分块法研究圆柱绕流升阻力

 使用新的分块耦合方法，分别对单圆柱和串列双圆柱绕流进行了数值 模拟. 对于单圆柱绕流，低$Re$下计算所得到的定常涡尺寸与实验非常接近. 对于 串列双圆柱绕流，研究分析了改变双圆柱中心间距对上下游圆柱的升阻力系数和脉动频率所 产生的影响，计算结果与实验非常吻合，为进一步研究涡致振动提供了依据.     

串列双圆柱绕流问题的数值模拟

本文运用有限体积方法,对绕串列放置的双圆柱的二维不可压缩流动进行了数值计算。为研究两圆柱不同间距对圆柱相互作用和尾流特征的影响,选取间距比Ｌ／Ｄ（Ｌ为两圆柱中心间的距离,Ｄ为圆柱直径）在１．５～５．０之间每隔０．５共八个有代表性的间距进行了计算模拟。计算均在Ｒｅ＝２００条件下进行。计算结果表明：对该绕流问题,流动特征在很大程度上取决于间距的大小。且间距存在一临界值,间距比从小于临界值变化到大于临界     

串列双圆柱绕流下游圆柱两自由度涡致振动研究

数值研究了串列双圆柱绕流下游圆柱两自由度涡致振动问题，研究发现：(1) 双自由度的圆柱振幅峰值及出现振峰的频率比都比单自由度的大；(2) 尾流圆柱中的升力远大于均匀来流的，而阻力却相反；(3) 下游圆柱的位移响应对于频率比的变化没有均匀来流中的"敏感"；(4) 尾流中，在频率比1.16和0.87之间，出现了明显的"拍"现象，即圆柱的振幅响应包含不同的频率，而在均匀来流中，并无明显的"拍"现象. 采用ALE方法，计算网格采用H-O非交错网格系统，结合分块耦合方法. N-S方程的对流项和扩散项分别采用三阶迎风紧致格式和四阶中心紧致格式离散. 圆柱振动采用弹簧柱体阻尼器模型，柱体的振动方程采用龙格-库塔法求解. 通过模拟柱体和流体之间的非线性耦合作用，成功地捕捉到了"拍"和"相位开关"等现象.     

圆柱绕流教学中常被忽略的一个问题

笔者所见不少流体力学教材在讲述无环量圆柱绕流时,大都这样处理:1)推出速度势(?)(或流函数(?))满足的 Laplace 方程 ▽~2(?)=0(或▽~2(?)=0);2)给出若干个基本解;3)将均匀流与偶极子两个基本解叠     

并列双圆柱绕流特性和互扰效应数值模拟研究

基于CFD方法,对不同间距比(1.25≤T*≤5)下的二维双圆柱绕流进行了数值模拟研究.初步研究结果表明:当1.25＜T*≤2.2时尾流不能达到稳定,两圆柱交替对下游流场起主导作用;当T*＞2.2时尾流能达到稳定且上下侧圆柱涡的脱落出现同步,即流场趋于稳定的临界值为T ≈2.2.进一步研究发现:当2.2＜T*≤3.3时...     

流向磁场作用下圆柱绕流的直接数值模拟

绕流是托卡马克装置中液态包层内常见的流动形态,对流场与热量分布有着重要的影响.本文通过直接数值模拟(DNS),研究了不同磁场强度下$Re=3900$的圆柱绕流,分析了磁场强度对于湍流尾迹的影响.无磁场情况下,直接数值模拟的结果与前人的实验及模拟结果吻合很好.圆柱下游的尾迹中,随着流向距离的增大, 流向速度剖面逐渐从U型进化呈V型, 并慢慢趋于平缓,这表明尾迹中的流动结构受圆柱影响逐渐减小.圆柱后方两侧的剪切层中,由于Kelvin-Helmholtz不稳定性的影响,可以清晰地看到小尺度剪切层涡的脱落.通过对无磁场的计算结果施加流向磁场,本文计算了哈特曼数($Ha$)分别为20, 40和80的工况,以研究磁场效应对于湍流的影响.结果表明磁场较弱时,流动依然呈三维湍流状态.随着磁场增强, 近圆柱尾流区受磁场抑制明显,回流区被拉长,剪切层失稳位置向下游转移.圆柱后方的涡结构由于受到竖直方向洛伦兹力的挤压作用,随着哈特曼数的增加尾迹区域逐渐变窄.相比于无磁场情况的涡结构,由于磁场的耗散作用,相应的涡结构尺度变小.该研究不仅扩展了现有磁场下湍流运动的参数范围,对于液态包层的设计及安全运行同样具有重要的理论指导意义和工程应用价值.      

双圆柱绕流特性的模拟研究

采用格子Boltzmann方法对低雷诺数下气体绕流圆柱的规律进行了研究. 对比计算了双圆柱在不同圆心距、不同Re数、不同来流速度与双圆柱圆心连线角度的情况下,各个圆柱的受力大小和曳力系数. 结果表明,若Re数为20, 改变圆柱间距,圆柱间距在1.2d和1.4d之间时,下游圆柱所受曳力有极小值;双圆柱间距为1.6d时,双圆柱受到总曳力最小;圆柱间距大于2d时,上游颗粒受到的曳力不再受到下游颗粒的影响. 若圆柱间距为1.2d, 改变雷诺数,Re数在30和40之间,下游圆柱所受曳力有极小值. 另外,来流速度角度对圆柱的受力影响很大. 上述规律为低Re数下圆柱绕流的深入研究与应用打下基础.      

非定常圆柱绕流的实验研究

本文采用流场显示方法研究单圆柱在非定常流动中的涡旋脱落规律。实验结果表明：在不同的ＫＣ数（ＫＣ＝Ｕ∞Ｔ／Ｄ）下，非常圆柱绕流的涡旋脱落特性是不相同的，一般随ＫＣ数值的增加，其涡旋脱落对数也增加，但有明显的阶梯性，同时圆柱分离点的周期变化后于流场的变化。     

混合层绕流圆柱旋涡脱落的数值研究

对平面混合层绕流圆柱时的旋涡脱落和流动结构进行了数值研究。方法是用一空间、时间三阶精度的有限差分格式解二维不可压Navier-Stokes方程和连续性方程。计算时雷诺数Re取为1000,混合层速度比Ra从0到1,混合层动量厚度θ由0.2到2。     

虚拟边界法研究正交双圆柱及串列双圆球绕流

把Goldstein等人提出的虚拟边界法推广到三维情况,研究了 Re=150时不同间距下正交双圆柱绕流,和Re=250时不同间距下串列双 圆球绕流流场. 对于正交双圆柱绕流,当间距比大于3,下游圆柱对上游圆柱尾流的影响只 限定在下游圆柱的尾流所扫过的范围之内;当间距比小于等于3,下游圆柱对上游圆柱尾流 的影响扩大,下游圆柱尾流扫过区上下出现两排三维流向二次涡结构. 对于串列圆球绕流, 研究发现,在小间距比(L/D≈ 1.5)的情况下,由于上下游圆球尾流区的相互抑 制消除了压力不稳定性,整个流场呈现稳 态轴对称特征;间距比为2.0时,周向压力梯度诱发出流体的周向输运,流场呈现稳态非对 称性,但流场中存在特定的对称面;间距比增大到2.5后,绕流场开始周期振荡,原有的对 称面依旧存在;在间距比3.5时下游圆球下表面的涡结构强度有所减弱,导致占优频率发生 交替;间距比增至7.0时,整个流场恢复稳态特征,两圆球尾部同时出现双线涡,这时流场 对称面的位置发生了变动.     

Three-dimensional computation for flow-induced vibrations of an upstream circular cylinder in two tandem circular cylinders

It is well known from a lot of experimental data that fluid forces acting on two tandem circular cylinders are quite different from those acting on a single circular cylinder. Therefore, we first present numerical results for fluid forces acting on two tandem circular cylinders, which are mounted at various spacings in a smooth flow, and second we present numerical results for flow-induced vibrations of the upstream circular cylinder in the tandem arrangement. The two circular cylinders are arranged at close spacing in a flow field. The upstream circular cylinder is elastically placed by damper-spring systems and moves in both the in-line and cross-flow directions. In such models, each circular cylinder is assumed as a rigid body. On the other hand, we do not introduce a turbulent model such as the Large Eddy Simulation (LES) or Reynolds Averaged Navier-Stokes (RANS) models into the numerical scheme to compute the fluid flow. Our numerical procedure to capture the flow-induced vibration phenomena of the upstream circular cylinder is treated as a fluid-structure interaction problem in which the ideas of weak coupling is taken into consideration.     

Three-dimensional simulations of flow past two circular cylinders in side-by-side arrangements at right and oblique attacks

The flow past two identical circular cylinders in side-by-side arrangements at right and oblique attack angles is numerically investigated by solving the three-dimensional Navier–Stokes equations using the Petrov–Galerkin finite element method. The study is focused on the effect of flow attack angle and gap ratio between the two cylinders on the vortex shedding flow and the hydrodynamic forces of the cylinders. For an oblique flow attack angle, the Reynolds number based on the velocity component perpendicular to the cylinder span is defined as the normal Reynolds number Re_N and that based on the total velocity is defined as the total Reynolds number Re_T. Simulations are conducted for two Reynolds numbers of Re_N=500 and Re_T=500, two flow attack angles of α=0° and 45° and four gap ratios of G/D=0.5, 1, 3 and 5. The biased gap flow for G/D=0.5 and 1 and the flip-flopping bistable gap flow for G/D=1 are observed for both α=0° and 45°. For a constant normal Reynolds number of Re_N=500, the mean drag and lift coefficients at α=0° are very close to those at α=45°. The difference between the root mean square (RMS) lift coefficient at α=0° and that at α=45° is about 20% for large gap ratios of 3 and 5. From small gap ratios of 0.5 and 1, the RMS lift coefficients at α=0° and 45° are similar to each other. The present simulations show that the agreement in the force coefficients between the 0° and 45° flow attack angles for a constant normal Reynolds number is better than that for a constant total Reynolds number. This indicates that the normal Reynolds number should be used in the implementation of the independence principle (i.e., the independence of the force coefficients on the flow attack angle). The effect of Reynolds number on the bistable gap flow is investigated by simulating the flow for Re_N=100–600, α=0° and 45° and G/D=1. Flow for G/D=1 is found to be two-dimensional at Re_N=100 and weak three-dimensional at Re_N=200. While well defined biased flow can be identified for Re_N=300–600, the gap flow for Re_N=100 and 200 changes its biased direction too frequently to allow stable biased flow to develop.     

Control of vortex shedding behind circular cylinder for flows at low Reynolds numbers

It has been observed by researchers in the past that vortex shedding behind circular cylinders can be altered, and in some cases suppressed, over a limited range of Reynolds numbers by proper placement of a second, much smaller, ‘control’ cylinder in the near wake of the main cylinder. Results are presented for numerical computations of some such situations. A stabilized finite element method is employed to solve the incompressible Navier–Stokes equations in the primitive variables formulation. At low Reynolds numbers, for certain relative positions of the main and control cylinder, the vortex shedding from the main cylinder is completely suppressed. Excellent agreement is observed between the present computations and experimental findings of other researchers. In an effort to explain the mechanism of control of vortex shedding, the streamwise variation of the pressure coefficient close to the shear layer of the main cylinder is compared for various cases, with and without the control cylinder. In the cases where the vortex shedding is suppressed, it is observed that the control cylinder provides a local favorable pressure gradient in the wake region, thereby stabilizing the shear layer locally. Copyright © 2001 John Wiley & Sons, Ltd.     

一种模拟动边界绕流的锐利界面浸入边界法

为避免复杂贴体网格的更新和畸形对动边界流场计算效率、精度的影响,以充分掌握结构场的受力特性,采用一种改进的锐利界面（sharp-interface）浸入边界法模拟具有动边界绕流的流动问题。该方法将计算域中的固体视为流体,固体边界离散为若干个拉格朗日网格点,通过在界面单元处插值重构流动参数（速度）,将其直接作为流动求解器的边界条件,由此来反映固体边界的影响。即通过构造“虚拟点—受力点—垂足点”的计算结构,借助双线性插值得到虚拟点的速度,再通过强制满足固体边界的无滑移条件计算出受力点的速度,以此为边界条件,最终求解基于浸入边界法的耦合系统方程,实现复杂动边界的流动数值模拟。采用C++编写该浸入边界法的数值程序,以单圆柱绕流为验证算例,通过与文献和实验结果的对比,验证了该方法的准确性和可靠性。在此基础上,对主动运动椭圆柱绕流问题进行了精细计算,探讨了不同轴长比(A_R)、不同攻角($ \theta $)下的椭圆柱对尾涡结构分布特征和水力不稳定现象的影响。捕捉到了反对称S型、“P+S” Ⅰ型、“P+S” Ⅱ型尾涡脱落模态,漩涡强度、涡脱频率和升阻比随A_R和$ \theta $的变化规律,以及确定了升阻比临界攻角(25°)。

     

NUMERICAL STUDY ON THE FLOW AROUND A CIRCULAR CYLINDER WITH SURFACE SUCTION OR BLOWING USING VORTICITY-VELOCITY METHOD

A vorticity-velocity method was used to study the incompressible viscous fluid flow around a circular cylinder with surface suction or blowing. The resulted high order implicit difference equations were effeciently solved by the modified incomplete LU decomposition conjugate gradient scheme (MILU-CG). The effects of surface suction or blowing ' s position and strength on the vortex structures in the cylinder wake, as well as on the drag and lift forces at Reynoldes number Re = 100 were investigated numerically. The results show that the suction on the shoulder of the cylinder or the blowing on the rear of the cylinder can effeciently suppress the asymmetry of the vortex wake in the transverse direction and greatly reduce the lift force; the suction on the shoulder of the cylinder, when its strength is properly chosen , can reduce the drag force significantly , too.     

管间距对串联双圆柱表面压力分布特性的影响研究

对气液两相流中串联双圆柱表面压力分布特性进行了系统的实验研究 ,着重讨论了管间距的影响。实验双圆柱的间距比分别为 2 .0 ,3.0 ,4.0 ;截面含气率的范围为0～ 0 .3;来流雷诺数的范围为 3× 1 0 4 ～ 8× 1 0 4 。     

侧柱与串列双柱绕流之间的干扰

本文给出了关于串列双柱与创柱间流动干扰的实验研究结果。当三个圆柱排成等边三角形并靠得很近时，由于三圆柱间强烈的缝隙流动，大大地改变了绕流其中的串列双圆柱的流态。特别，当三圆柱中心距等于二倍圆柱直径时，在串列双柱的前、后柱之间形成了强烈的偏斜的缝隙流，出现了独特的压力分布以及要比单柱高出三倍以上的旋涡脱落频率。     

Application of local DFD method to simulate unsteady flows around an oscillating circular cylinder

In this paper, the recently proposed local domain‐free discretization (DFD) method is applied to simulate incompressible flows around an oscillating circular cylinder. It is found that it is very easy for the local DFD method to handle such moving boundary flow problems. This is because it does not need to move the mesh, which is indeed needed in traditional methods. Numerical experiments show that the present numerical results agree very well with the available data in the literature, and that the local DFD method is an effective tool for the computation of moving boundary flow problems. Copyright © 2008 John Wiley & Sons, Ltd.