通道中串列旋转圆柱的格子Boltzmann-虚拟区域方法的模拟

通过结合格子Boltzmann方法(LBM)和虚拟区域(Fictitiou sDomain)思想,建立格子Boltzmann-虚拟区域(LB-DF/FD)方法.采用两套网格系统,欧拉网格用于流体,拉格朗日网格用于固体.原有的LBM在计算运动固体的受力方面存在数据振荡,LB-DF/FD方法改进了此缺陷.为验证该方法,模拟圆柱绕流、圆形颗粒在无限长通道中平动及在无限大流场中转动三种情况,结果与其他数值解及理论解符合得很好.利用该方法模拟低雷诺数下通道中串列旋转圆柱周围的流场,分析圆柱间距(g)及雷诺数(Re)对流场结构的影响.给出Re=0.001,0.1和10下,0.2≤g≤8.0的流线结构、圆柱升力、阻力以及力矩等数值结果.结果表明,g对流场的结构及圆柱的受力有显著影响,Re对圆柱阻力及Stokes单元数目的影响较大.     

A numerical study of transient flow around a cylinder and aerodynamic sound radiation

The fully 3D turbulent incompressible flow around a cylinder and in its wake at a Reynolds number Re = = 9×10⁴ based on the cylinder diameter and Mach number M = 0.1 is calculated using Large Eddy Simulations (LES). Encouraging results are found in comparison to experimental data for the fluctuating lift and drag forces. The acoustic pressure in far-field is commutated through the surface integral formulation of the Ffowcs Williams and Hawkings (FWH) equation in acoustic analogy. Five different sound sources, the cylinder wall and four permeable surfaces in the flow fields, are employed. The spectra of the sound pressure are generally in quantitative agreement with the measured one though the acoustic sources are pseudo-sound regarding the incompressible flow simulation. The acoustic component at the Strouhal number related to vortex shedding has been predicted accurately. For the broad band sound, the permeable surfaces in the near wake region give qualitative enough accuracy level of predictions, while the cylinder wall surface shows a noticeable under-prediction. The sound radiation of the volumetric sources based on Lighthill tensors at vortex shedding is also studied. Its far-field directivity is of lateral quadrupoles with the weak radiations in the flow and cross-flow directions.     

Cylinder wake in lattice gas hydrodynamics

We measure, using a lattice gas algorithm, the time development of a wake behind a cylinder at Reynolds numbers equal to 30 and 60. We measure also drag at these values and determine Re90 to be the Reynolds number where vortex shedding starts. Agreement with experimental results is good. We also measure vortex shedding frequency at Re = 108 from transverse velocity and at Re = 180 from lift on the cylinder. The corresponding Strouhal numbers are obtained and discussed.     

Hydrodynamic binary coalescence of droplets under air flow in a hydrophobic microchannel

Based on the volume of fluid(VOF) method, we conduct a numerical simulation to study the hydrodynamic binary coalescence of droplets under air flow in a hydrophobic rectangular microchannel. Two distinct regimes, coalescence followed by sliding motion and that followed by detaching motion, are identified and discussed. Additionally, the detailed hydrodynamic information behind the binary coalescence is provided, based on which a dynamic mechanical analysis is conducted to reveal the hydrodynamic mechanisms underlying these two regimes. The simulation results indicate that the sliding motion of droplets is driven by the drag force and restrained by the adhesion force induced by the interfacial tension along the main flow direction. The detachment(i.e., upward motion) of the droplet is driven by the lift force associated with an aerodynamic lifting pressure difference imposed on the coalescent droplet, and also restrained by the adhesion force perpendicular to the main flow direction. Especially, the lift force is mainly induced by an aerodynamic lifting pressure difference imposed on the coalescent droplet. Two typical regimes can be quantitatively recognized by a regime diagram depending on Re and We. The higher Re and We respectively lead to relatively larger lift forces and smaller adhesion forces acting on the droplet, both of which are helpful to detachment of the coalesced droplet.     

过渡流下近壁插入圆柱对壁面强化传热的影响

本文利用基于复合网格系统的计算方法,对Re=50~1200的近壁插入圆柱流场进行数值模拟,研究过渡流状态下在壁面附近插入圆柱对下游壁面传热强化的影响。并基于低速循环水槽流动实验台,采用粒子成像测试法(PIV)对Re=100~500的近壁插入圆柱流场进行可视化实验研究,验证了数值模拟方法的可靠性。研究结果表明:近壁插入圆柱流场在Re=100时进入过渡流状态;Re直接影响圆柱尾流中周期性涡脱和壁面涡岛的发生位置及其洗刷效应的大小,随着Re的增大,洗刷效应明显增强,因而,过渡流范围内Re越大,圆柱下游壁面传热强化越大。     

电磁力控制湍流边界层分离圆柱绕流场特性数值分析

在亚临界区高雷诺数Re=1.4×105下,采用脱体涡模拟结合湍流分离的方法对弱电解质中电磁力作用下湍流边界层分离圆柱绕流场及其升(阻)力特性进行了数值模拟和分析.结果表明,电磁力可以提高圆柱体湍流边界层内的流体动能,延缓圆柱体湍流边界层的流动分离,减弱圆柱体湍流绕流场中在流向和展向上大尺度漩涡的强度,减小圆柱体阻力时均值及其升力脉动幅值.当电磁力作用参数大于某个临界值后,湍流边界层流动分离消失,在圆柱体尾部产生射流现象,从而电磁力对圆柱体产生净推力作用,出现负阻力现象,而且升力脉动幅值接近于零,出现圆柱体升力消失现象.     

Structure-based interpretation of the Strouhal-Reynolds number relationship

The wake in the flow past a circular cylinder has posed a long-standing challenge to scientists since the late 19th century. Many aspects of this seemingly simple phenomenon remain unexplained. Of particular interest is the relationship between the dimensionless vortex shedding frequency (the Strouhal number St) and the ratio of inertial to viscous forces in the fluid (the Reynolds number Re). We propose a new St-Re relation based on the observations of the structure of a vortex street in flowing soap films. The measurements suggest a simple two-parameter form St=1/(A+B/Re) that describes laminar vortex shedding remarkably well for bulk fluids as well as for two-dimensional flowing soap films.     

高超临界雷诺数区间内二维圆柱绕流的实测研究

实测强风工况下高度167 m的徐州彭城电厂冷却塔的表面风荷载,并归纳历史上其他研究人员给出的实测结果,以丰富高超临界雷诺数(Re)区间二维圆柱绕流的试验成果.在低湍流度均匀流场和高湍流度大气边界层流场中分别开展4种风速8类粗糙度条件下的冷却塔刚性模型测压风洞试验,通过对比低雷诺数(Re=2.1×10~5—4.19×10~5)条件下的风洞试验结果和高雷诺数(Re=5.4×10~7—1×10~8)条件下的现场实测结果研究各种静动态绕流特征随雷诺数的变化规律,重点考察雷诺数无关现象的产生条件.研究结果表明,对于物表相对粗糙度在0.01以上的圆柱绕流,雷诺数不相关现象存在于很宽的雷诺数范围(2×10~5Re1×10~8)内;增大来流湍流度亦能引起的雷诺数无关现象,但此时该现象可能仅存在于一个较窄的低雷诺数范围内.     

串列对转双圆柱的多块LB模拟

结合格子玻尔兹曼方法(Lattice Boltzmann Method,简称LBM)和多块网格(Multi-Block Grid)技术,数值研究较小雷诺数(Re=100)下均匀来流绕串列对转双圆柱问题,综合分析圆柱中心间距比S/D和圆柱无量纲旋转速度α对流场结构的影响,考察前、后圆柱的涡脱落形态和升阻力特性.结果表明:当间距比为1.2时存在一临界旋转比α_c,转速超过这一值后前圆柱产生负Magnus效应;间距比为2时,流场出现类似单旋转圆柱时的第二不稳定模态;当间距比较大(S/D=4、6)时,前、后柱之间存在涡脱落,后柱尾涡中出现2S、2S^*、2P、P+S等多种形态.     

圆柱绕流问题的初期流动数值模拟

本文继Zheng-huan Teng[1]中介绍的解Navier-stokes方程的椭圆涡团法,研究了一种新的变形涡团法,用以模拟不可压粘性流体绕圆柱的不定常流动。圆柱在静止流体中突然起动并做匀速直线运动。对整个流动区域构造完全Navier-Stokes方程的解并不容易,近十年出现很多数值研究,本文对算法有所推进。把本文的方法称作变形涡团法是因为圆柱边界附近的流体中用椭圆涡团,远离边界时用圆形涡团。计算圆柱绕流比平板绕流在满足附着条件上更为困难,本文分析了怎样在圆柱边界上给出适当的附着条件的数值方法。在算例中雷诺数分别取200、550、3000,得到了不定常边界层分离,二次涡等复杂的物理现象,这些数值结果与近年实验结果[2]是一致的。     

Solving Navier–Stokes equation for flow past cylinders using single-block structured and overset grids

Many complex fluid motions are driven by physical processes of instability, transition and turbulence dependent upon nonlinear mechanisms. Here, we solve the flow past cylinder(s) using single-block structured and overset grids by computing Navier–Stokes equation in two-dimensions. The suitability of a compact scheme in discretizing convection and diffusion terms are investigated first by looking at relevant numerical properties. Also, for the overset grid method, one of the methods is identified that shows the best results in minimizing interpolation error at sub-domain boundaries for an analytical test function. We provide extensive comparisons with experimental and other computational results for flow past a single cylinder, utilizing both single-block structured and Chimera or overset grids. Apart from showing instability of this flow calculated by these methods, we also compare the computed vorticity and velocity data using these two grids by employing the proper orthogonal decomposition (POD). We have analyzed and developed an overset grid method with compact scheme that does not need any filtering to control error. This has been ascertained by performing POD analysis. To show that the developed method is capable of handling complex geometries, we have computed flow past two cylinders in side-by-side arrangement. Results obtained capture the known flow characteristics for this arrangement well using relatively fewer number of grid points.     

Flow characteristics around a rotating grooved circular cylinder with grooved of different depths

The present paper describes the flow characteristics around a rotating grooved circular cylinder with grooves of different depths. The surface structure of a circular cylinder was varied by changing the depths of 32 arc grooves on the surface. The surface pressure on the cylinder is measured for theRe range of from 0.4×10⁵ to 1.8×10⁵ and for rotations of from 0 to 4500 rpm. The drag coefficient of a grooved cylinder increases as the spin rate ratio α (= rotational speed of the cylinder surface/uniform velocity) increases forRe>1.0×10⁵. As the groove depth increases, the drag coefficient of a grooved cylinder is independent from the spin rate ratio α. The direction of the lift force of a smooth cylinder is opposite to the Magnus force forRe>1.0×10⁵. However, the direction of the lift force of a grooved cylinder is the same as that of the Magnus force for allRe>1.0×10⁵. As the groove depth increases, the increase in the slope of the lift coefficient becomes small. These phenomena are related to the positions of the flow separation points, which are clarified from the pressure distribution and flow visualization by the spark tracing method. In addition, in the present study, the flow around a rotating grooved cylinder is clarified by flow visualization.     

低Re数非线性绕流问题的一种算法

本文对低Re数非线性绕流问題提出了一种算法。做法是以Oseen线化方程为基础通过迭代修正来考虑非线性的惯性项的影响,即先将Navier-Stokes方程写作Oseen方程外加一"强迫函数"项的形式,然后用基本解‘Oseenlet’的积分形式给出其解式,它是一个含物面积分和流动空间积分的非线性积分微分方程。设沿边界上的‘Oseenlet’分布强度为待求量,由边条件加以确定。空间体积分由于有赖于流场,而它在求出解前是未知的,故采用迭代修正的做法进行处理。迭代过程从线性流场开始进行,直到算出达到规定精度要求的收敛解为止。作为算例,对圆柱绕流问題进行了计算,给出了圆柱阻力随Re数的变化规律,并同实验及有关计算资料做了比较。结果表明,本文方法是令人满意的。     

Flow pattern and lift evolution of hydrofoil with control of electro-magnetic forces

The initial responses and evolutions of the flow pattern and lift coefficient of a hydrofoil under the action of electro-magnetic (Lorentz) force have been studied experimentally and numerically, and trace particle methods are employed for them. With the introduction of BVF (boundary vortex flux), the quantitative relation among Lorentz forces, BVF and lifts is deduced. The influences of flow patterns on the hydrofoil lift coefficient have been discussed based on the BVF distribution, and the flow control mechanism of Lorentz force for a hydrofoil has been elucidated. Our results show that the flow pattern and lift of the hydrofoil vary periodically without any force. However, with the action of streamwise Lorentz forces, the separation point on the hydrofoil surface moves backward with a certain velocity, which makes the flow field steady finally. The streamwise Lorentz force raises the foil lift due to the increase of BVF intensity. On the other hand, Lorentz force also increases the hydrofoil surface pressure, which makes the lift decrease. However, the factor leading to the lift enhancement is determinant, therefore, the Lorentz force on the suction side can increase the lift, and the stronger the Lorentz force, the larger the lift enhancement. Our results also show that the localized Lorentz force can also both suppress the flow separation and increase the hydrofoil lift coefficient, furthermore, the Lorentz force located on the tail acts better than that located on the front.     

Use of the wake of a small cylinder to Control unsteady loads on a circular cylinder

Simultaneous measurements of lift and drag forces have been performed in order to study passive control of unsteady loads induced on a circular cylinder. For this purpose, an aerodynamic balance has been developed. The balance, developed for a cylinder of 25.4 mm in diameter, was designed to operate in the subcritical regime (Re=32000). This instrument is characterized by its sensitive element that forms a small central part of the cylinder. The static and dynamic calibrations of the balance show the appropriateness of the present design. Moreover, qualification experiments carried out with a single cylinder gave results (mean and rms values of the lift and drag coefficients) that are in good agreement with those found in the literature. The purpose of this paper is to present a passive control experiment performed by means of the wake of a smaller cylinder interacting with a larger one. Firstly, a parametric study was performed by varying the following: i) the diameterdS of the small cylinder for one large cylinder diameterd (7 values in the range 0.047≤d _S /d≤0.125); ii) the center-to-center spacingS/d (11 values in the range 1.375≤S/d≤2.5); and iii) the stagger anglea(0≤a≤90 with a fine angular stepDa fora≤15). A maximum mean drag reduction of about 48% is achieved. Ata=4 to 8, one can observe a peak of mean lift coefficient. Then unsteady fluid forces, vortex shedding frequency and flow pattern were systematically investigated for the small cylinder having a diameterdS=2.4 mm (0.094d). Reductions of 78% and 56% of the rms lift and drag fluctuations respectively were obtained with the small cylinder placed at a slight stagger angle in the range 6<a<9. This leads to an instantaneous force vector that exhibits more steadiness both in angle and amplitude. Moreover, at these stagger angles, the energy of the lift fluctuations at the shedding frequency is significantly reduced compared to the single cylinder case.     

A non-linear fluid force model for vortex-induced vibration of an elastic cylinder

Even under the assumption of a sinusoidal lift and drag force at a single frequency for a stationary cylinder in a cross flow, higher harmonics that represent non-linearity in the fluid-structure interaction process are present. This fact is considered in the formulation of a non-linear fluid force model for a freely vibrating cylinder in a cross flow. The force model is developed based on an iterative process and the modal analysis approach. The fluid force components in the model can be evaluated from measured vibration data with the help of the auto-regressive moving averaging (ARMA) technique. An example is used to illustrate that non-linear (higher order) force components are present at resonance, even for a case with relatively weak fluid-structure interaction. Further analysis reveals that the fluid force components are dependent on structural damping and mass ratio. The non-linear fluid force model is further modified by taking these considerations into account and is used to predict the dynamic characteristics of a freely vibrating cylinder over a range of Reynolds numbers, mass and structural damping ratios. On comparison with measurements obtained from four different experiments and predictions made by previous single-degree-of-freedom model, good agreement is found over a wide range of these parameters.     

Moment and forces exerted on the inner cylinder in eccentric annular flow

This paper presents results of numerical modeling for analysis of the moment and forces exerted on an eccentrically positioned rotating inner cylinder due to the annular flow between two cylinders with parallel axes. Laminar stationary fully developed flows of Newtonian and power law fluid flows are considered. An impact of annulus geometry, flow regime, and fluid characteristics are studied. The study indicates that the moment exerted on the inner cylinder increases monotonically with the eccentricity. Forces acting on the inner cylinder include pressure and viscous friction. The pressure forces provide a predominant contribution. When eccentricity does not exceed a certain critical value, the radial force pushes the inner cylinder to the channel wall. When eccentricity is large enough, the radial force reverses its sign, and the inner cylinder is pushed away from the outer wall. Circumferential component of the force has always the same direction and induces precession of the inner cylinder.     

利用顶部柔膜颤振抑制三维方形棱柱气动力

通过风洞实验研究了高宽比H/d=5的正方形棱柱顶部柔性薄膜颤振对柱体气动力特性的影响规律.模型宽度为40 mm,来流风速4~20 m/s,对应Reynolds数为10 960~54 800.柔性薄膜为厚度0.04 mm的高压聚乙烯膜,长度l=0.5~4 cm不等.实验发现,柔性薄膜的振动状态对三维方柱气动力特性有显著影响.低风速下,柔性薄膜不发生颤振,其对方柱气动力影响不大;随风速的增加,柔性薄膜发生颤振,方柱的时均阻力、脉动阻力和脉动侧向力最多分别减少约5%,25%和60%.柔膜发生颤振的临界风速随其长度增大而减小.只要柔膜发生颤振,其对柱体气动力的影响都是类似的,与其长度无关.流动可视化实验发现,薄膜的颤振改变了方柱绕流场,使柱体上半部分的反对称展向漩涡变为受膜拍动控制的对称结构,且越靠近自由端,此现象越明显.      

Open loop control of vortex-induced vibration of a circular cylinder

In this paper both numerical and experimental investigations have been carried out to suppress the vortex-induced vibration (VIV) of a circular cylinder in an electrically low-conducting fluid. The electromagnetic forces (Lorentz forces) in the azimuthal direction were generated through the mounted electrodes and magnets locally on the surface of the cylinder, which have been proved having an accelerating effect to the fluid on the surface of the cylinder. Results of computations are presented for synchronous vibration phenomenon of a cylinder at Re=200, which are in good agreement with previous computational results. With the Lorentz forces loaded, the VIV of the cylinder has been suppressed successfully. Experimental results have also shown the same tendency and are in reasonable agreement with the numerical results.     

Numerical study of the influence of flow blockage on the aerodynamic coefficients of models in low-speed wind tunnels

With the use of ANSYS Fluent software and ANSYS ICEM CFD calculation grid generator, the flows past a wing airfoil, an infinite cylinder, and 3D blunted bodies located in the open and closed test sections of low-speed wind tunnels were calculated. The mathematical model of the flows included the Reynolds equations and the SST model of turbulence. It was found that the ratios between the aerodynamic coefficients in the test section and in the free (unbounded) stream could be fairly well approximated with a piecewise-linear function of the blockage factor, whose value weakly depended on the angle of attack. The calculated data and data gained in the analysis of previously reported experimental studies proved to be in a good agreement. The impact of the extension of the closed test section on the airfoil lift force is analyzed.