PROPULSIVE PERFORMANCE AND VORTEX SHEDDING OF A FOIL IN FLAPPING MOTION

1 .　INTRODUCTIONInsectsandfishhaveexperiencedabillions yearprocessofevolutionwithnaturalselectionfortheirsurvivalandhavedevelopedtheirsuperiorandcomplete performanceofflightandswim ming .Usually ,aflappingmotionisabasicmodeoflocomotionforinsects ,birds ,andfish .Thrustandliftaregeneratedwhentheflappingwingsortailsinteractwiththesurroundingfluids .Becauseofthehighlyunsteadynatureofviscousflowaroundaflappingwing ,itisfarfromsatisfactorytounderstandthephysicalmechanismandvortexsheddinginuns…     

圆柱尾迹流场中横向振荡翼型绕流的数值模拟

该文数值模拟了静止圆柱尾迹中横向振荡翼型绕流的涡结构、力及频谱特性,揭示前方尾迹流场对振荡翼绕流特性的影响，以了解鱼类游动时自主利用前方流场有利干扰的流体力学机理。研究表明圆柱尾迹的影响可导致翼型前缘涡脱落模态发生变化；翼型尾迹的频谱特性更加复杂;在一定的参数下翼型阻力系数可以整体下降。     

NUMERICAL ANALYSIS OF THE UNSTEADY FORCE IN INSECT FORWARD FLIGHT

1. INTRODUCTION Although the quasi-steady analysis provides a simple framework for quantitative prediction of the forces in insect flight, the predicted forces and power requirements of insects in hovering or forward flight cannot prove that insects use t…     

绕圆柱非定常周期性涡旋脱落的数值模拟

利用非定常流函数涡量方程数值模拟圆柱突然起动尾流涡旋的形成及周期性脱落过程。对求解的流函数的一阶导数即速度项采用四阶精度的Hermitian公式，而方程的对流项则采用四阶精度的差分格式，并利用ADI方法迭代求解差分方程组。当雷诺数Re不大于40时，圆柱尾流为附体的两个对称涡，为定常解。当Re大于40后流动为非定常及非对称的，圆柱尾流呈现周期性涡旋交替脱落而形成著名的Karman涡街。选择Re=100为例，在初始条件未加任何扰动情况下，成功地模拟了圆柱非定常涡旋形成与脱落的完整过程（无量纲时间算到t=250及以上）。所计算的阻力系数与实验结果及其它数值方法的计算结果一致。约在t=200形成严格的Karman涡街。对涡量方程ADI求解方法的稳定性进行了分析。对流项采用四阶精度差分格式，若应用于定常问题，将极大提高数值求解的精度，若应用于非定常问题的求解，将对求解精度有所改善，其中时间空间两阶混合偏导数的处理是关键，有待进一步的数值实验。     

NUMERICAL SIMULATION OF AN OSCILLATING FLOW PAST A CIRCULAR CYLINDER IN THE VICINITY OF A PLANE WALL

Oscillating flow around a circular cylinder in the vicinity of a plane wall was investigated by solving the two-dimensional incompressible Navier-Stokes equations with a finite element Galarkin residual method. The effect of the gap G/D between the cylinder surface and the wall on the flow behavior was studied. For the case of G/D 〈 0.25, the periodicity in the flow is attributed to both the outer shear layer instability and the oscillating frequency. As G/D 〉 0.25, vortex shedding occurs and the periodicity in the flow is mainly due to the competition of the oscillating frcqucncy and the vortex shedding frequency from an isolated stationary cylinder.     

附体与主体接合部的流场及不同连接形式对流场影响的试验研究

本文用氢气泡流场显示技术,实时观察了随时间变化的附体与主体相互干扰的马蹄涡流动的形成、发展和变化过程。采用的模型是用3:2椭圆作端部的NACA 0020翼型附体与一平板主体相连接,这是国际上最常用的模型。从实验的观察和分析出发,本文从物理意义上解释了接合部处产生低频振荡马蹄涡的机理,而且由此出发提出了控制低频振荡马蹄涡的一个简便方法,就是在附体与主体连接处以某一几何形状构造连接方式;对五种不同连接方式进行试验研究,发现弓形不对称连接方式为最佳,不但减弱了振荡同时也降低了涡强,从而证实了这种简便方法的实用性。     

再附着剪切层中的大涡结构

在空气流动中利用流场显示技术对二维后向台阶流动的分离和再附着剪切层中的大涡结构作了实验研究，发现当分离前为层流边界层时，分离后有一平直段，其长度随来流速度和台阶高度的增大而缩短，剪切层的发展受层中大涡支配，涡脱落频率随来流速度的提高而加快，在再附着点之前有涡的归并现象出现，再附着长度与台阶高度的比值仅与来流速度有关，再附着点附近涡的尺度与台阶高度相当，大涡碰壁后有时被撕裂成两个，有时被撞碎，有时弹离壁面向下游运动，流动很不规则，难以预测。     

STUDIES ON THE NUMERICAl SIMULATION OF UNSTEADY FLOWS

ＳＴＵＤＩＥＳＯＮＴＨＥＮＵＭＥＲＩＣＡｌＳＩＭＵＬＡＴＩＯＮＯＦＵＮＳＴＥＡＤＹＦＬＯＷＳ￥ＬｕＸｉ－ｙｕｎ（ＵｎｉｖｅｒｓｉｔｙｏｆＳｃｉｅｎｃｅａｎｄＴｅｃｈｎｏｌｏｇｙｏｆＣｈｉｎａ，Ｈｅｆｅｉ２３００２６，Ｐ．Ｒ．Ｃｈｉｎａ）Ｓｕｐｅｒｖｉ...     

STUDY OF VORTEX CHARACTERISTICS OF THE FLOW AROUND A HORIZONTAL CIRCULAR CYLINDER AT VARIOUS GAP-RATIOS IN THE CROSS-FLOW

1. INTRODUCTION The flow around a horizontal circular cylinder near a plane boundary (fixed wall) is of great significance in Hydrodynamics. For instance, sewagepipelines in water body of rivers in environmental engineering, conveying pipelines and optica…     

A STUDY OF OSCILLATING FLOWS OVER PROPAGATING RIPPLES:PART Ⅱ. VORTICITY DYNAMICS

1 .　ＩＮＴＲＯＤＵＣＴＩＯＮＡｈｏｒｉｚｏｎｔａｌｂａｃｋ ａｎｄ ｆｏｒｔｈｍｏｔｉｏｎｐｒｏｄｕｃｅｄｎｅａｒａｓａｎｄｂｅｄｂｙ ｐｒｏｇｒｅｓｓｉｖｅｇｒａｖｉｔｙｗａｖｅｓｐｒｏｐａｇａｔｉｏｎｉｎｓｈａｌｌｏｗｗａｔｅｒｕｓｕａｌｌｙｇｉｖｅｓｒｉｓｅｔｏｎｅａｒｌｙｓｙｍｍｅｔｒｉｃａｌｂｅｄ ｆｏｒｍｓｃｏｍ ｍｏｎｌｙｋｎｏｗｎａｓｒｉｐｐｌｅｓ .Ｏｓｃｉｌｌａｔｉｎｇｆｌｏｗｓｏｖｅｒｐｅｒｉｏｄｉｃｒｉｐｐｌｅｓａｒｅｏｆｐｒａｃｔｉｃａｌａｓｗｅｌｌａｓｓｃｉ ｅｎｔｉｆｉｃ…     

NUMERICAL INVESTIGATION ON OSCILLATING FLOWSPAST A HALF CIRCULAR CYLINDER ON A PLANE

l.INTRoDUCTIoNTheunsteadyflowpastabluffbodyhasreceivedagreatdealofattentionowingmainlytoitstheoreticalandpracticalsignificance.Foranoscillatingflowpastacircularcylinder,thetwoparametersgoverningthedevelopmentoftheflowaretheReynoldsnumber,de-finedbyRe=2U,R/v,WhereU,isthemaximumvelocityoftheambientflow,visthekine-maticviscosityandRistheradiusofthecircularcylinder;andtheKeulegan-Carpenternum-ber,definedasKC=U,T/2R,whereTistheoscillationperiod.Todescirbethisproblem,earlierstudiesofviscou…     

STABILITY OF VORTEX STREET IN GAS-LIQUID TWO-PHASE FLOW

The stability of the Karmen vortex street in gas-liquid two-phase flow was studied experimentally and theoretically. The values of the parameter h/l characterizing the vortex street structure (i. e. , the ratio of the vortex street width to the distance between two vortexes) for a stable vortex street in gas-liquid two-phase flow were obtained for the first time. The parameter h/l was prpved to be a variable, not a constant as in single-phase flow, h/l is related to the upstream fluid void fraction. In gas-liquid two-phase fluid flow to form a steady vortex street is more difficult than in a single-phase fluid flow. Because in the unsteady vortex shedding the vortex shedding band frequency is broader than the one in the single phase fluid flow, so it is easier to induce the cross-cylinder resonance than in the single phase fluid flow, and this case should give rise to the attention of engineers.     

利用分离盘控制隔水管涡激振动的数值模拟

涡激振动是导致深海隔水管疲劳失效的重要因素.为有效控制隔水管涡激振动,提出了利用分离盘控制涡激振动的方法.对加装0.25～2.0倍隔水管直径的分离盘后的隔水管流动进行了二维数值模拟.研究了亚临界状态下各种模型旋涡脱落的流场结构、脱落模态、隔水管升阻力系数和旋涡泄放频率的变化特征.结果表明:分离盘加装前与经典的实验和计算结果吻合很好;不同尺寸的分离盘对隔水管涡激振动控制效果具有明显差异;当分离盘的长度为1.0至1.5倍的隔水管直径时,平均阻力系数可以减小20%,尾迹涡街的频率也有了较大的减小,达到控制涡激振动的最优效果.     

明渠柱体上游马蹄涡的运动学特征研究

马蹄涡是水下柱体结构周围床面冲刷的主要动力。为掌握马蹄涡的运动学特征,引入了基于旋转强度的马蹄涡识别方法,提出了基于椭圆涡拟合的马蹄涡特征提取方法,其中,椭圆涡由Oseen涡和倾斜单向剪切流叠加而成。基于上述方法,通过开展明渠圆柱绕流PIV实验,对柱体上游对称面时均流场中马蹄涡的运动学特征进行了研究。结果表明,在柱体雷诺数大于104的紊流柱体绕流中,马蹄涡上游的流动分离点随柱体雷诺数的增大而缓慢向下游移动,但马蹄涡的位置保持不变,涡中心距柱体中轴约0.67D(D为柱体直径或宽度),距床面约0.06D;随着柱体雷诺数的增大,马蹄涡的半径不变而强度增加,使得马蹄涡作用于床面的剪切应力增大。研究结果建立了客观提取马蹄涡运动学特征的数学方法,得到了流动分离点、马蹄涡位置等运动学特征随水流条件的变化规律,揭示了明渠柱体绕流与其它流动中柱体绕流的差异,初步构建了马蹄涡的动力作用概念模型。     

海山后尾迹涡形成因素的数值研究

该文采用regional ocean modeling system(ROMS)海洋模式,对理想channel中的海山模型,研究均匀分层流作用下,不同的伯格数和来流条件对海山诱导的尾迹涡及垂向流动的影响。结果表明,伯格数越大,越有利于海山后尾迹涡的产生,涡的强度不断增大,且涡脱落频率加快;随流速的增加,海山后尾迹涡的强度增大,涡脱落频率加快,且海山周围的垂向流动增强;当来流速度较低(0.05 m/s)及较高(0.8 m/s)时,均不利于在海山后形成稳定的垂向流动。     

FLOW PAST TWO ROTATING CIRCULAR CYLINDERS IN A SIDE-BY-SIDE ARRANGEMENT

Measurements were performed using Particle Image Velocimetry(PIV) to analyze the modification of flow by the combined effects of the rotation and the Reynolds number on the flow past two rotating circular cylinders in a side-by-side-arrangement at a range of 425 ≤ Re≤ 1130,0 ≤α≤ 4(α is the rotational speed) at one gap spacing of T / d =1.11(T and d are the distance between the centers of two cylinders and the cylinder diameter,respectively).A new Immersed-Lattice Boltzmann Method(ILBM) scheme was used to st...     

民机不同后体空间流态水洞实验研究

利用荧光黄纳及片光在水槽中对具有不同收缩比的民机后体的复杂绕流进行流动显示，探讨了后体的分离尾涡特性及参数影响．实验发现后体收缩比改变时对应了两种典型的尾涡流态，即稳定流向尾涡和周期性尾涡，并进一步探讨了尾翼对后体尾涡的影响。     

弧形闸门前漩涡特性研究

吸气漩涡是水利工程中进水口前常见的水力学问题,模型试验是研究进水口前漩涡特性的常用方法。目前关于漩涡特性的研究主要集中在淹没水深较大、进水口结构不变的泄洪洞以及电站进水口等,而对于弧形闸门局部开启时闸前漩涡特性的研究较少。为了研究弧形闸门前漩涡的水力特性以及黏性力和表面张力对弧形闸门前漩涡的影响,以某泄洪闸弧形闸门为研究对象,采用2个不同比尺的模型试验及理论分析方法,对弧形闸门局部开启时闸前吸气漩涡的水力特性进行了研究。结果表明,闸前漩涡是行进水流横向和纵向突然收缩共同作用的结果。弧形闸门前水面紊动较大,所以闸前漩涡频率较高,持续时间较短,闸门开度越大,闸前吸气漩涡越强。若按照弗劳德数准则设计模型试验,当来流雷诺数和韦伯数大于某一临界值时,可以忽略黏性力和表面张力影响。     

两层黏性流体中直立圆柱体绕流的三维数值模拟

研究了两层流体中直立贯底圆柱体的三维黏性绕流问题.以不可压缩Navier-Stokes方程为控制方程,应用VOF方法追踪两层流体的内界面,建立了该问题的数值模拟方法.成功地数值模拟了两层流体中圆柱体黏性绕流产生的三维尾涡特征,表明了流体的密度分层效应对直立贯底圆柱体的尾涡特性和阻力系数都是有影响的.在海洋立管涡激振动的研究中,考虑流体的密度分层效应是重要的.     

The method to control the submarine horseshoe vortex by breaking the vortex core

The quality of the inflow across the propeller is closely related with the hydrodynamic performance and the noise characteristics of the propeller. For a submarine, with a horseshoe vortex generated at the junction of the main body and the appendages, the submarine wake is dominated by a kind of highly non-uniform flow field, which has an adverse effect on the performance of the submarine propeller. In order to control the horseshoe vortex and improve the quality of the submarine wake, the flow field around a submarine model is simulated by the detached eddies simulation （DES） method, and the vortex configuration is displayed using the second invariant of the velocity derivative tensor. The state and the transition process of the horseshoe vortex are analyzed, then a modified method to break the vortex core by a vortex baffle is proposed. The flow numerical simulation is carried out to study the effect of this method. Numerical simulations show that, with the breakdown of the vortex core, many unstable vortices are shed and the energy of the horseshoe vortex is dissipated quickly, and the uniformity of the submarine wake is improved. The submarine wake test in a wind tunnel has verified the effect of the method to control the horseshoe vortex. The vortex baffle can improve the wake uniformity in cases of high Reynolds numbers as well, and it does not have adverse effects on the maneuverability and the speed ability of the submarine.