A VORTICITY–STREAMFUNCTION FORMULATION FOR STEADY INCOMPRESSIBLE TWO-DIMENSIONAL FLOWS

A vorticity–streamfunction formulation for incompressible planar viscous flows is presented. The standard kinematic field equations are discretized using centred finite difference schemes and solved in a coupled way via a Newton-like linearization scheme. The linearized system of partial differential equations is handled through the restarting linear GMRES algorithm, preconditioned by means of an incomplete LU approximate factorization. The proposed solution technique constitutes a fast and robust algorithm for treating laminar flows at high Reynolds numbers. The pressure field is obtained at a subsequent step by solving a convection– diffusion equation in terms of the stagnation pressure, which presents certain advantages compared with the widely used static pressure Poisson equation. Results are shown for a wide variety of applications including internal and external flows.     

轴流式水轮机三维非定常湍流计算及压力脉动预测

水轮机的稳定性研究是目前工程上大型水轮机组流动分析的重要课题,本文以某水电站的大型轴流转桨式水轮机原型为基础建立计算模型,完成了从蜗壳进口到尾水管出口整个流道的非定常湍流计算,预测了固定导叶进口、转轮后、尾水管进口等多个位置的压力脉动,并利用FFT变换进行分析得到了压力脉动的频率和幅值,探讨了水轮机全流道内压力脉动产生和传播的原理．     

热声波数值模拟的虚假振荡研究

采用可压缩流动的SIMPLE算法对一维封闭空腔内由边界突然加热所引起的非稳态热声波进行了数值模拟,对流-扩散项采用了中心差分、一阶迎风差分、QUICK、及MIJSCL等不同格式。计算表明各种格式均存在不同程度的虚假振荡现象,其大小与热声波的强度及离散格式的形式等多种因素有关。这些结果对热声波的进一步研究及高效可靠的对流差分格式的开发具有重要意义。     

A control volume finite element method for three-dimensional three-phase flows

A novel control volume finite element method with adaptive anisotropic unstructured meshes is presented for three-dimensional three-phase flows with interfacial tension. The numerical framework consists of a mixed control volume and finite element formulation with a new P₁DG-P₂ elements (linear discontinuous velocity between elements and quadratic continuous pressure between elements). A “volume of fluid” type method is used for the interface capturing, which is based on compressive control volume advection and second-order finite element methods. A force-balanced continuum surface force model is employed for the interfacial tension on unstructured meshes. The interfacial tension coefficient decomposition method is also used to deal with interfacial tension pairings between different phases. Numerical examples of benchmark tests and the dynamics of three-dimensional three-phase rising bubble, and droplet impact are presented. The results are compared with the analytical solutions and previously published experimental data, demonstrating the capability of the present method.     

A discontinuous Galerkin immersed boundary solver for compressible flows: Adaptive local time stepping for artificial viscosity–based shock-capturing on cut cells

We present a higher-order cut cell immersed boundary method (IBM) for the simulation of high Mach number flows. As a novelty on a cut cell grid, we evaluate an adaptive local time stepping (LTS) scheme in combination with an artificial viscosity–based shock-capturing approach. The cut cell grid is optimized by a nonintrusive cell agglomeration strategy in order to avoid problems with small or ill-shaped cut cells. Our approach is based on a discontinuous Galerkin discretization of the compressible Euler equations, where the immersed boundary is implicitly defined by the zero isocontour of a level set function. In flow configurations with high Mach numbers, a numerical shock-capturing mechanism is crucial in order to prevent unphysical oscillations of the polynomial approximation in the vicinity of shocks. We achieve this by means of a viscous smoothing where the artificial viscosity follows from a modal decay sensor that has been adapted to the IBM. The problem of the severe time step restriction caused by the additional second-order diffusive term and small nonagglomerated cut cells is addressed by using an adaptive LTS algorithm. The robustness, stability, and accuracy of our approach are verified for several common test cases. Moreover, the results show that our approach lowers the computational costs drastically, especially for unsteady IBM problems with complex geometries.     

颗粒流的动力学模型和实验研究进展

首先叙述了颗粒流研究的背景和基本概念,接着概述了颗粒流动力学研究的三种基本数学力学模型:颗粒动理论(kinetic theory)模型、摩擦塑性模型和离散元模拟模型.介绍了它们的基本原理,并分析了各个模型的优缺点和适用范围.随之,对颗粒流的实验研究情况作了简要的介绍,列举了研究中几种常用的试验方法和观测手段,以及观测到的一些典型现象.最后,我们简述了颗粒流研究的主要困难,并提出了某些有待解决的研究课题.      

湍流的诱导及其对瓦斯爆炸过程中火焰和爆炸波的作用

在实验的基础上,研究了管内瓦斯爆炸过程中湍流的诱导及其对瓦斯爆炸过程中火焰和爆炸波的影响作用.研究结果表明,管道面积突变对瓦斯爆炸过程中湍流的产生具有重要影响.管道面积突变(变大、变小)时,产生附加湍流,并使下游火焰气流的湍流度增加,瓦斯爆炸过程中火焰的传播速度迅速提高,并可诱导激波的产生.在80×80mm等截面直管中(瓦斯浓度为理论上最猛烈的爆炸浓度9.5%),瓦斯爆炸最大火焰传播速度为40.8m/s,管内各点均为压力波信号,当管道加装一Φ300mm圆管形成面积突扩11倍和突缩11倍两断面后,面积突扩处(L/D=22)火焰速度增大5.05倍,达到64.4m/s,面积突缩处(L/D=28)火焰速度为156.0m/s, 增大4.55倍,并在L/D=48倍处形成激波(超压1.6976atm、波速416.7m/s),在L/D=98倍处,激波强度最大.在面积突变管内加装加速环可使瓦斯爆炸过程中湍流度加剧,火焰的传播速度更高,激波生成的位置(L/D=28)、最强点位置(L/D=70)均前移,激波强度增大.研究结果对指导现场如何防治瓦斯爆炸,减轻瓦斯爆炸的威力具有一定的指导意义.     

5.12汶川大地震诱发大型崩滑灾害动力特征初探

对乙炔氧气混合气体中爆轰波与激波的正面对撞现象的实验研究是以高速摄影获 取两波对撞的x-t纹影图,以烟迹板记录对撞中的爆轰胞格图案,并基于激波 理论和经典CJ爆轰理论求解了两波对撞的稳态解并探寻其规律. 研究发现透射波系包括一道激波和爆轰波, 以及紧随爆轰波后的稀疏波区,这一结果对应于一维理论分析中的CJ解. 透射波系基本不受 初始压强影响;初始温度也只成比例地改变流场整体速度,温度越高,速度越快;对波系起 实质影响作用的是入射激波强度,激波越强,则整个透射流场呈现偏向激波的趋势;理论分 析还指出,稀疏波区的出现不可避免,当激波强度趋于声波稀疏波区趋于消失,激波越强则 疏波区趋于扩大. 两波对撞存在一个有限的转变阶段,透射爆轰首先减缓,接着迅速迸发为 过驱爆轰,然后再逐渐平衡为CJ爆轰. 对于强不稳定的燃气,对撞后爆轰波在空间上的发展 极不均衡,一些区域发生火焰面与诱导激波的严重脱离,随后的火焰面失稳发展为诱导激波 区内的爆轰波,实验观察到了这种爆轰在烟迹板上留下的极为精细的迹线.     

高焓湍流边界层壁面摩阻产生机制分析

高超飞行器在中低空以极高马赫数飞行时,飞行器表面会遇到湍流与高温非平衡效应耦合作用的新问题.这种高焓湍流边界层壁面摩阻产生机制是新型高超声速飞行器所关注的基础科学问题,厘清此产生机制可以为减阻方法的设计提供指导,具有重要的工程实用价值.本文选取高超声速飞行时楔形体头部斜激波后的高焓流动状态,开展了考虑高温非平衡效应的湍流边界层直接数值模拟研究,并设置同等边界层参数下的低焓完全气体湍流边界层流动作为对比,采用RD (Renard&Deck)分解技术研究了高焓湍流边界层摩阻的主要产生机制,对摩阻产生的主要贡献项积分函数分布进行了详细分析,研究了高温非平衡效应对摩阻产生的影响规律;采用象限分析技术,研究了摩阻分解湍动能生成项的主导流动事件.计算结果表明,高温非平衡效应会使得壁面摩阻脉动条带的流向和展向尺寸均减小.分子黏性耗散项和湍动能生成项是高焓湍流边界层摩阻生成的主要流动过程.分子黏性耗散项主要作用在近壁区,高焓流动的分布与低焓流动存在差异.象限分析表明,上抛和下扫运动是影响摩阻分解中湍动能生成项的主导事件.     

解非定常不可压缩N-S方程的迭代压力Poisson方程法

１．引 言 数值求解不可压缩流体流动问题可以采用原始变量的方程作为控制方程,也可以用涡量一流函数方程作为控制方程．直接求解原始变量的不可压缩 Ｎａｖｉｅｒ—Ｓｔｏｋｅｓ方程存在一个主要困难：速度向量在每一时刻都必须满足零散度约束条件,即不可压缩性连续方程．用涡量一流函数方程求解时,连续方程自动满足,所以不存在约束条件的问题,但涡量的边界条件比较难处理,且不易应用于三维问题和带有自由表面或其它流体交界面的问题． 解决上述速度向量必须满足零散度约束条件的困难的方法有：人工压缩法［３,１７］;压力Ｐｏｉｓ…