圆柱绕流电磁控制影响因素的实验研究

实验与数值模拟表明,利用电介质溶液中圆柱体侧表面附近分布的电磁场产生电磁力可有效改变圆柱流体边界层,控制圆柱绕流.本文对圆柱绕流电磁控制过程中的主要影响因素进行了实验研究,电磁包覆在圆柱表面后部时,其控制效果与全部包覆相当,而包覆在其它位置时,消涡效果较差;电磁极板窄的圆柱绕流控制效果较极板宽的消涡与减阻效果好.另外,电磁作用参数N愈大,消涡减阻的效果愈好.     

圆柱绕流的电磁控制

流体绕过非流线物体时，在物体尾部形成涡街，使其表面周期性变化的阻力和升力增加，从而导致物体振荡，产生噪音。本文通过实验和计算，研究圆柱绕流的电磁控制，阐述浸于弱电介质溶液中，表面包覆电磁激活板的圆柱，在电磁力作用下的流体控制原理，讨论电磁力的消涡、减阻和减振过程。     

流体边界层上电磁力的控制效应研究

利用作用于流体边界层上的电磁体积力改变流体边界层的结构，研究电磁力对流场的控制 作用效果. 电极与磁极交替分布的电磁场激活板包覆在圆柱体表面置于流动的电解质溶液 中，产生的电磁力沿圆柱体表面分布，可以改变流体边界层的结构，从而实现对流场的控制. 用电磁屏蔽和时域控制的方法调整电磁力的时空分布参数，圆柱绕流分离点可以在前驻点和 后驻点之间变动，产生不同的控制效果. 流体边界层上的电磁力能连续控制圆柱绕流、尾流 涡街的形态. 正向电磁力具有较好的消涡、减震和减阻控制效应. 反向电磁力具有明显的增 涡控制效应，具有较强的制动控制效应，此时圆柱体表面涡量分布的对称性和稳定性被破坏.     

翼型绕流的电磁力控制

将表面包覆电磁激活板的翼型,按一定的攻角,置于流动的弱电介质溶液中,电磁激活板可产生作用于流体的切向电磁力(Lorentz力),从而改变流体边界层的结构. 在转动水槽中,对翼型绕流及电磁力控制下的绕流形态进行了实验研究. 结果表明,未加电磁力时,前缘涡的脱落点是不确定的,与流场具体条件有关,而后缘涡仅在尖角处脱落. 前缘涡与后缘涡相互影响,并周期性的脱体,在尾部形成涡街. 施加电磁力后,当力的方向与流动方向相同时,可以在一定程度上抑制分离,消除涡街,其效果与减小攻角类似. 加反向电磁力时,则相当于加大攻角,在翼型体的背风面形成涡街.      

圆柱表面包覆电磁场消涡与增涡实验研究

利用电磁体积力改变流体边界层的结构，用作用于流体边界层上的电磁力进行消涡与增涡控制。交替分布的电极和磁极包覆在圆柱体的表面置于电介质溶液中，简单调整电磁力的时间与空间分布可以方便地控制圆柱绕流的形态。通过理论分析和数值模拟确定了实验控制的关键参数，实现了电磁力消涡和增涡的连续控制，电磁力作用下的圆柱绕流的分离点可以在前驻点和后驻点之间变动。     

隔离板对流向振荡圆柱尾流旋涡脱落的抑制

用隔离板对直径为D, 沿流向振荡的圆柱后涡脱落进行抑制. 隔离板放于圆柱尾流中心线上,控制参数包括隔离板长度L/D以及隔离板前缘到柱体振荡中心的距离G/D. 实验的雷诺数范围Re=V_∞D/v=1.01×10⁴～1.69×10⁴,柱体折减振频范围f_eD/V_∞=0～0.03, 柱体振幅固定为A/D=0.2. 风洞烟线显示和热线测量结果表明:当 G/D位于一个有效区域内时,可有效抑制振荡柱体尾流的旋涡脱落. 该有效区的大小随着隔离板板长的增大而增大, 随着Re数和圆柱振荡频率的增大而减小.     

圆柱和平板磁绝缘传输线的横向空间电荷流

根据电磁场理论及电子运动守恒方程导出传输线横向空间电荷流的数值模型和磁绝缘临界条件，对圆柱和平板情况进行了计算和讨论。     

晃板的力学

分析晃板表演的动力学原理. 将晃板简化为3个刚体以纯滚动约束和可控圆柱铰相联系的系 统. 人体控制木板的摆动, 使圆筒和人体受激励产生微幅周期响应, 从而使系统的不稳定状态转化为稳定.     

关于压电介质的守恒方程和路径无关积分

借鉴分析动力学中的Ｊａｃｏｂｉ积分和循环积分概念，以及电磁场理论中的能量矩概念，导出了压电介质在静态场中的守恒方程形式，由这些守恒方程即可得在位错，断裂力学和其他缺陷理论中应用广泛的路径无关积分。     

磁场对铁磁结构动态特性的影响

研究了两种铁磁材料结构(板梁和圆柱壳)在不同的磁场下固有频率的变化。频谱分析仪产生的扫频正弦信号通过压电陶瓷片对试件进行激励;压电薄膜作为感知元件再把感知信号送回频谱分析仪。根据共振原理,从频谱分析仪的频率响应曲线可以得到试件的各阶固有频率。通过改变外加磁场的大小,用同样的方法可以得到不同磁场下的固有频率。实验结果表明,铁磁板梁固有频率的变化很大程度上依赖于所施加磁场的方向:当磁场顺着板梁的长度方向时,板梁的固有频率会随着外加磁场的增大而增大;然而当磁场施加在板梁的厚度方向时,固有频率是先降低然后再升高。对铁磁材料圆柱壳,当磁场顺着圆柱壳的轴线方向增大时固有频率是逐渐增大然后达到一个饱和值。     

Numerical study of the suppression mechanism of vortex-induced vibration by symmetric Lorentz forces

In this paper, the electro-magnetic control of vortex-induced vibration (VIV) of a circular cylinder is investigated numerically based on the stream function–vorticity equations in the exponential–polar coordinates attached on the moving cylinder for Re=150. The effects of the instantaneous wake geometries and the corresponding cylinder motion on the hydrodynamic forces for one entire period of vortex shedding are discussed using a drag–lift phase diagram. The drag–lift diagram is composed of the upper and lower closed curves due to the contributions of the vortex shedding but is magnified, translated and turned under the action of the cylinder motion. The Lorentz force for controlling the vibration cylinder is classified into the field Lorentz force and the wall Lorentz force. The symmetric field Lorentz force will symmetrize the flow passing over the cylinder and decreases the lift oscillation, which, in turn, suppresses the VIV, whereas the wall Lorentz force has no effect on the lift. The cylinder vibration increases as the work performed by the lift dominates the energy transfer. Otherwise, the cylinder vibration decreases. If the net transferred energy per motion is equal to zero, the cylinder will vibrate steadily or be fixed.     

Instability of cylinder wake under open-loop active control

Instability of a wake controlled by a streamwise Lorentz force is investigated through a Floquet stability analysis. The streamwise Lorentz force, which is a two-dimensional control input created by an electromagnetic actuator located on the cylinder surface,adjusts the base flow to affect the three-dimensional wake instability and achieve wake stabilization and transition delay. The instability mode at a Reynolds number Re = 300 can be transformed from B to A with N = 1.0, where N is an interaction number representing the strength of the Lorentz force relative to the inertial force in the fluid. The wake flow is Floquet stable when N increases to 1.3. The spanwise perturbation wavelengths are 3.926 D and 0.822 D in the modes A and B, respectively, where D is the cylinder diameter. In addition, the oscillating amplitudes of drag and lift are reduced with the increase in the interaction number. Particle tracing is used to explore the essential physical mechanism for mode transformation. The path lines show that suppression of flow separation hinders the fluid deformation and rotation, leading to the decrease in elliptic and hyperbolic instability regions, which is the material cause of mode transformation.All of the results indicate that wake stabilization and transition delay can be achieved under open-loop active control via the streamwise Lorentz force.     

电磁极宽度对圆柱绕流场影响的数值分析

在雷诺数Re=200的情况,利用Maxwell方程直接数值计算表面包覆电极与磁极圆柱体产生的电磁力分布,将其加入到动量方程中,在各种电磁力作用参数和电磁极宽度的组合下,对表面覆盖电磁极圆柱体在弱电解质中的绕流场结构及其升阻力特性进行了数值模拟与分析.结果表明,当电磁极宽度较小时,圆柱体绕流场的分离点越容易接近后驻点,而电磁力对总阻力的影响并不明显,但对压差和摩擦阻力均有明显影响.当电磁极宽度较大时,圆柱体尾部区域越容易产生射流现象,而且总阻力随电磁力作用参数和电磁极宽度增大而减小.在电磁力尚不足以完全抑制周期性涡脱落的情况下,升力幅值随电磁力作用参数增大而减小,但随电磁极宽度则先减小后略有增加,升力脉动频率则均随电磁力作用参数和电磁极宽度增大而增加.研究表明,电磁力可以有效地改善圆柱体绕流场结构,达到减小圆柱体阻力并抑制其脉动升力之目的,因此是圆柱型结构的一种有效流动控制手段.     

Optimal control of cylinder wake flow by electro-magnetic force based on adjoint flow field

A nonlinear adjoint-based optimal control approach of cylinder wake flow by using electro-magnetic forcing has been proposed and investigated numerically in the paper. A cost functional representing the balance between the enstrophy Ω² and the interaction parameter N has been developed, and its corresponding adjoint equations have been derived. The sensitivity of the cost functional is found to be a simple function of the adjoint stream function in the adjoint field. Under the action of optimal force, N(t), the flow separation is suppressed successfully, the oscillations of drag and lift disappear and the total drag coefficient decreases dramatically. Furthermore, the global enstrophy is proved to be equal to the total squared strain during the control, which validates the conservation rule of Okubo-Weiss function.     

On the development of lift and drag in a rotating and translating cylinder

The two-dimensional flow around a rotating cylinder is investigated numerically using a vorticity forces formulation with the aim of analyzing quantitatively the flow structures, and their evolutions, that contribute to the lift and drag forces on the cylinder. The Reynolds number considered, based on the cylinder diameter and steady free stream speed, is Re=200, while the non-dimensional rotation rate (ratio of the surface speed and free stream speed) selected was α=1 and 3. For α=1 the wake behind the cylinder for the fully developed flow is oscillatory due to vortex shedding, and so are the lift and drag forces. For α=3 the fully developed flow is steady with constant (high) lift and (low) drag. Each of these cases is considered in two different transient problems, one with angular acceleration of the cylinder and constant speed, and the other one with translating acceleration of the cylinder and constant rotation. We characterize quantitatively the contributions of individual fluid elements (vortices) to aerodynamic forces, explaining and quantifying the mechanisms by which the lift is generated in each case. In particular, for high rotation (when α=3), we explain the relation between the mechanisms of vortex shedding suppression and those by which the lift is enhanced and the drag is almost suppressed when the fully developed flow is reached.     

Electromagnetic control of seawater flow around circular cylinders

We investigate numerically the electromagnetic control of seawater flows over an infinitely long circular cylinder. Stripes of electrodes and magnets, wrapped around the cylinder surface, produce a tangential body force (Lorentz force) that stabilizes the flow. This mechanism delays flow separation, reduces drag and lift, and finally suppresses the von Kármán vortex street. Results from two-dimensional simulations of the Navier–Stokes equations in a range 10<Re<300 and Lorentz force calculations are presented. Emphasis is placed on the disclosure of physical phenomena as well as a quantitative detection of the flow field and forces. It is shown that the drag strongly depends on the geometry of the electromagnetic actuator and on its location at the cylinder surface. The effect of flow control increases with larger Reynolds numbers, since the boundary layer thickness and the penetration depth of the Lorentz force are closely connected.