表面台阶引起的高超声速湍流边界层分离

介绍了圆柱、方柱和二维台阶前干扰热流分布及油流和液晶热图的实验结果。来流马赫数Ｍ＿１＝５—９，雷诺数Ｒｅ＝（２—５）×１０￣７／ｍ，台阶高度与边界层厚度比ｈ／δ＝０．０６—　２．５．实验发现干扰压力和热流高峰值出现在台阶前０．１５倍台阶高度处的再附点附近，方柱台阶前压力和热流最高峰值不在中心线上，而在两侧角之内０．５倍台阶高度处附近，结果还表明干扰区几何特征参数，如分离距离、热流峰值和谷值点位置，与马赫数、雷诺数和台阶展宽无关，只随台阶高度线性增加。     

《小问题》上期解答

《小问题》上期解答答２８７．１．Ｊｃ＝Ｊ０－ｍｌ２＝１２－１６９π２ｍｒ２，本题为单自由度，取广义坐标θ．粗糙时：速度瞬心为Ｐ１，ＪＰ１＝Ｊｃ＋ｍＣＰ１２＝３２－８３πｃｏｓθｍｒ２．动能Ｔ＝１２ＪＰ１θ２＝１２３２－８３πｃｏｓθｍｒ２θ２．图３图...     

《小问题》上期解答

《小问题》上期解答答２８７．１．Ｊｃ＝Ｊ０－ｍｌ２＝１２－１６９π２ｍｒ２,本题为单自由度,取广义坐标θ．粗糙时：速度瞬心为Ｐ１,ＪＰ１＝Ｊｃ＋ｍＣＰ１２＝３２－８３πｃｏｓθｍｒ２．动能Ｔ＝１２ＪＰ１θ２＝１２３２－８３πｃｏｓθｍｒ２θ２．图３图...     

一端弹性支撑的串列双柱和并列双柱在气流中自由端的振幅响应

本文研究了一端弹性支撑的并列双柱和串列双圆柱在气流中自由端的振幅响应，与单圆柱相比，在小间距时，串列双圆柱中前柱的横向振幅受到较强的激励。而在大间距时，振幅受到抑制，特别当Ｌ／Ｄ＝３．５和４．０时，其振幅响应仅为单柱的１／３左右，而对于后柱，则在大间距时，纵向振幅响应有所增大，而且后柱的振幅响应要比前柱的大得多，而并列双圆柱的自由端振幅基本上受到抑制，在Ｔ／Ｄ＞３．０之后，干扰很快减小到接近单个圆     

亚临界情形下非线性裂纹转子的分叉与浑沌

分析非线性涡动裂纹转子中刚度变化比ΔＫ，裂纹角β，不平衡参数Ｕ对系统分叉及浑沌行为的影响。在转速区Ω＝２Ωｃ／３附近，当ΔＫ较大时，会出现分叉及浑沌现象，β对这些行为有很大影响，在Ω＝Ωｃ／２附近，当ΔＫ很大时，无论β为何值，将由拟周期通向浑沌，Ｕ作为一种外部因素，将使系统的非线性行为得到激发或抑制。     

流线型轴对称钝体尾迹特性及其声激励控制

在Ｒｅ＝３．０×１０３～１．０×１０５的范围内实验研究了流线型轴对称钝体尾迹特性，并采用声激励手段对尾迹进行控制．研究表明，自然状态下流线型轴对称钝体尾涡无量纲脱落频率在Ｒｅ＝５．０×１０４～１．０×１０５的范围内具有普适性，适当频率的声激励可以减小回流区，该结果对工程减阻具有实际应用价值．实验中脉冲热线的成功应用为复杂流场的测量提供了新的技术手段．     

小问题

３２５．设系统的运动方程为：　求： （１）此系统自由振动第一谐波的振幅ａ１与其频率ｗ的关系． （２）画出当ｍ＝ｃ＝１,Ｆ０＝π,ｘ（０）＝０．５;ｘ（０）＝０时的时程曲线和相平面图．（来源：密歇尔斯基理论力学习题集,［俄］第３６版,５７．５题）（李银山,太原理工大学应用力学研究所,太原０３００２４） ３２６．古埃及人在四千年前就已懂得了摩擦学的原理,他们曾用滚子和滑板来搬运重物．有一幅浮雕反映了奴隶们搬运一台石雕巨像的情景（约公元前１９００年）．仔细观察可以发现：巨像放在滑板上,由１７２个奴隶拉着…     

混合层中的流向涡与Rayleigh离心不稳定

用有限差分方法求解三维Ｎａｖｉｅｒ－Ｓｔｏｋｅｓ方程和连续性方程，对时间发展的平面混合层中流向涡的产生原因进行了分析．将Ｒａｙｌｅｉｇｈ的轴对称无粘离心不稳定理论推广应用于分析混合层的二维基本流，并导出一无量纲量Ｒａｙ＝－（ｒ／νθ）νθ／ｒ，其中νθ为一流体质点相对于平均速度的速度，ｒ为该质点迹线的曲率半径．当Ｒａｙ＞１．０时就会发生离心不稳定．采用这一判别式后发现混合层中展向涡的周围，尤其是在辫带区，的确存在离心不稳定区域，而过去的实验结果也表明三维不稳定产生于展向涡之间的辫带区．因此有理由认为，除非雷诺数特别小，离心不稳定是流向涡产生和发展的主要原因     

俯抑振荡三角翼前缘涡破碎的流动显示

前缘后掠角为６５°和７０°的两个平板三角翼作俯仰振荡运动１，前缘涡破碎的流动显示实验研究在南京航空航天大学１米低速风洞中进行。俯仰振荡运动的攻角范围为０°－６０°，折合频率为０．０３和０．０６。采用四氯化钛发烟技术显示前缘涡核轨迹及涡破碎位置。     

理论力学教学讨论点滴

理论力学教学讨论点滴对“瞬时静止时刻平面运动刚体的速度瞬心”一文的商榷（杨越宁沈阳工业学院，沈阳１１００１５）从文［１］引用速度瞬心点Ｐ的坐标（ＸＰ，ＹＰ）公式ＸＰ＝Ｘ０－Ｙ０φ·，ＹＰ＝Ｙ０＋Ｘ０φ·可看出，当ｔ＝０时，刚体角速度φ·及刚体上任一点...     

Time-resolved velocity field measurements of separated flow in front of a vertical fence

 An experimental investigation of the velocity fields of the turbulent shear flow in front of a vertical fence was performed using a hybrid particle tracking velocimetry technique. The vertical fence, with an aspect ratio of 10, was embedded in a thin laminar boundary layer. The Reynolds number of the flow based on the fence height was about 5,000. The measured instantaneous velocity fields and flow visualization results revealed a complex but organized flow structure in front of the fence. The stagnation point detached from the surface of the fence, increasing the streamline curvature near the fence. Time resolved turbulence statistics were obtained and compared with those of a 3D junction flow. Received: 23 August 2000/Accepted: 1 February 2001     

Surface pressure and flow field behind an oscillating fence submerged in turbulent boundary layer

Phase-locked PSP and PIV measurements were used to study the evolution of three-dimensional disturbances produced by an oscillating fence actuator immersed in a zero pressure gradient turbulent boundary layer. For the single fence frequency studied, strong three-dimensionality is observed in the vortical structure that varies along the span of the fence soon after the fence enters the flow. At the midspan, the structure grows, weakens, and convects faster than at other locations. As the fence height increases, the data indicate that the vortical structure terminates near the edge of the fence. In contrast, the vortex structure terminates on the plate surface adjacent to the fence edge as the fence descends, similar to a wake vortex of a stationary obstacle. This study demonstrates that the combined use of surface and flow-field diagnostics provide a link between flow field and surface features, yielding an understanding of the flow that would have not been possible with any one technique.     

Process of formation of a vortex street in the wake behind a flat plate

The process of the formation of a vortex street in the wake behind a flat plate set parallel to a uniform flow was investigated in a low speed wind tunnel. The vorticity distributions in the wake were calculated from the measured velocities using Taylor's hypothesis.

Just behind the plate, the equi-vorticity lines were nearly parallel to the free stream. At locations somewhat downstream, sinusoidal contour lines appeared near the wake center. Further downstream, some closed contour lines appeared in the figures mapped. The arrangement of the closed lines suggests the existence of a vortex street. The maximum value for vorticity in a wave length of the fundamental velocity fluctuation decreased in the downstream direction; the concentration of vorticity, however, increased in a region the further downstream it was. Meanwhile, the value for circulation obtained by the surface integral of vorticity within the closed contours of a vortex increased until the vortex street was established.     

Symmetry groups and similarity analysis for boundary layer control over a wedge using electric forces

In this paper, we consider the control of laminar, incompressible boundary layer for the ionized air flow over a flat plate and wedges through the application of an electric field. Group theory is used to find equivalence transformations of the resulting boundary layer equations. These transformations in turn reveal forms of the electric field function which lead to reductions of the equations via similarity variables. Differences in boundary layer thickness growth and velocity profiles as well as wall shear stresses are shown for the specific choices of the electric field function. The effects of suction and blowing type of boundary conditions are also demonstrated. The applied electric fields lead to a thinning of boundary layer thickness and an increase in wall shear stress. They are also shown to stabilize the velocity component profiles and suppress flow separation in reentrant corners.     

Impingement of a low Reynolds number turbulent circular jet onto a flat plate at normal incidence

The velocity field of a circular water jet impinging onto a flat plate has been measured using particle image velocimetry, or PIV. The velocity field has been recorded at several instants in time, producing thousands of simultaneous two-dimensional velocity measurements for each realization. The instantaneous velocity, vorticity and rate-of-strain fields reveal the interaction of vortices near the impinging wall within the radial wall jet downstream from the stagnation point. An ensemble average of the instantaneous fields produces a mean velocity field of the jet flow, which reveals many of the processes leading to boundary layer separation and vortex breakaway within the wall jet. The PIV system extracts the velocity measurements using a two-dimensional autocorrelation method, and can obtain thousands of highly accurate velocity measurements within a few minutes. The structure found in these experiments may be similar to the ground level structure of atmospheric microburst phenomena.A version of this paper was presented at the 11th Symposium on Turbulence, University of Missouri-Rolla, 17–19 October 1988     

A numerical exploration of secondary separation in starting flow around a flat plate by the discrete vortex model

In the present work, a further numerical simulation of the starting flow around a flat plate normal to the direction of motion in a uniform fluid has been made by means of the discrete vortex method. The secondary separation occurring at rear surface of the plate is explored, and predicted approximately using Thwait's method. The calculated results show that in the early stages of the flow secondary separation does occur. The evolution of flow field, the vortex growing process and the characteristics of secondary vortices have been described. The time dependent drag coefficients, the vorticity shed from the edges and rear surface, and the separation positions are calculated as well as the distributions of velocity and pressure on the plate. In the case of flow normal to the plate, the calculated secondary vortices are weak. Their existence will change the local velocity distributions and affect pressure distributions. However, the effect on drag coefficient is negligible.     

Hole diameter effect on flow characteristics of wake behind porous fences having the same porosity

The hole diameter effect on the flow characteristics of wake behind porous fences has been investigated experimentally in a circulating water channel having a test section of 300^w×200^h×1200^l (mm). Three porous fences having different hole diameters of d=1.4,2.1,2.8 mm were tested in this study, but they have the same =38.5% geometric porosity. One thousand instantaneous velocity fields for each fence were measured consecutively by the hybrid PTV system employing a high-speed CCD camera. Free stream velocity was fixed at 10 cm/sec and the corresponding Reynolds number based on the fence height was Re=2,985. Consequently, the fence with the smallest hole diameter d=1.4 mm (d_1.4) decreases the streamwise velocity component and increases the vertical velocity component. Among the three hole diameters tested in this study, the d_1.4 fence has the largest turbulence intensity in the shear layer developed from the fence top. Regardless of the hole diameter, however, all three fences having the same porosity reduce the reduction of turbulent intensity in the lower region below the fence height (y/H<1).     

激波在平板上诱导的旋涡强度的测量

运动激波通过两个等攻角平板后诱导出两个同向旋转的旋涡，这两个旋涡在随当地气流向下游运动的同时，绕涡核连线中心旋转。本文通过测量涡对的转动角度速度，获得了每个旋涡的强度。实验结果表明，由此测得的旋涡强度不同用于小攻角平板起动涡公式计算了起动涡强度。     

Flow development upstream of a fence

The effect of Reynolds number on the flow development upstream of a rigid, non-porous, static fence is investigated experimentally. The flow field is measured using time-resolved, two-component particle image velocimetry at Reynolds numbers based on fence height of 18000, 36000, and 54000. The results show that a laminar separation bubble forms upstream of the junction vortex at the base of the fence. The mean extent of the bubble decreases with increasing Reynolds number, with mean separation moving downstream and mean reattachment moving upstream. In the aft portion of the bubble, shear layer vortices form and are shed at scaled frequencies and wavelengths that are comparable to laminar separation bubble shedding in low Reynolds number airfoils and flat plates with an imposed adverse pressure gradient. The strong periodicity of the associated coherent structures and the proximity of shear layer roll-up relative to the fence should be taken into consideration in the relevant designs due to potential implications to structural loading. A simple flow separation prediction model combining inviscid fence flow solution with Thwaites’ method is introduced and shows good agreement with the experimental results for the Reynolds number range considered.     

GROUND EFFECT OF FLOW AROUND AN ELLIPTIC CYLINDER IN A TURBULENT BOUNDARY LAYER

The flow characteristics around an elliptic cylinder with an axis ratio of AR=2 located near a flat plate were investigated experimentally. The elliptic cylinder was embedded in a turbulent boundary layer whose thickness is larger than the cylinder height. For comparison, the same experiment was carried out for a circular cylinder having the same vertical height. The Reynolds number based on the height of the cylinder cross-section was 14000. The pressure distributions on the cylinder surface and on the flat plate were measured for various gap distances between the cylinder and the plate. The wake velocity profiles behind the cylinder were measured using hot-wire anemometry. In the near-wake region, the vortices are shed regularly only when the gap ratio is greater than the critical value of G/B=0·4. The critical gap ratio is larger than that of a circular cylinder. The variation of surface pressure distributions on the elliptic cylinder with respect to the gap ratio is much smaller than that on the circular cylinder. This trend is more evident on the upper surface than the lower one. The surface pressures on the flat plate recover faster than those for the case of the circular cylinder at downstream locations. As the gap ratio increases, the drag coefficient of the cylinder itself increases, but the lift coefficient decreases. For all gap ratios tested in this study, the drag coefficient of the elliptic cylinder is about half that of the circular cylinder. The ground effect of the cylinder at small gap ratio constrains the flow passing through the gap, and restricts the vortex shedding from the cylinder, especially in the lower side of the cylinder wake. This constraint effect is more severe for the elliptic cylinder, compared to the circular cylinder. The wake region behind the elliptic cylinder is relatively small and the velocity profiles tend to approach rapidly to those of a flat plate boundary layer