Investigation of the flow around a circular cylinder under the influence of an electromagnetic force

An investigation of the effect of the local electromagnetic body force on the flow behavior around a circular cylinder is conducted. The electromagnetic force is applied locally on the cylinder surface in the range of 70–130° from the stagnation point along the cylinder circumference in both clockwise and counterclockwise directions. The numerical results predict that the Lorentz force applied in the circumferential direction on the cylinder moves the separation point rearward, and reduces the drag. To validate the numerical results, an experiment is conducted with a circular cylinder of 5 cm diameter. The electrodes and permanent magnets are flush mounted on the cylinder in such a way that the Lorentz force is generated in the circumferential direction. Flow visualization with polystyrene particles and direct drag measurement using strain gages are made. The fluid used is natural sea water of electric conductivity of about 4 (Ω m)^-1. Induction effect can be neglected in the present investigation due to the low flow speed and the Lorentz force is proportional to E×B where E is an applied electric field and B is a magnetic field. Received: 7 June 1998/Accepted: 28 April 1999     

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.     

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.     

包覆电磁场激活板的圆柱尾迹的数值研究

利用电磁场作用于电介质溶液的Lorentz力可以控制溶液的流动.对置于弱电介质溶液中,包覆电磁场激活板的圆柱周围的绕流进行数值研究,讨论了电磁场激活板的安装位置,激活方式等对圆柱尾迹的影响.     

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.     

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.     

The influences of wall Lorentz force and field Lorentz force on the cylinder drag reduction

In this paper,the effects of Lorentz force on drag reduction for a circular cylinder have been studied experimentally and numerically.Based on its effects on drag reduction,the Lorentz force is found to be classified into two parts:one acts directly on the cylinder,named as the wall Lorentz force,and the other called the field Lorentz force acts on the fluid inside the boundary layer.The wall Lorentz force leads to the generation of a thrust,whereas the field Lorentz force results in drag increase.Since the former dominates the drag variation,the drag would reduce accordingly and even turn into negative (thrust) with the application of Lorentz force.     

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.      

电磁力控制翼型绕流分离的增升减阻效率研究

电磁力可有效对流体流动进行控制,增升减阻,抑制流动分离,制约其推广应用的瓶颈为控制效率问题.为提高其控制效率,基于翼型绕流的电磁力控制,对电磁力增升减阻的控制效率问题进行数值研究. 根据能量守恒定律,推导电磁力控制能耗的比,基于升力和阻力计算节省能量. 定义电磁力的控制效率为能量节省与电磁力控制所需能耗的比值,研究不同工况下电磁力增升减阻的控制效率. 发现在控制开始阶段,电磁力能量主要消耗在增加边界层流体的动能上,电磁力控制效率非常低,但电磁力控制效率会随着电磁力工作时间的增长而增加;电磁力控制效率随着来流速度的增加呈指数下降;通过增加电磁力激活板的输入能量可增强电磁力的控制效果,但无法明显增加其控制效率.      

Numerical simulation of flow around a square cylinder in uniform-shear flow

This paper presents results obtained from a numerical simulation of a two-dimensional (2-D) incompressible linear shear flow over a square cylinder. Numerical simulations are performed, using the lattice Boltzmann method, in the ranges of 50⩽Re⩽200 and 0⩽K⩽0.5, where Re and K are the Reynolds number and the shear rate, respectively. The effect of the shear rate on the frequency of vortex shedding from the cylinder, and the lift and drag forces exerted on the cylinder are quantified together with the flow patterns around the cylinder. The present results show that vortex structure behind the cylinder is strongly dependant on both the shear rate and Reynolds number. When Re=50, a small K can disturb the steady state and cause an alternative vortex shedding with uneven intensity. In contrast, a large value of K will suppress the vortex shedding from the cylinder. When Re>50, the differences in the strength and size of vortices shed from the upper and lower sides of the cylinder become more pronounced as K increases. Vortex shedding disappears when K is larger than a critical value, which depends on Re. The flow patterns around the cylinder for different Re tend towards self-similarity with increasing K. The lift and drag forces exerted on the cylinder, in general, decrease with increasing K. Unlike a shear flow past a circular cylinder, the vortex shedding frequency past a square cylinder decreases with increasing the shear rate. A significant reduction of the drag force occurs in the range 0.15<K<0.3.     

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.     

Non-uniform suction control of flow around a circular cylinder

In the present study, numerical investigations were performed with optimisation to determine efficient non-uniform suction profiles to control the flow around a circular cylinder in the range of Reynolds numbers 4 < Re < 188.5. Several objectives were explored, namely the minimisation of the separation angle, total drag, and pressure drag. This was in an effort to determine the relationships between the characteristics of the uncontrolled flow and the parameters of the optimised suction control. A variety of non-uniform suction configurations were implemented and compared to the benchmark performance of uniform suction. It was determined that the best non-uniform suction profiles consisted of a distribution with a single locus and compact support. The centre of suction on the cylinder surface for the optimised control, and the quantity of suction necessary to achieve each objective, varied substantially with Reynolds number and also with the separation angle of the uncontrolled flows. These followed predictable relationships. Surprisingly, the location of optimised suction to eliminate separation did not follow the separation point as it moved with Re, but rather it moved in opposition to it towards the trailing edge of the cylinder. Non-uniform suction profiles were much more efficient at eliminating boundary layer separation, requiring the removal of less than half the volume of fluid as uniform control to achieve the same objective. Regardless of the method of control, less net suction was needed to minimise total drag than to eliminate separation, except at low Re. The results suggest that controlling the dynamic aspects of the flow has the most impact for reducing drag. This reinforces the usefulness of other studies that focus on the elimination of vortex shedding. The results show that the balance of drag components must be an important consideration when designing flow control systems and that, when done appropriately, substantial improvement can be seen in the flow characteristics.     

MODELING THE UNSTEADY LIFT AND DRAG ON A FINITE-LENGTH CIRCULAR CYLINDER IN CROSS-FLOW

Semi-empirical models for unsteady lift and drag are developed to predict the spectral features of the unsteady forces on a finite-length, right circular cylinder in cross-flow. In general, the models consist of two parts; the spatial variation of r.m.s wall pressure on the cylinder, and the correlation lengths which describe the spatial extent of the correlation of the unsteady wall pressures. Experiments were conducted in a low noise wind tunnel as a function of cylinder diameter Reynolds number (19 200<Re<32 000) and the Strouhal number (0·05< St<3·33), to measure the statistics of the unsteady wall pressures on a model cylinder. These results are incorporated into the theoretical models, and predictions of the spectral characteristics of the lift and drag are made. The r.m.s. wall pressures on the cylindrical surface are found to have the largest amplitude near the cylinder end-cap, and on the rearward portion of the cylinder body. The high levels in these locations are attributed to the separated flow region over the end-cap. The circumferential and axial length-scales decrease exponentially with Strouhal number. Both length-scales exhibit maxima near the Strouhal shedding frequency of St=0·21. The axial length-scales are found to depend on the measurement reference location due to the three-dimensional flow and separated flow region near the end-cap. The unsteady lift and drag predictions using the models developed in this work agree well with previously measured unsteady force data measured on inertial hydrophones exposed to flow. The broadband unsteady lift is found to be greater than the broadband unsteady drag by nominally 3dB.     

Drag Reduction of a Circular Cylinder Using an Upstream Rod

Experimental studies on the drag reduction of the circular cylinder were conducted by pressure measurement at a Reynolds number of 82 000 (based on the cylinder diameter). A rod was placed upstream of and parallel to the cylinder to control the flow around the cylinder. The upstream rod can reduce the resultant force of the cylinder at various spacing between the rod and the cylinder for α < 5^∘(α defined as the staggered angle of the rod and the cylinder). For α > 10^∘, the resultant force coefficient has a large value, so the upstream rod cannot reduce the force on the cylinder any more. For α = 0^∘ and d/D = 0.5 (where d and D are the diameter of the rod and the cylinder, respectively), the maximum drag of the cylinder reduces to 2.34% that of the single cylinder. The mechanism of the drag reduction of the cylinder with an upstream rod in tandem was presented by estimating the local contributions to the drag reduction of the pressure variation. In the staggered arrangement, the flow structures have five flow patterns (they are the cavity mode, the wake splitting mode, the wake merge mode, the weak boundary layer interaction mode and the negligible interaction mode) according to the pressure distribution and the hydrogen bubble flow visualization. The half plane upwind of the cylinder can be divided to four regions, from which one can easily estimates the force acting on the circular cylinder with an upstream rod in staggered arrangement.     

Flow behavior around a square cylinder subject to modulation of a planar jet issued from upstream surface

The flow behavior in the up- and downstream regions of a square cylinder subject to the modulation of a planar jet issued from the cylinder׳s front surface was studied using the laser-assisted smoke flow visualization method and hot-wire anemometer measurement. Reynolds numbers were from 1628 to 13 000. The drag force experienced by the square cylinder was obtained by measuring the surface pressures on the up- and downstream faces. The temporally evolving smoke flow patterns in the up- and downstream regions were synchronously revealed through the smoke flow visualization. The frequency characteristics of the instability waves in the up- and downstream regions were synchronously detected by the two hot-wire anemometers. Four characteristic flow modes were observed within the different ranges of the injection ratios. At the low injection ratios (IR<1), the ‘swinging jet’ appeared. The jet swung periodically leftward and rightward and formed a fluid bubble on the front surface. The fluid bubble contained a pair of counter-rotating vortices and presented a periodic variation in its height. At moderately low injection ratios (1<IR<4.3), the ‘deflected oscillating jet’ appeared. The jet was deflected in either the left or the right direction and wrapped around one of the edges of the square cylinder. Both the swinging and oscillating motions of the jet in the swinging jet and deflected oscillating jet modes were induced by the periodic feedback pressure signals generated by the vortex shedding in the wake. At the moderately high (4.3<IR<8.3) and high (IR>8.3) injection ratios, the ‘deflection jet’ and ‘penetrating jet’ appeared. The jet detached from the cylinder׳s front surface and penetrated a long distance into the upstream region due to large jet momentum. Neither periodic jet oscillation in the upstream region nor vortex shedding in the wake was observed. The drag coefficient was found to be decreasing quickly with increasing the injection ratio.     

Rotating cylinder drag balance with application to riblets

Experimental results are reported and discussed for a rotating cylinder drag balance designed to predict drag reduction by surfaces like riblets. The apparatus functions by measuring the torque applied to the inner cylinder by a fluid, such as water, that is set in motion by the controlled rotation of the outer cylinder. The instrument was validated by calibration for laminar flow and comparison of turbulent flow results to the those of G. I. Taylor. The ability to predict drag reduction was demonstrated by testing 114 m symmetric sawtooth riblets, which gave a maximum reduction of about 5% and an overall drag reduction range of 5<S ⁺<20, both of which are in excellent comparison to results reported in literature. The most suitable conditions for testing riblets are to apply the riblets only to the inner cylinder surface and to use cylinders for which the curvature of the flow is minimized. Received: 2 February 1999/Accepted: 1 October 1999     

Potential Flow Past a Slightly Deformed Porous Circular Cylinder

A uniform potential flow past a porous circular cylinder with a core of different permeability is discussed. The porous circular cylinder is slightly deformed whose radius is r=r₁(1+ecosm q){r=r_1(1+\epsilon \cos m \theta)} , where | e | << 1{\mid\epsilon\mid\ll 1} and m is a positive integer. Here r, θ are the polar coordinates and r ₁ is the characteristic radius of the cylinder. The drag force exerted by the exterior flow on the surface of the cylinder is calculated and it depends on the thickness of the porous material and on the permeabilities of the two porous regions. As special cases, porous cylinder with hollow core, rigid core, and deformed cylinder is discussed.     

Wakes of elliptical cylinders at low Reynolds number

The wakes of elliptical cylinders are numerically investigated at a Reynolds number Re_D = 150. ANSYS-Fluent, based on the finite volume method, is used to simulate two-dimensional Newtonian fluid flow. The cylinder cross-sectional aspect ratio (AR) is varied from 0.25 to 1.0 (circular cylinder), and the angle of attack (α) of the cylinder is changed as α = 0° – 90°. With the changes in AR and α, three distinct wake patterns (patterns I, II, III) are observed, associated with different characteristics of fluid forces. Steady wake (pattern I) is characterised by two steady bubbles forming behind the cylinder, occurring at AR < 0.37 and α < 2.5°. Time-mean drag and fluctuating lift coefficients are small. Pattern II refers to Karman wake followed by steady wake (AR ≥ 0.37 – 0.67, depending on α) with the Karman street transitioning to two steady shear layers downstream. An inflection angle α_i is identified where the time-mean drag of the elliptical cylinder is identical to that of a circular cylinder. Pattern III is the Karman wake followed by secondary wake (AR ≤ 0.67, α > 52°), where the Karman street forming behind the cylinder is modified to a secondary vortex street with a low frequency. The Time-mean drag coefficient is maximum for this pattern.     

The effect of a wake-mounted splitter plate on the flow around a surface-mounted finite-height circular cylinder

The influence of a wake-mounted splitter plate on the flow around a surface-mounted circular cylinder of finite height was investigated experimentally using a low-speed wind tunnel. The experiments were conducted at a Reynolds number of Re=7.4×10⁴ for cylinder aspect ratios of AR=9, 7, 5 and 3. The thickness of the boundary layer on the ground plane relative to the cylinder diameter was δ/D=1.5. The splitter plates were mounted on the wake centreline with negligible gap between the base of the cylinder and the leading edge of the plate. The lengths of the splitter plates, relative to the cylinder diameter, ranged from L/D=1 to 7, and the plate height was always equal to the cylinder height. Measurements of the mean drag force coefficient were obtained with a force balance, and measurements of the vortex shedding frequency were obtained with a single-component hot-wire probe situated in the wake of the cylinder–plate combination. Compared to the well-studied case involving an infinite circular cylinder, the splitter plate was found to be a less effective drag-reduction device for finite circular cylinders. Significant reduction in the mean drag coefficient was realized only for the finite circular cylinder of AR=9 with intermediate-length splitter plates of L/D=1–3. The mean drag coefficients of the other cylinders were almost unchanged. In terms of its effect on vortex shedding, a splitter plate of sufficient length was able to suppress Kármán vortex shedding for all of the finite circular cylinders tested. For AR=9, vortex shedding suppression occurred for L/D≥5, which is similar to the case of the infinite circular cylinder. For the smaller-aspect-ratio cylinders, however, the splitter plate was more effective than what occurs for the infinite circular cylinder: for AR=3, vortex shedding suppression occurred for all of the splitter plates tested (L/D≥1); for AR=5 and 7, vortex shedding suppression occurred for L/D≥1.5.     

Shockwave�Cturbulent boundary layer interaction control using magnetically driven surface discharges

This study demonstrates the potential for shockwave?Cturbulent boundary layer interaction control in air using low current DC constricted surface discharges forced by moderate strength magnetic fields. An analytical model describing the physics of magnetic field forced discharge interaction with boundary layer flow is developed and compared to experiments. Experiments are conducted in a Mach 2.6 indraft air tunnel with discharge currents up to 300?mA and magnetic field strengths up to 5?Tesla. Separation- and non-separation-inducing shocks are generated with diamond-shaped shockwave generators located on the wall opposite to the surface electrodes, and flow properties are measured with schlieren imaging, static wall pressure probes and acetone flow visualization. The effect of plasma control on boundary layer separation depends on the direction of the Lorentz force (j × B). It is observed that by using a Lorentz force that pushes the discharge upstream, separation can be induced or further strengthened even with discharge currents as low as 30?mA in a 3-Tesla magnetic field. If shock-induced separation is present, it is observed that by using Lorentz force that pushes the discharge downstream, separation can be suppressed, but this required higher currents, greater than 80?mA. Acetone planar laser scattering is used to image the flow structure in the test section and the reduction in the size of recirculation bubble and its elimination are observed experimentally as a function of actuation current and magnetic field strength.