Dependence of flow characteristics of rectangular cylinders near a wall on the incident velocity

Numerically simulated results are presented for a family of rectangular cylinders with aspect ratios r ₁ (=b/a with height a and width b) ranging from 0.1 to 1.0 (square cylinder) to gain a better insight into the dependency of the aerodynamic characteristics on the operational dimensionless parameters, namely Reynolds number Re and aspect ratio r ₁. This work describes the flow from a long cylinder of rectangular cross-section placed parallel to a wall and subjected to a uniform shear flow. The flow is investigated in the laminar Reynolds number range (based on the incident stream at the cylinder upstream face and the height of the cylinder) at cylinder to wall gap height 0.5 times the cylinder height. The governing unsteady Navier?CStokes equations are solved numerically through a finite volume method on a staggered grid system using QUICK scheme for convective terms. The resulting equations are then solved by an implicit, time-marching, pressure correction-based SIMPLE algorithm for Reynolds number up to 1,000. The critical Reynolds numbers at which vortex shedding from the cylinder is started are specified for both the cases: far from the wall and near to the wall. It is reported that the vortex shedding from the rectangular cylinder of lower aspect ratio r ₁ (???0.25) becomes regular and insensitive to the Reynolds number, while the aerodynamic characteristics of the rectangular cylinders with higher aspect ratio r ₁ (???0.5) are strongly dependent on the Reynolds number.     

二维矩形柱体表面风压频域特性的雷诺数效应研究

在低紊流度的均匀流场中分别对截面宽厚比B/D(B为模型的顺风向宽度,D为迎风面厚度)为2、2.5、3、3.5和4的二维矩形柱体模型进行了测压试验,雷诺数(Re)的变化范围为1.1×105~6.8×105。分析了各模型表面的三个典型测点的脉动风压系数的功率谱特性,研究了各模型侧面的风压相关性。研究结果表明气流再附于模型侧面会影响模型侧面和背风面典型测点的风压系数功率谱,且B/D=4模型的三个典型测点的风压系数功率谱随雷诺数变化明显。B/D=2模型侧面的风压相关性较好,但B/D=2.5、3和3.5模型的侧面靠近前边缘的测点与气流再附区的测点的风压相关性较强,侧面中部区域的测点与侧面前后部区域的测点的风压相关性较差。B/D=4模型侧面的风压相关性受雷诺数影响明显,在雷诺数Re=6.8×105时出现了明显的突变。     

Aerodynamic forces on two stationary and oscillating square prisms in tandem and side by side arrangements

Abstract

In order to gain further understanding of aerodynamic forces and their effects on groups of high‐rise buildings, this study used wind‐tunnel experiments. Two square prisms were arranged both in tandem and side‐by‐side arrangement with different spacings in between. Similar experiments were carried out to study the interactions of aerodynamics between the two prisms when both were stationary, when only one prism oscillated, and finally, when both prisms oscillated.

The results showed that the aerodynamic responses were either enhanced or suppressed by the spacing ratios, the oscillating frequencies, and the mutual influences of the two square prisms in various arrangements. The aerodynamics also changed due to the occurrences of different flow patterns, such as channel flow, deflected flow, pulsating flow, and so on. Obviously, the aerodynamics of the flow patterns of the two square prisms in tandem and side‐by‐side arrangements proved to be more complex than those of a single square prism.     

PARTICLE DISPERSION BY COHERENT STRUCTURES IN FREE SHEAR FLOWS

ABSTRACT

The dispersion of particles in turbulent flows is poorly understood. Previous approaches to this problem have been found to be inadequate for nonisotropic turbulent flows. An approach involving a new physical concept is presented. This approach assumes that coherent vortex structures control the particle dispersion process in free shear flows. A simple computational model employing Stuart's vortices is used to simulate particle motion in a two-dimensional free shear layer. The results of this simulation are in reasonable agreement with previous experiments. For the first time, experimental observations indicating particle dispersion rates greater than fluid dispersion rates in free shear flows can be plausibly explained.     

Computational study of unsteady viscous flow around a transversely and longitudinally oscillating circular cylinder in a uniform flow at high reynolls numbers

A finite difference simulation method for the time dependent viscous incompressible flow around a transversely and longitudinally oscillating circular cylinder at Reynolds numbers of Re=4×10³ and 4×10⁴ is presented. The Navier-Stokes equations in finite difference form are solved on a moving grid system, based on a time dependent coordinate transformation. Solution of the vortex street development behind the cylinder is obtained when the cylinder remains stationary and also when it is oscillating. Time eholution of the flow configuration is studied by means of stream lines, pressure contours and vorticity contours. The computer results predict the lock-in phenomenon which occurs when the oscillation frequency is close to the vortex shedding frequency in the transverse mode or around double the vortex shedding frequency in the longitudinal mode. The time dependent lift and drag coefficients are obtained by the integration of the pressure and shear forces around the body. The drag, lift and the displacement relations are also discussed.     

Effects of an airfoil and shock waves on vortex shedding process behind a square cylinder

Summary Effects of an airfoil and shock waves on vortex shedding process behind a square cylinder have been examined experimentally at a Mach number of about 0.91 and at a Reynolds number (based on the side lengthD of the square cylinder) of about 4.2×10⁵. The main experimental parameter is the spacing ratioL/D, and is varied from 1.125 to 5.5, whereL is the spacing between the square cylinder and the airfoil.It is found that similarly to the case at subcritical Mach numbers at the supercritical Mach number there exist three patterns of the flow around the square cylinder and airfoil arranged in tandem depending upon the spacing ratioL/D: In the first flow pattern with small spacing ratio, the downstream airfoil is enclosed completely in the vortex formation region of the square cylinder. In the second flow pattern, the shear layers separating from the square cylinder reattach to the airfoil. In the third flow pattern with large spacing the shear layers roll up upstream of the airfoil. The Strouhal number at the supercritical Mach number is higher than that at the subcritical Mach numbers. Shock waves hasten the vortex shedding behind the square cylinder by decreasing the area of asymmetrical part of the vortex formation region with respect to the wake axis, and let the streamwise length of the separating shear layers longer than otherwise.With 8 Figures     

Vortex shedding suppression for laminar flow past a square cylinder near a plane wall: a two-dimensional analysis

Summary A numerical study on the uniform shear flow past a long cylinder of square cross-section placed parallel to a plane wall has been made. The cylinder is considered to be within the boundary layer of the wall. The maximum gap between the plane wall to the cylinder is taken to be 0.25 times the cylinder height. We investigated the flow when the regular vortex shedding from the cylinder is suppressed. The governing unsteady Navier-Stokes equations are discretized through the finite volume method on staggered grid system. A pressure correction based iterative algorithm, SIMPLER, has been used to compute the discretised equations iteratively. We found that the critical value of the gap height for which vortex shedding is suppressed depends on the Reynolds number, which is based on the height of the cylinder and the incident stream at the surface of the cylinder. At high Reynolds number (Re ≥ 500) however, a single row of negative vortices occurs for wall to cylinder gap height L ≥ 0.2. The shear layer that emerges from the bottom face of the cylinder reattaches to the cylinder itself at this gap hight.     

Dynamic response of an elastic fence under wind action

Abstract

The along‐wind response of a surface‐mounted elastic fence under the action of wind was investigated numerically. In the computations, two sets of equations, one for the simulation of the unsteady turbulent flow and the other for the calculation of the dynamic motion of the fence, were solved alternatively. The resulting time‐series tip response of the fence as well as the flow fields were analyzed to examine the dynamic behaviors of the two.

Results show that the flow is unsteady and is dominated by two frequencies: one relates to the shear layer vortices and the other one is subject to vortex shedding. The resulting unsteady wind load causes the fence to vibrate. The tip deflection of the fence is periodic and is symmetric to an equilibrium position, corresponding to the average load. Although the along‐wind aerodynamic effect is not significant, the fluctuating quantities of the tip deflection, velocity and acceleration are enhanced as the fundamental frequency of the fence is near the vortex or shedding frequency of the flow due to the occurrence of resonance. In addition, when the fence is rather soft, a higher mode response can be excited, leading to significant increases in the variations of the tip velocity and acceleration.     

箱型吊杆的风致振动特性

该文通过风洞试验研究了箱型吊杆的风振特性,并进一步分析了紊流对吊杆气动性能的影响。箱型吊杆会发生涡振与驰振失稳,高宽比对涡振特性存在较显著影响,对改善驰振性能作用不大。紊流对箱型吊杆涡振起振风速及锁定区间有较大影响,但紊流的作用并不总是有利的。相应的数值模拟显示,紊流是通过改变结构的St数与涡脱力的大小来影响结构的涡振特性。     

Effect of Static Deformation on Basic Flow Patterns in Thermocapillary-Driven Free Liquid Film

A series of terrestrial, parabolic-flight and on-orbit experiments on thermocapillary-driven flows in free liquid films are carried out. We focus on the basic flow patterns induced in the film formed in a rectangular hole by varying the film volume in order to make a comparison with the results of the fluid physics experiments under microgravity conditions conducted by one of the authors, Pettit, on the International Space Station. The free liquid film is formed in a rectangular hole of O(0.1 mm) in thickness under a designated temperature difference between the end walls. The temperature dependence of the surface tension results in a non-uniform surface tension distribution over the free surfaces. A liquid generally has a negative temperature coefficient of surface tension; i.e., the fluid over a free surface is driven from a higher-temperature region to a lower-temperature region. In the case of a thin free liquid film with two free surfaces, however, an unusual flow pattern is realized. That is, the fluid seems to be driven toward the heated region from a colder region. In order to understand the physical mechanism of this behavior in the free liquid film, a series of on-orbit and ground experiments were conducted. We indicate several flow patterns in the film and corresponding film profiles as well as the surface temperature distribution. We also try to illustrate the cross-sectional flow structures in the thin free liquid film with two free surfaces.     

Shear flow past a square cylinder near a wall

A numerical study on the wake behind a square cylinder placed parallel to a wall has been made. The cylinder is considered to be within the boundary layer of the wall, so that the outside flow is taken to be due to uniform shear. Flow has been investigated in the laminar Reynolds number (based on the cylinder height) range. The interaction of wall boundary layer on the vortex shedding at Reynolds number up to 1400.0 has been investigated for cylinder to wall gap height 0.5 and 0.25 times the cylinder height. The gap flow between the cylinder and wall during a period of vortex shedding has been obtained. The governing unsteady Navier–Stokes equations are discretised through the finite volume method on staggered grid system. An algorithm SIMPLE has been used to compute the discretised equations iteratively. Our results show that at the gap height 0.5 times the cylinder height the vortex shedding occurs at a Strouhal number greater than for an isolated cylinder. Vortex shedding suppression occurs and wake becomes steady up to a certain value of Reynolds number at gap height 0.25 time the cylinder height. At higher Reynolds number the formation of a single row of negative vortices behind the cylinder when it is in close proximity to wall is noteworthy. Due to the shear, the drag experienced by the cylinder is found to decrease with the reduction of gap height.     

SPLASH program for three dimensional fluid dynamics with free surface boundaries

This paper describes a three dimensional computer program SPLASH that solves Navier-Stokes equations based on the Arbitrary Lagrangian Eulerian (ALE) finite element method. SPLASH has been developed for application to the fluid dynamics problems including the moving boundary of a liquid metal cooled Fast Breeder Reactor (FBR). To apply SPLASH code to the free surface behavior analysis, a capillary model using a cubic Spline function has been developed. Several sample problems, e.g., free surface oscillation, vortex shedding development, and capillary tube phenomena, are solved to verify the computer program. In the analyses, the numerical results are in good agreement with the theoretical value or experimental observance. Also SPLASH code has been applied to an analysis of a free surface sloshing experiment coupled with forced circulation flow in a rectangular tank. This is a simplified situation of the flow field in a reactor vessel of the FBR. The computational simulation well predicts the general behavior of the fluid flow inside and the free surface behavior. Analytical capability of the SPLASH code has been verified in this study and the application to more practical problems such as FBR design and safety analysis is under way.     

倒角切角对方柱气动性能影响的大涡模拟研究

倒角和切角措施对方柱的气动力及流场影响很大,常作为方柱流动控制的手段,采用大涡模拟方法,以雷诺数22000的方柱为研究对象,考虑了角部措施(角部变化率10%)的影响,对均匀流场下标准方柱、倒角和切角方柱周围流场及气动性能进行了模拟研究。通过将标准方柱大涡模拟结果与相关文献的试验和数值模拟结果对比,验证了该方法及参数取值的有效性;研究分析了倒角和切角措施对方柱风压分布和气动力的影响,并着重从时均流场和瞬态流场角度分析了角部处理措施对方柱气动性能的影响机理。结果表明,倒角和切角措施对方柱表面风压分布和气动力均有一定影响,其中对方柱表面流动分离区的风压系数影响更为显著。采用角部处理措施后,方柱前缘角区的流动分离受到影响,分离剪切层扩散角更小,侧面的分离涡更贴近壁面,从而在方柱侧面形成再附,尾流变窄,旋涡脱落频率成分更为复杂,使得方柱的平均阻力系数更小,气动力脉动强度更弱,旋涡脱落频率更高、强度更弱。     

Effects of a small airfoil-shaped splitter plate on vortex shedding from a square prism at subsonic Mach numbers

Summary The present study is concerned with effects of a small airfoil-shaped splitter plate (NACA 0018, chord lengthc=20 cm) on the vortex shedding from a single square prism (side lengthD=20 mm) at free-stream Mach numbers between 0.15 and 0.91 and a constant spacing ratioL/D=2.0, whereL is the central distance between the square prism and airfoil arranged in tandem.It is found that while there exist no shock waves in the flow the vortex shedding from a single square prism is retarded by the small airfoil due to the interaction with the upper and lower separating shear layers: The Strouhal number for the square prism and airfoil being arranged in tandem, is almost independent of the Mach number and takes about 0.11. This is smaller than the value of 0.13 known for a single square prism. However, as soon as shock waves appear in the flow, the Strouhal number increases suddenly and then increases with increasing the Mach number. It is inferred that the sudden increase of the Strouhal number is primarily caused by shock waves appearing above and below the vortex formation region, for the shock waves make the vortex formation region small and symmetrical with respect to the common axis of the square prism and airfoil. That means, the small airfoil causes only a secondary effect on the vortex shedding from a single square prism under the presence of shock waves in the flow.     

Faceted design of channels for low-dispersion electrokinetic flows in microfluidic systems

A novel methodology for designing microfluidic channels for low-dispersion, electrokinetic flows is presented. The technique relies on trigonometric relations that apply for ideal electrokinetic flows, allowing faceted channels to be designed using common drafting software and a hand calculator. Flows are rotated and stretched along the abrupt interface between adjacent regions having differing specific permeability--a quantity with dimensions of length that we introduce to derive the governing equations. Two-interface systems are used to eliminate hydrodynamic rotation of bands injected into channels. Regions bounded by interfaces form faceted flow "prisms" with uniform velocity fields that can be combined with other prisms to obtain a wide range of turning angles and expansion ratios. Lengths of faceted prisms can be varied arbitrarily, simplifying chip layout and allowing the ability to reduce unwanted effects such as transverse diffusion and Joule heating for a given faceted prism. Designs are demonstrated using two-dimensional numerical solutions of the Laplace equation.     

倒角化处理对于矩形高层建筑风荷载特性的影响机理研究

通过风洞测压试验,对比不同风向下、不同倒角半径的矩形高层建筑表面风压分布、整体风力及斯托罗哈数St;采用PIV试验,给出建筑的近尾流流动特性,并从流场作用角度,揭示倒角化处理对于矩形高层建筑风荷载特性的影响机理.研究表明:临界风向下,在建筑一侧分离的剪切层发生流动再附,形成分离泡;此时,建筑的阻力达谷值,升力和St达最...     

Simulations of Vortex Evolution and Phase Slip in Oscillatory Potential Flow of the Superfluid Component of 4He Through an Aperture

The evolution of semicircular quantum vortex loops in oscillating potential flow emerging from an aperture is simulated in some highly symmetrical cases. As the frequency of potential flow oscillation increases, vortex loops that are evolving so as eventually to cross all of the streamlines of potential flow are drawn back toward the aperture when the flow reverses. As a result, the escape size of the vortex loops, and hence the net energy transferred from potential flow to vortex flow in such 2π phase-slip events, decreases as the oscillation frequency increases. Above some aperture-dependent and flow-dependent threshold frequency, vortex loops are drawn back into the aperture. Simulations are performed using both radial potential flow and oblate-spheroidal potential flow.     

Flows induced by a plate moving normal to stagnation-point flow

The flow generated by an infinite flat plate advancing toward or receding from a normal stagnation-point flow is obtained as an exact reduction of the Navier?CStokes equations for the case when the plate moves at constant velocity V. Both Hiemenz (planar) and Homann (axisymmetric) stagnation flows are considered. In each case, the problem is governed by a Reynolds number R proportional to V. Small and large R behaviors of the shear stress parameters are found for both advancing and receding plates. Numerical solutions determined over an intermediate range of R accurately match onto the small and large R asymptotic behaviors. As a side note, we report an interesting exact solution for plates advancing toward or receding from an exact rotational stagnation-point flow discovered by Agrawal (1957).     

Periodic Oscillations of Josephson-Vortex Flow Voltage in Stacked Intrinsic Josephson Junctions

Using the coupled sine-Gordon equations, we study the magnetic oscillation behavior of the Josephson-vortex flow voltage (JVFV) in stacked intrinsic Josephson junctions (IJJs). It is found that the periodic oscillations of the JVFV with the magnetic field are determined by the lattice structure. In narrow IJJs with a high density of vortices, the boundary interaction is favorable for forming a rectangular lattice structure and inducing H ₀ oscillations. The oscillation period is equal to the magnitude of the field needed to add one vortex quantum per one intrinsic Josephson junction. In wide IJJs, the shearing inter-layered interaction is favorable for forming a triangular lattice structure and inducing H ₀/2 oscillations. In this case, with increasing magnetic field, a transformation from the triangular (with period H ₀/2) to rectangular (with period H ₀) configurations is also obtained in a long lateral size. Besides, from the magnetic oscillation characteristics of the JVFV in wide IJJs, the oscillating inversions have also been obtained.