Investigation of a hybrid approach combining experimental tests and numerical simulations to study vortex-induced vibration in a circular cylinder

In this study, a hybrid approach based on computational fluid dynamics (CFD) was used to investigate the aerodynamic forces associated with vortex-induced vibration (VIV) in a circular cylinder. The circular cylinder and the flow field were considered as two substructures of a system. Circular cylinder motion was produced in a wind tunnel test of the VIV prior to the numerical simulation; this motion was used as a known cylinder boundary condition and applied to the flow field. The flow field with the known moving boundary condition was then numerically simulated by the ANSYS CFX code. The transient aerodynamic coefficients of the circular cylinder with predetermined motion were obtained from the numerical simulation. To verify the feasibility and accuracy of the proposed hybrid approach and to calculate cylinder vibrations, the transient aerodynamic coefficients were applied to a single degree of freedom (SDOF) model of the circular cylinder. The oscillation responses of the circular cylinder from the calculated (SDOF model) and experimental results were compared, and the results indicate that the hybrid approach accurately simulated the transient aerodynamic coefficients of the circular cylinder. For further comparison, a nonlinear aerodynamic coefficient model based on a nonlinear least square technique was applied to the SDOF model. The nonlinear aerodynamic model can predict well the amplitude and lock-in region of the VIV of the circular cylinder model.     

Electro-Magnetic Control of Vortex Shedding Behind a Circular Cylinder

Both open- and closed-loop control algorithms have been developed for suppressing of vortex shedding behind a circular cylinder in an electrically low-conducting fluid. For the open-loop control, the localized Lorentz forces are generated parallel to the cylinder surface, which have the accelerated effect to the fluid. Furthermore, two closed-loop control methods have been derived from the equations of motion capable of determining at all times the intensity of the Lorentz force to control the vortex shedding of a cylinder.     

翼型绕流电磁控制的实验和数值研究

分布在弱电介质溶液中的电磁力(Lorentz力),可以有效地控制边界层的流动.利用以转动水槽为主的实验系统和基于双时间步Roe格式的数值方法,对翼型绕流的电磁控制进行了实验和数值研究.结果表明,对于一定攻角的翼型,电磁力可以控制其绕流形态.当电磁力方向与流动方向相同时,可以抑制分离,消除涡街,其效果与减小攻角类似.当电磁力的方向与流动方向相反时,可在流场中形成大涡组成的涡街,增强流体的混合能力,其效果与增大攻角类似. 关键词： 电磁力 翼型绕流 流体控制     

翼型绕流电磁控制的实验和数值研究

分布在弱电介质溶液中的电磁力(Lorentz力)，可以有效地控制边界层的流动.利用以转动水槽为主的实验系统和基于双时间步Roe格式的数值方法，对翼型绕流的电磁控制进行了实验和数值研究.结果表明，对于一定攻角的翼型，电磁力可以控制其绕流形态.当电磁力方向与流动方向相同时，可以抑制分离，消除涡街，其效果与减小攻角类似.当电磁力的方向与流动方向相反时，可在流场中形成大涡组成的涡街，增强流体的混合能力，其效果与增大攻角类似.     

电磁力滤波与快速反射镜光学补偿在潜航器光轴稳定控制中的应用

搭载在潜航器上的光电桅杆是光电跟瞄的重要装置.当潜航器在水下高速行进时,海水会在物体表面形成脱体边界层和涡街,涡街的生成和脱体会引起阻力和升力的大幅度波动,从而对光轴稳定性产生极大的扰动.本文首先基于电磁场和流体力学的基本控制方程,通过层次结构网格下的有限体积法探讨了电磁流体表面控制对潜航器绕流流场的影响和消涡减振效果;其次,分析并获得了快速反射镜(fast steering mirror,FSM)的结构特性、传递函数和PID控制策略;最后,以潜航器光路模型为研究背景,结合电磁流体的滤波特性和FSM的传递函数,论证了复合控制对潜载光电跟瞄系统稳定性提高的效果.结果表明,壁面流向电磁力能很好地调控潜航器绕流边界层,抑制涡激振动、减少光学系统的输入噪声,在此基础上通过FSM实现二次补偿,可以进一步提高光学系统跟踪的精度.本研究是电磁流体控制在光电领域的探索,也是对传统流体力学实验方法的拓展,因此具有一定的科学意义和实用价值.     

基于有限差分法刚性圆柱体涡激振动响应特性数值研究

基于尾流振子模型对刚性圆柱体涡激振动响应特性进行数值研究.建立圆柱体结构振子及尾流振子之间的耦合方程,基于二阶精度中心差分格式对耦合模型先离散后迭代进行求解.对不同质量比及不同阻尼比圆柱体涡激振动响应的无量纲位移、无量纲升力、频率比及锁定区间等参数进行分析.结果表明数值方法可以很好地模拟刚性圆柱体的涡激振动响应特性.随着质量比的增加,锁定开始点逐渐延后,锁定结束点逐渐提前,锁定区间宽度逐渐变窄.     

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.     

基于浸入边界-多松弛时间格子玻尔兹曼通量求解法的流固耦合算法研究

针对流固耦合问题,发展了基于浸入边界-多松弛时间格子玻尔兹曼通量求解法(immersed boundary method multi-relaxation-time lattice Boltzmann flux solver,IB-MRT-LBFS)的弱耦合算法.依据多尺度Chapman-Enskog展开,建立不可压宏观方程状态变量和通量与格子玻尔兹曼方程中粒子密度分布函数之间的关系;采用强制浸入边界法处理流固界面使固壁表面满足无滑移边界条件,根据修正的速度求解动量方程力源项;结构运动方程采用四阶龙格-库塔法求解.格子模型与浸入边界法的引入使流固耦合计算可以在笛卡尔网格下进行,无需生成贴体网格及运用动网格技术,简化了计算过程.数值模拟了单圆柱横向涡激振动、单圆柱及串列双圆柱双自由度涡激振动问题.结果表明,IB-MRT-LBFS能够准确预测圆柱涡激振动的锁定区间、振动响应、受力情况以及捕捉尾流场结构形态,验证了该算法在求解流固耦合问题的有效性和可行性.     

Effects of uniform surface roughness on vortex-induced vibration of towed vertical cylinders

The present study was motivated by a desire to understand the vortex-induced vibration (VIV) of cylindrical offshore structures such as spars in strong currents. In particular, the consequences of marine growth on the surface as well as natural surface roughness that occurs with years in service are studied. Of special interest is the effect of surface roughness on the response amplitudes and the forces experienced by these structures while undergoing VIV.The experimental apparatus employed for the present study consisted of an elastically mounted rigid vertical cylinder with no end plates, towed along the length of a water tank. The cylinder was attached to a parallel linkage mechanism allowing motion in the transverse direction only. The cylinder surface was covered by sandpapers with known mean particle diameters, thus providing controlled values of roughness coefficient from 0.28×10⁻³ to 1.38×10⁻². The tests covered the subcritical range of Reynolds number from 1.7×10⁴ to 8.3×10⁴, and a reduced velocity range from 4 to 16.It was found that as the roughness of the cylinder was increased the maximum response amplitude and the maximum mean drag coefficient decreased, levelling off to constant values. The onset of lock-in was progressively delayed for rougher cylinders, and the width of the lock-in region showed remarkable reduction at higher roughness values. The Strouhal number was found to display a modest increase with roughness. The dynamic mean drag of the rough cylinders was also found to be lower than that for a smooth cylinder. It is felt that uniform roughness such as caused in marine environments may act favorably to lower VIV incidence and effects in the range of Reynolds number tested.     

Power extraction using flow-induced vibration of a circular cylinder placed near another fixed cylinder

We conducted an experiment in a towing tank to investigate the performance of an energy extraction system using the flow-induced vibration of a circular cylinder. This experiment tested three different cases involving the following arrangements of cylinder(s) of identical diameter: the upstream fixed–downstream movable arrangement (case F); the upstream movable–downstream fixed arrangement (case R); and a movable isolated cylinder (case I). In cases F and R, the separation distance (ratio of the distance between the centers of the two cylinders to their diameters) is fixed at 1.30. Measurement results show that while cases F and I generate vortex-induced vibration (VIV) resonance responses, case R yields wake-induced vibration (WIV) at reduced velocity over 9.0, which is significantly larger than that of the VIV response, leading to the induction of higher electronic power in a generator. Accordingly, primary energy conversion efficiency is higher in the case involving WIV.     

Electromagnetic flow control in poor conductors

Lorentz force-based flow control in materials with low electrical conductivity has a long history back to the first half of the 19th century. This review will focus on developments during the last two decades, collecting results from numerical simulations and laboratory experiments. Typically, the actuators consist of permanent magnets and electrodes flush-mounted with the surface, generating Lorentz forces in the fluid layers adjacent to the wall. We will discuss the application of Lorentz forces to reduce friction drag in turbulent boundary layers and to delay boundary layer transition. The control of separated flows and shear layers is another key aspect of the review. Energetic efficiency, one of the main criteria for flow control, and its relation to typical operating conditions will be analyzed as well. Lorentz forces can be successfully used to control a broad range of flow phenomena and are a versatile tool for basic fluid dynamics research. However their current applicability in large scale systems is hampered by the low electrical to mechanical efficiency intrinsic to actuators based on the magnetic fields delivered by today’s permanent magnets.     

结合振动控制的柱面纵向梯度线圈目标场设计方法

在磁共振成像系统的工作过程中,噪声主要是由梯度线圈系统产生的.梯度线圈置于高均匀度超导磁体的室温孔内,并工作于脉冲状态,频繁的开启和关闭会使线圈中电流急剧随时间变化,变化的电流导致线圈受到变化的洛伦兹力作用,从而产生振动,这种高频振动所发出的噪声会对病人产生刺激,严重时甚至会对病人的听觉神经产生损伤.梯度场的场强越强、切换速度越快,所产生的噪声就越大.降低噪声的最根本方法是通过有效的梯度线圈设计,降低洛伦兹力的空间分布.本文针对纵向梯度线圈,在原经典目标场设计方法基础上,加入对振动参量,从而能够有效地降低线圈工作时所产生的噪声.其具体方法是将振动控制函数作为约束条件,通过目标场法建立数学模型,利用MATLAB进行电磁验算.计算结果表明,所提数学模型可有效地降低线圈振动的最大振幅.     

Flow-induced instability under bounded noise excitation in cross-flow

Flow-induced vibration of a single cylinder in a cross-flow is mainly due to vortex shedding, which is usually considered as a forced vibration problem. It is shown that flow-induced vibration of a cylinder in the lock-in region is a combination of forced resonant vibration and fluid-damping-induced instability, which leads to time-dependent-fluid-damping-induced parametric resonance and constant-negative-damping-induced instability. The time-dependent fluid damping can be modeled as a bounded noise. The dynamic stability of a two-dimensional system under bounded noise excitation with a narrow-band characteristic is studied through the determination of the moment Lyapunov exponent and the Lyapunov exponent. The case when the system is in primary parametric resonance in the absence of noise is considered and the effect of noise on the parametric resonance is investigated. For small amplitudes of the bounded noise, analytical expansions of the moment Lyapunov exponents and Lyapunov exponents are obtained, which are shown to be in excellent agreement with those obtained using Monte Carlo simulation. The theory of stochastic stability is applied to explore the stability of a cylinder in a cross-flow. The analytical and numerical results show that the time-dependent-fluid-damping-induced parametric resonance could occur, which suggests that parametric resonance also contributes to the vibration of the cylinder in the lock-in range.     

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.     

Two-dimensional radiative equilibrium of a gray medium between concentric cylinders

A method for calculating radiative equilibrium temperature and surface heat-flux distributions is developed for an absorbing-emitting medium between concentric cylinders. The cylinder walls are assumed to be gray diffuse absorbers and emitters and have arbitrary temperature distributions along their peripheries. Heat generation may take place within the medium. As a first approximation, the problem is solved for optically-thick systems by using the differential approximation. To obtain accurate results also for the optically-thin and intermediate regimes, the differential approximation is subsequently improved by a number of geometric parameters, as has been discussed in a previous paper. As examples, two cares are presented in detail: (1) a hollow cylinder with uniform internal heat generation and uniform surface temperature and (2) a hollow cylinder with a cosine temperature distribution imposed on the wall, with no internal heat generation. Comparison with some numerical results generated by Hottel's zonal method shows excellent agreement.     

Experimental and theoretical investigation of high-pressure arcs.II. The magnetically deflected arc (three-dimensional modeling)

While Part I deals with cylindrical arcs, Part II studies the influence of transverse magnetic fields on the arc column for ambient pressures of 0.1-5.0 MPa. If exposed to a magnetic induction of several millitesla, the column of an arc is deflected by the Lorentz forces. In this paper, heat transfer and fluid flow with coupled electromagnetic forces are modeled for the magnetically deflected arc. To verify the predictions, the three-dimensional temperature distributions of the arc column are determined by line and continuum radiation measurements using tomographic methods. These temperature maps are compared with the results of the numerical simulations. To gain insight into the physical professes of the discharge and to make the arc properties available which are not readily measured, a self-consistent numerical model of the arc column is applied to the time-dependent and three-dimensional case. The temperature, velocity, pressure, and current densities are predicted by solving the conservation equations for mass, momentum, and energy, and Ohm's and Biot-Savart's law using material functions of the plasma. A control volume approach facilitates a numerically conservative scheme for solving the coupled partial differential equations. The predictions are in fair agreement with experimental results. A time-dependent fully implicit simulation of the arc was used to investigate the arc instabilities for large magnetic inductions     

A class of Cartesian grid embedded boundary algorithms for incompressible flow with time-varying complex geometries

We present a class of numerical algorithms for simulating viscous fluid problems of incompressible flow interacting with moving rigid structures. The proposed Cartesian grid embedded boundary algorithms employ a slightly different idea from the traditional direct-forcing immersed boundary methods: the proposed algorithms calculate and apply the force density in the extended solid domain to uphold the solid velocity and hence the boundary condition at the rigid-body surface. The principle of the embedded boundary algorithm allows us to solve the fluid equations on a Cartesian grid with a set of external forces spread onto the grid points occupied by the rigid structure. The proposed algorithms use the MAC (marker and cell) algorithm to solve the incompressible Navier-Stokes equations. Unlike projection methods, the MAC scheme incorporates the gradient of the force density in solving the pressure Poisson equation, so that the dipole force, due to the jump of pressure across the solid-fluid interface, is directly balanced by the gradient of the force density. We validate the proposed algorithms via the classical benchmark problem of flow past a cylinder. Our numerical experiments show that numerical solutions of the velocity field obtained by using the proposed algorithms are smooth across the solid-fluid interface. Finally, we consider the problem of a cylinder moving between two parallel plane walls. Numerical solutions of this problem obtained by using the proposed algorithms are compared with the classical asymptotic solutions. We show that the two solutions are in good agreement.     

An experimental investigation of dual-resonant and non-resonant responses for vortex-induced vibration of a long slender cylinder

Experimental results of the dual-resonant and non-resonant responses are presented for vortex-induced vibrations(VIV)of a long slender cylinder.The cylinder has a diameter of 10mm and a length of 3.31 m,giving an aspect ratio of 331.The cylinder was towed by a carriage with the velocity up to 1.5 m/s,with the Reynolds number varying from 2500 to 38000.Three different weights were used to provide the initial tension.Dual resonance means that resonance occurs simultaneously in both the cross-flow(CF)and in-line(IL)directions.The experiments were conducted in two stages.At the first stage,dual-resonant dynamic features of the cylinder subjected to vortex-induced excitation were investigated.The features of CF and IL vibration amplitude,motion orbits,phase angle differences,dominant frequencies and mode order numbers are presented.At the second stage of the experiments,particular emphasis was placed on non-resonant dynamic features.The variation of multi-mode modal displacement amplitudes was investigated in detail.     

A novel mesh regeneration algorithm for 2D FEM simulations of flows with moving boundary

A novel mesh regeneration algorithm is proposed to maintain the mesh structure during a finite element simulation of flows with moving solid boundary. With the current algorithm, a new body-fitted mesh can be efficiently constructed by solving a set of Laplace equations developed to specify the displacements of individual mesh elements. These equations are subjected to specific boundary conditions determined by the instantaneous body motion and other flow boundary conditions. The proposed mesh regeneration algorithm has been implemented on an arbitrary Lagrangian–Eulerian (ALE) framework that employs an operator-splitting technique to solve the Navier–Stokes equations. The integrated numerical scheme was validated by the numerical results of four existing problems: a flow over a backward-facing step, a uniform flow over a fixed cylinder, the vortex-induced vibration of an elastic cylinder in uniformly incident flow, and a complementary problem that compares the transient drag coefficient for a cylinder impulsively set into motion to that measured on a fixed cylinder in a starting flow. Good agreement with the numerical or experimental data in the literature was obtained and new transient flow dynamics was revealed. The scheme performance is further examined with respect to the parameter employed in the mesh regeneration algorithm.     

流动/运动松耦合与紧耦合计算方法及稳定性分析

为了更加精确地模拟流动/运动耦合问题, 建立了耦合动态混合网格生成非定常流场计算和六自由度运动方程求解的一体化计算方法, 并在统一框架内同时实现了松耦合与紧耦合方法.通过圆柱涡致自激振荡(vortex induced vibration, VIV)的模拟, 对不同时间精度的松耦合和紧耦合算法的优劣及适用范围进行了评估和分析; 通过引入附加质量的概念, 对耦合算法的稳定性进行了理论分析.研究表明:在流体的密度与物体的密度接近时, 松耦合方法是不稳定的, 必须采用紧耦合方法.最后利用耦合算法对二维鱼体的自主游动和钝锥三自由度自由飞过程进行了数值模拟, 证实了理论分析的结论.