可压缩流向涡与激波轴对称干扰的数值模拟

用ＮＳ方程数值模拟了可压缩流向涡和激波轴对称相互作用现象。数值模拟包括定常和非定常两种情况,计算结果分别与相应的实验进行了比较,结果表明数值模拟成功地捕捉到了激波和旋涡相互作用过程中发生的激波波面变形,激波振荡,涡核变大以及激波波后出现驻点、回流区等流场特征。提出了判断流向涡与运动激波相互作用中旋涡破碎的准则。     

可压缩流向涡与激波轴对称干扰的数值模拟

用ＮＳ方程数值模拟了可压缩流向涡和激波轴对称相互作用现象．数值模拟包括定常和非定常两种情况,计算结果分别与相应的实验进行了比较．结果表明数值模拟成功地捕捉到了激波和旋涡相互作用过程中发生的激波波面变形,激波振荡,涡核变大以及激波波后出现驻点、回流区等流场特征．提出了判断流向涡与运动激波相互作用中旋涡破碎的准则     

可压缩流向涡与激波轴对称干扰的数值模拟1)

用NS方程数值模拟了可压缩流向涡和激波轴对称相互作用现象.数值模拟包括定常和非定常两种情况,计算结果分别与相应的实验进行了比较.结果表明数值模拟成功地捕捉到了激波和旋涡相互作用过程中发生的激波波面变形,激波振荡,涡核变大以及激波波后出现驻点、回流区等流场特征.提出了判断流向涡与运动激波相互作用中旋涡破碎的准则.     

旋涡与行进表面水波相互作用的实验研究

采用振动板式造波器在二维水槽中生成近似单色的行进表面水波，采用夹板式涡发生器生成稳定上浮的涡对，在Ｆｒｏｕｄｅ数约为０．５的条件下，得到了水下涡对与不同波长和振幅的行进表面水波相互作用时的干扰图象，以实验方法验证了理论分析和数值计算的结果，并为进一步研究旋涡与行进表面水波的相互作用提供了一种实验研究方法     

激波与涡对相互作用的实验研究

在方截面激波管中进行了平面运动激波和涡对的二维相互作用实验，研究了激波与同向涡对、激波与反向涡对相互作用的非定常过程．根据实验照片，分析讨论了作用过程中激波的变形，二次激波和三波点的形成、演变，激波与激波的相互作用，以及旋涡结构的变化等．实验表明，激波通过涡核时，激波发生剧烈变形，旋涡强度增大，涡核形状改变．     

旋涡、湍流与自由表面的相互作用

旋涡及自由表面湍流与自由表面的非定常、非线性相互作用是当前流体力学中一个十分活跃的前沿领域的研究课题,它具有深刻的理论意义和重要的实用背景．旋涡、湍流与自由表面的相互作用是一个非常复杂的非线性、非定常过程,涉及到涡－涡、波－涡相互作用,旋涡在自由表面处的断裂、重联、合并及自由表面湍流中的准拟序结构的形成、发展等复杂动力学过程．关于这一领域的研究已经取得了一些重要成果．对旋涡与自由表面的相互作用的基本过程有了较为清楚的认识;但对湍流与自由表面的相互作用中的许多基本物理过程还知之甚少．本文综述此领域的实验、理论和数值模拟研究的最新进展和主要结果,讨论若干有待解决的问题,并指出进一步的研究方向      

三维涡环与自由表面的粘性相互作用

本文用ＶＯＦ方法数值模拟了三维涡环与自由表面的粘性相互作用，重点考察了Ｆｒｏｕｄｅ数、初始扰动及密度分层对其影响。结果表明：粘性的作用主要体现在涡量的耗散和新的二次涡环的产生：对自由表面变形而言，Ｆｒｏｕｄｅ数是主要的控制参数；当存在初始周向扰动时，水下涡环环量的的非均匀分布会在自由表面上留下与纵向沟槽相类似的水面痕迹；自由表面的存在可削弱流场中的扰动，涡环的不稳定性有抑制作用；密度分层对涡环与自     

可压缩流向涡与反向运动激波相互作用的实验

对可压缩流向涡与反向运动激波相互作用的现象进行了实验研究．实验在９４ｍｍ×９４ｍｍ的方截面激波管中进行．在实验段上游安装了一个有限翼展平直机翼．当入射激波通过机翼后，波后２区气流在模型翼尖诱导出一条流向涡．入射激波在激波管端壁反射后，形成的反射激波在观察窗处和流向涡发生作用．实验中拍摄了激波与流向涡作用全过程的纹影照片，观察到了一些和定常激波与旋涡相互作用不同的现象，并与数值计算结果进行了初步比较     

斜激波和可压缩流向涡相互作用的实验观察

激波和可压缩流向涡相互作用现象近年来成为流体力学研究中的一个热点。本文在激波风洞中研究了可压缩流向涡与斜激波相互作用的现象。实验发现,相互作用后激波和旋涡均发生不同程度变形,但旋涡未发生明显破碎。并且发现在干扰点附近,从涡核发出一束膨胀波,这些膨胀波与斜激波作用,使得激波干扰后发生弯曲。     

非定常流动的快速拉格朗日涡方法数值模拟

采用快速拉格朗日涡方法数值模拟有复杂旋涡运动的非定常流动. 利用离散涡元模拟旋涡的产生、聚集和输送过程. 拉格朗日描述法用来计算离散涡元的移动,而移动速度则利用广义毕奥-萨伐尔公式结合快速多极子展开法计算,修正的涡半径扩散模型用来模拟离散涡元的黏性扩散. 突然起动圆柱和大攻角下突然起动翼型的非定常有涡流动的数值模拟,及其与试验结果的对比验证了方法的有效性. 另外,大攻角下突然起动翼型的计算结果给出了翼型起动后吸力面旋涡的产生、发展,周期性非定常流动的形成,以及尾流旋涡结构等一些重要的流动特征.[关键词] 非定常流有涡流动快速涡方法      

Dynamics of the vortex structure of a jet impinging on a convex surface

Smoke–wire flow visualization is used to investigate the behavior of a round jet issuing from a straight tube and impinging on a convex surface. Video analysis of the impinging jet shows the initiation and growth of ring vortices in the jet shear layer and their interaction with the cylindrical surfaces. Effects of relative curvature, nozzle-to-surface distance, and Reynolds number on vortex initiation, vortex separation from the surface and vortex breakup are described. Examples of vortex merging are discussed.     

Hydrodynamics of flow over a transversely oscillating circular cylinder beneath a free surface

In this paper, hydrodynamic force coefficients and wake vortex structures of uniform flow over a transversely oscillating circular cylinder beneath a free surface were numerically investigated by an adaptive Cartesian cut-cell/level-set method. At a fixed Reynolds number, 100, a series of simulations covering three Froude numbers, two submergence depths, and three oscillation amplitudes were performed over a wide range of oscillation frequency. Results show that, for a deeply submerged cylinder with sufficiently large oscillation amplitudes, both the lift amplitude jump and the lift phase sharp drop exist, not accompanied by significant changes of vortex shedding timing. The near-cylinder vortex structure changes when the lift amplitude jump occurs. For a cylinder oscillating beneath a free surface, larger oscillation amplitude or submergence depth causes higher time-averaged drag for frequency ratio (=oscillation frequency/natural vortex shedding frequency) greater than 1.25. All near-free-surface cases exhibit negative time-averaged lift the magnitude of which increases with decreasing submergence depth. In contrast to a deeply submerged cylinder, occurrences of beating in the temporal variation of lift are fewer for a cylinder oscillating beneath a free surface, especially for small submergence depth. For the highest Froude number investigated, the lift frequency is locked to the cylinder oscillation frequency for frequency ratios higher than one. The vortex shedding mode tends to be double-row for deep and single-row for shallow submergence. Proximity to the free surface would change or destroy the near-cylinder vortex structure characteristic of deep-submergence cases. The lift amplitude jump is smoother for smaller submergence depth. Similar to deep-submergence cases, the vortex shedding frequency is not necessarily the same as the primary-mode frequency of the lift coefficient. The frequency of the induced free surface wave is exactly the cylinder oscillation frequency. The trends of wave length variation with the Froude number and frequency ratio agree with those predicted by the linear theory of small-amplitude free surface waves.     

Resonance in flow past oscillating cylinder under subcritical conditions

Flow around an oscillating cylinder in a subcritical region are numerically studied with a lattice Boltzmann method(LBM). The effects of the Reynolds number,oscillation amplitude and frequency on the vortex wake modes and hydrodynamics forces on the cylinder surface are systematically investigated. Special attention is paid to the phenomenon of resonance induced by the cylinder oscillation. The results demonstrate that vortex shedding can be excited extensively under subcritical conditions, and the response region of vibration frequency broadens with increasing Reynolds number and oscillation amplitude. Two distinct types of vortex shedding regimes are observed. The first type of vortex shedding regime(VSR I) is excited at low frequencies close to the intrinsic frequency of flow, and the second type of vortex shedding regime(VSR II)occurs at high frequencies with the Reynolds number close to the critical value. In the VSR I, a pair of alternately rotating vortices are shed in the wake per oscillation cycle,and lock-in/synchronization occurs, while in the VSR II, two alternately rotating vortices are shed for several oscillation cycles, and the vortex shedding frequency is close to that of a stationary cylinder under the critical condition. The excitation mechanisms of the two types of vortex shedding modes are analyzed separately.     

Interactions of three-dimensional vicous axisymmetric vortex rings with a free surface

The unsteady nonlinear interaction of three-dimensional vortices with a free surface is a great challenge in fluid mechanics, which has deep theoretical significance and important practical background. Applying the three-dimensional VOF method, the interactions of three-dimensional axisymmetric vortex rings with a free surface in an incompressible viscous fluid are numerically simulated. The influence of the Froude number and the surface tension are studied and the evolution of the vorticity, the trajectories of the vortex rings and the baroclinic vorticity on the surface are obtained. The results agreed well with the experiments reported in the literature. The project supported by the National Natural Science Foundation of China     

Branching of a horseshoe vortex around surface-mounted rectangular cylinders

Flow visualizations and surface pressure measurements are performed to study the branching phenomenon of a horseshoe vortex upstream of a series of rectangular cylinders with aspect ratios ranging from 0 to 17. The Reynolds numbers are 500 for visualization experiments and 1990 to 6650 for wind tunnel surface pressure measurements. The flow visualization results indicate that a horseshoe vortex will first evolve into a wavy structure and for aspect ratios which are equal or larger than 10, the wavy horseshoe vortex will branch itself into smaller regular vortices. The waviness disappears as soon as branching occurs. The number of the branched smaller vortices increases as the aspect ratio increases further. Received: 31 March 1998/Accepted: 29 June 1999     

A model of stratified entrainment using vortex persistence

A new model is proposed for the entrainment rate by vortices across stratified interfaces. In the model, different entrainment regimes are distinguished by the conventional parameters Richardson, Reynolds, and Schmidt number as well as a new parameter, the “vortex persistence”. Vortex persistence is defined as the number of rotations a vortex makes during the time it moves its own diameter with respect to the interface. It is further proposed that the concept of vortex persistence is important whenever a vortex is near any kind of surface, either stratified or solid. The model is in accord with most field and laboratory observations in a variety of stratified and bounded flows, including measurements of wall heat transfer and vortex formation in starting jets.     

Two-degree-of-freedom vortex induced vibration of low-mass horizontal circular cylinder near a free surface at low Reynolds number

Two-degree-of-freedom vortex induced vibration (VIV) of a low-mass zero-damping circular cylinder horizontally placed near a free surface at Re = 100 was numerically studied with an adaptive Cartesian cut-cell/level-set method. Two Froude numbers and various normalized submergence depths were considered. The results reveal that the Froude number affects the critical normalized submergence depth and possible physical mechanisms are proposed. The in-line vibration amplitude cannot be neglected. Proximity to a free surface strengthens and suppresses the VIV for low and high Froude numbers, respectively; increases the occurrence of amplitude modulation; and in general enhances the magnitude of the time-averaged lift coefficient, which is always negative. The phase lag of the transverse displacement behind the lift coefficient jumps at some reduced velocity, which strongly depends on the Froude number and normalized submergence depth. Regular trajectories exist only in cases with a small vibration amplitude or a large normalized submergence depth. The vortex structures in any case with large transverse amplitude basically originate from the alternative vortex shedding with the negative vortex weaker than the positive one. For the higher Froude number, an extra free surface positive vortex interacts with the vortices from the cylinder surface. The vibration frequency deviates from the natural structure frequency in fluids in the large-amplitude regime.     

Stability and capillary dynamics of circular vortex sheets

We investigate the motion of circular vortex sheets with surface tension. A linear stability analysis shows that high modes of the circular vortex sheet are stabilized by surface tension, and the sheet is stable if surface tension is larger than a critical value. The modes of perturbations, n = 1 and 2, are always stable, regardless of surface tension, and the mode n = 3 is also stable for large surface tension. The numerical results show that a stable vortex sheet rotates and oscillates weakly. The oscillations of each mode of the interface mainly consist of two travelling waves of different frequencies in time. The amplitude and the period of the oscillation depend on the mode of the perturbation and surface tension. We also perform long-time computations for the unstable evolution of circular sheets. For a high Weber number, ripples are produced on the sheets, as well as pinching and self-intersection. It is found that the appearance of ripples is associated with the growth of noise. For an intermediate Weber number, the sheet evolves to an exotic structure with small spikes on the fingers, while for a low Weber number, it is nonlinearly stable.     

Free surface waves in equilibrium with a vortex

Finite amplitude solitary waves of uniform depth which interact with a stationary point vortex are considered. Waves both with and without a submerged obstacle are computed. The method of solution is collocation of Bernoulli's equation at a finite number of points on the free surface coupled with equations for equilibrium of a point vortex. The stream function and vortex location are found by computing a conformal map of the flow domain to an infinite strip. For a given obstacle the solutions are parametrized with respect to Froude number and vortex circulation. When no obstacle is present there are two families of solutions, in one of which the amplitude of the wave increases by increasing the circulation, while in the other amplitude increases by decreasing the circulation. Beyond a certain critical Froude number the maximum amplitude wave has a sharp crest with an angle of 120 degrees. Similar behavior is observed for the flow past a submerged obstacle except that there is a critical Froude number below which there is no solution at all.     

网格自适应技术在复杂外形流场模拟中的应用

建立了一套适用于非结构混合网格自适应方法,针对激波和涡的不同特征采用不同加密探测器,各向异性加密棱柱单元并沿物面法向方向剖分所有棱柱层,各向异性剖分四面体单元,并保证四面体与棱柱交界面上网格协调。构造Hermit插值近似投影物面新加网格点和基于Laplacian光滑方法对空间网格进行优化。通过网格自适应加密,使用Roe格式计算高超声速球头绕流的红玉现象得到明显减轻。F16飞机含激波和脱体涡的流场自适应计算表明,网格加密集中在激波面和涡核附近区域,激波和涡计算更准确。