under-resolved spatial scales is by a relaxation regularization, e.g. Stolz and Adams [S. Stolz, N.A. Adams, An approximate deconvolution procedure for large eddy simulation, Phys. Fluids II (1999) 1699-1701]. We consider the analogous approach based on time scales, time filtering and damping of under-resolved temporal features. The paper explores theoretical and practical aspects of temporally damped fluid-flow simulations. We prove existence of solutions to the resulting continuum model. We also establish the effect of the damping of under-resolved temporal features as the energy balance and dissipation and prove that the time fluctuations → 0 in a precise sense. The method is then demonstrated to obtain both steady-state and time-dependent coarse-grid solutions of the Navier-Stokes equations.     

Characterization of the Reynolds-stress and dissipation-rate decay and anisotropy from DNS of grid-generated turbulence

Grid–generated turbulence is a classical but still controversial topic, one open issue being the spatial decay rate of turbulent energy. We study the influence of grid geometry on the Reynolds–stress and dissipation–rate tensors, including the range and exponent of their self–similar spatial decay. DNS using a validated lattice Boltzmann code at mean–flow Reynolds numbers up to 1400 are performed, comparing square grids with blockage ratios from 0.05 to 0.49. A clear picture of spatial distribution and self–similarity emerges for the statistics of interest: Axisymmetry is excellently confirmed. A consistent power law decay is found in the self–similar decay region beyond 10 grid stride lengths downstream. Its exponent of –5/3 can be obtained, for weak turbulence, from a spatial flux balance reminiscent of the constant transport through the inertial range of isotropic turbulence. In the near–grid region, on the other hand, differences in Reynolds stress components are pronounced while those between dissipation tensor components are only recognizable very close to the grid, where a strong dependence on grid porosity is found. A normalization with respect to porosity is proposed that collapses the data from all runs. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Direct numerical simulation of turbulent non-Newtonian flow using OpenFOAM

Understanding transition and turbulence in the flow of shear-thinning non-Newtonian fluids remains substantially unresolved and additional research is required to develop better computational methods for wall-bounded turbulent flows of these fluids. Previous DNS studies of shear-thinning fluids mainly use purpose-built codes and simple geometries such as pipes and channels. However in practical application, the geometry of mixing vessels, pumps and other process equipment is far more complex, and more flexible computational methods are required. In this paper a general-purpose DNS approach for shear-thinning fluids is undertaken using the OpenFOAM CFD library. DNS of turbulent Newtonian and non-Newtonian flow in a pipe flow are conducted and the accuracy and efficiency of OpenFOAM are assessed against a validated high-order spectral element-Fourier DNS code – Semtex. The results show that OpenFOAM predicts the flow of shear-thinning fluids to be a little more transitional than the predictions from Semtex, with lower radial and azimuthal turbulence intensities and higher axial intensity. Despite this, the first and second order turbulence statistics differ by at most 16%, and usually much less. An assessment of the parallel scaling of OpenFOAM indicates that OpenFOAM scales very well for the CPUs from 8 to 512, but the intranode scalability is poor for less than 8CPUs. The present work shows that OpenFOAM can be used for DNS of shear-thinning fluids in the simple case of pipe flow, and suggests that more complex flows, where flow separation is often important, are likely to be simulated with accuracies that are acceptably good for engineering application.     

Analysis of turbulent diffusion in the temperature variance dissipation rate equation

Results of a new direct numerical simulation (DNS) for the Rayleigh‐Bénard convection at Prandtl number Pr = 0.025 and Rayleigh number Ra = 100,000 are used to analyse the turbulent diffusion term in the transport equation for the temperature variance dissipation rate. These DNS results are also used to investigate the performance of statistical models for this turbulent diffusion term. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Analysis of the cell boundary element methods for convection dominated convection-diffusion equations

We introduce two kinds of the cell boundary element (CBE) methods for convection dominated convection-diffusion equations: one is the CBE method with the exact bubble function and the other with inexact bubble functions. The main focus of this paper is on inexact bubble CBE methods. For inexact bubble CBE methods we introduce a family of numerical methods depending on two parameters, one for control of interior layers and the other for outflow boundary layers. Stability and convergence analysis are provided and numerical tests for inexact bubble CBEs with various choices of parameters are presented.     

反映强流动曲率效应的非线性湍流模型

首先定性地分析了流线曲率效应对流场湍流结构的影响,然后以U型槽道流为典型算例,对多种湍流模型进行了评估．评估的模型包括:线性涡粘性模型,二阶和三阶非线性涡粘性模型,二阶显式代数应力模型和Reynolds应力模型．评估结果表明,性能良好的三阶非线性涡粘性模型,如黄于宁等人发展的HM模型以及CLS模型,可以较好地描述流线的曲率效应对湍流结构的影响,如凸曲率作用下内壁附近湍流强度的衰减和凹曲率作用下外壁附近湍流的增强,并且较好地确定了管道下游的分离点位置和分离泡长度,其预测的结果和实验符合较好,与Reynolds力模型的结果十分接近,因此可以较好地应用于具有曲率效应的工程湍流的计算．     

Turbulent diffusion: Direct numerical simulation

Direct numerical simulations (DNS) of a vibrating grid turbulence at two different Reynolds numbers (based on an amplitude and frequency of the grid) Re = 500, 1000 are presented. The evolution of a turbulent/non-turbulent interface was detected and compared to the theoretical results obtained in the paper [1] using Lie group analysis of the governing equations. Good agreement was found between DNS and theory. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)