On the study of vortex-induced vibration of a cylinder with helical strakes

While the effect of helical strakes on suppression of Vortex-Induced Vibrations (VIV) has been studied extensively, the mechanism of VIV mitigation using helical strakes is much less well documented in the literature. In the present study, a rigid circular cylinder of diameter d=80 mm attached with three-strand helical strakes of dimensions of 10d in pitch and 0.12d in height was tested in a wind tunnel. It was found that the helical strakes can reduce VIV by about 98%. Unlike the bare cylinder, which experiences lock-in over the reduced velocity in the range of 5-8.5, the straked cylinder does not show any lock-in region. In exploring the mechanism of VIV reduction by helical strakes, measurements in stationary bare and straked cylinder wakes using both a single X-probe at four different Reynolds numbers, i.e. Re=10 240, 20 430, 30 610 and 40 800, and two X-probes with variable separations in the spanwise direction at Re=20 430 were conducted. It was found that vortices shed from the straked cylinder are weakened significantly. The dominate frequency varies by about 30% over the range of x/d=10-40 in the streamwise direction while that differs by about 37.2% of the averaged peak frequency over a length of 3.125d in the spanwise direction. The latter is supported by the phase difference between the velocity signals measured at two locations separated in the spanwise direction. The correlation length of the vortex structures in the bare cylinder wake is much larger than that obtained in the straked cylinder wake. As a result, the straked cylinder wake agrees more closely with isotropy than the bare cylinder wake. Flow visualization on the plane perpendicular to the cylinder axis at Reynolds number of about 300 reveals small-scale vortices in the shear layers of the straked cylinder wake. However, these vortices do not roll up and interact with each other to form the well-organized Karman-type vortices. Flow visualization on the plane parallel to the cylinder axis shows vortex dislocation and swirling flow, which should be responsible for the variations of the peak frequency in the streamwise as well as spanwise directions.     

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.     

Control of separated flows by time periodic Lorentz forces

Electromagnetic, i.e. Lorentz forces, may be used to influence the flow of electrically conducting fluids. The present paper investigates the application of time periodic Lorentz forces to the control of the suction side flow on a NACA 0015 hydrofoil. Experimental results, consisting of flow visualizations and force measurements, characterizing the control effect in the low Reynolds number range of 10⁴Re10⁵, are presented. A comparison of the forcing effect with stationary Lorentz forces on one hand and conventional oscillatory blowing on the other hand is given as well.     

VIV suppression of a two-degree-of-freedom circular cylinder and drag reduction of a fixed circular cylinder by the use of helical grooves

Experimental investigations have been carried out to examine the effects of triple-starting helical grooves on the drag of fixed circular cylinders and the vortex-induced vibration of elastically supported cylinders. For the elastically supported cylinder, the Reynolds number varied from 1.3×10⁴ to 4.6×10⁴, whilst for the fixed cylinder from 3.1×10⁴ to 3.75×10⁵. A comparative approach which allows direct comparisons of the results was adopted where two cylinders of identical dimensions and physical properties with or without helical surface grooves were tested in exactly same experimental set-ups. In the elastically supported cylinder tests, the cylinders were attached to a vertically cantilevered supporting rod and towed in a towing tank. Both the in-line and cross-flow vibrations were permitted. In the fixed cylinder tests, the cylinders were supported on rigid vertical struts and towed horizontally in the same towing tank. It is found that for the case investigated the helical grooves were effective in suppressing the vortex-induced cross-flow vibration amplitudes with the peak amplitude reduced by 64%. Drag reductions of up to 25% were also achieved in the sub-critical Reynolds number range tested in the study for the fixed cylinders.     

Development of a generalised active vibration suppression strategy for a cantilever beam using internal resonance

In this paper, the potential to utilise modal coupling effects in the formulation of a generalised vibration suppression algorithm is investigated. The plant, a flexible cantilever beam undergoing first mode oscillation, is modelled by a second order differential equation with a spring constant and damping coefficient that are representative of the first mode flexibility and material damping of the beam, respectively.In order to establish an internal resonance condition, a second equation, designated the supplementary equation or controller, is appended to the plant to render a two-degree-of-freedom system. The objective is to generate an internally resonant pair. Upon successful completion of this task, a suppression technique is implemented whereby energy is removed from the plant via the supplementary system.The introduction of the supplementary system results in a set of design parameters which are employed to realise a state of internal resonance and to establish the desired dynamic response. The choice of 2:1 internal resonance models results in a unidirectional control torque making this technique particularly attractive for systems using thrusters or tendons as actuators. A similar structural configuration regulated under a PD (Proportional-Derivative) control law is compared to the proposed control scheme via simulation.     

受电磁激励的连续转子-轴承系统非线性振动与分岔

研究了受横向不平衡电磁激励的转子．轴承系统的非线性振动响应。首先将转子．轴承系统简化为带有质量不平衡并受横向激励的连续梁,由于短轴承的油膜力和电磁力的共同激励,系统振动具有强非线性特性。用Galerkin方法把偏微分控制方程离散为常微分方程组,采用四阶Runge—Kutta法对该系统进行数值仿真研究。其次比较了转轴分别在电磁力、油膜力单独作用和两种力共同作用下的振动特性,研究表明电磁力和油膜力对转子系统的非线性振动和分岔有着不同的贡献：油膜力的存在抑制了拟周期运动的发生,延长了稳定运行区域;电磁力拉长了拟周期发生的区域,降低了转子系统发生突发性破坏的风险。最后给出了系统响应随转速、电磁参数、油膜粘度等控制参数变化的分岔图,表明：系统在两个方向的运动随控制参数的变化趋势基本相同,经历了周期、倍周期、拟周期等非线性运动交替出现的过程;且油膜粘度的增大有利于转子系统的安全运行。     

介电弹性作动器主动隔振模糊控制试验研究

采用模糊控制策略,开展介电弹性作动器的主动隔振性能试验研究。基于介电弹性材料的Maxwell应力模型建立了作动器的力电耦合模型,分析了作动器的非线性特性;针对隔振系统设计了Mamdani型模糊控制器,建立了控制电压信号与振动响应之间的关系;在此基础上,开展了介电弹性作动器主动隔振试验研究,并与加速度反馈控制进行了对比。试验结果表明,在相同驱动电压的情况下,基于模糊控制策略的主动隔振性能要优于加速度反馈控制,且能够显著降低由于非线性驱动力导致的倍频响应幅值,有助于提高隔振系统的稳定性。     

Approximate analytical solution of the self-excited vibration of piecewise-smooth systems induced by dry friction —The vibration mechanism of the Chinese cultural relic Dragon Washbasin

Based on our previous work, a mathematical model of piecewise-smooth systems is established by means of phase-plane orbit analysis, and it is then used to study the intersting phenomena of Chinese cultural relic Dragon Washbasin. The mechanism of nonlinear damping is analyzed; the approximate analytical solution of self-excited vibration of piecewise-smooth nonlinear systems induced by dry friction is derived by means of KB Method, the results of which agree well with that of the numerical solution. Therefore, the method presented in this paper is proved to be very efficient in analyzing the self-excited vibration of piecewise-smooth systems induced by dry friction.     

The resonances of parametric vibration with forced vibration of the stator system of hydroelectric-generator stimulated by electromagnetism

The resonances of parametric vibration with forced vibration is analyzed, the bifurcation equation of the system is obtained and the singularity analysis is made. Some of the laws and phenomena are revealed. The transition variety and bifurcation diagram of the physical parametric plane are given. The results can be used in engineering. Supported by National Natural Science Foundation and Doctoral Programme Foundation of Institution of Higher Education of China.     

Suppression of lift fluctuations on a circular cylinder by inducing the symmetric vortex shedding mode

The flow around a stationary circular cylinder modified by two synthetic jets positioned at the mean separation points is numerically studied. The Reynolds number based on the free-stream velocity and the circular cylinder diameter is Re=500. The focus is to present a novel way to suppress the lift fluctuations by changing the vortex shedding mode, and thus particular attention is paid to the interactions between the synthetic jets and wake shear layers and the resulting vortex dynamics. The overall influences of both momentum coefficient and excitation frequency are discussed. In some simulated cases, the vortex lock-on phenomenon is discovered, which causes the typical Kàrmàn type vortex shedding to be converted into the symmetric shedding modes, leading to the complete suppression of lift fluctuations. In other cases, the asymmetric shedding mode still dominates the wake evolution. Detailed vortical evolution for each typical wake pattern is analyzed to reveal the control mechanism. Additionally, the control effectiveness is evaluated, indicating that the present control strategy contributes an effective way to suppress the lift fluctuations and reduce the mean drag.     

Mechanism of drag reduction by spanwise oscillating Lorentz force in turbulent channel flow

Numerical simulations and experimental research are both carried out to investigate the controlled effect of spanwise oscillating Lorentz force on a turbulent channel flow. The variations of the streaks and the skin friction drag are obtained through the PIV system and the drag measurement system, respectively. The flow field in the near-wall region is shown through direct numerical simulations utilizing spectral method. The experimental results are consistent with the numerical simulation results qualitatively, and both the results indicate that the streaks are tilted into the spanwise direction and the drag reduction utilizing spanwise oscillating Lorentz forces can be realized. The numerical simulation results reveal more detail of the drag reduction mechanism which can be explained, since the spanwise vorticity generated from the interaction between the induced Stokes layer and intrinsic turbulent flow in the near-wall region can make the longitudinal vortices tilt and oscillate, and leads to turbulence suppression and drag reduction.     

Numerical experiments on the stability of leading edge boundary layer flow: A two-dimensional linear study

A numerical study is performed in order to gain insight to the stability of the infinite swept attachment line boundary layer. The basic flow is taken to be of the Hiemenz class with an added cross-flow giving rise to a constant thickness boundary layer along the attachment line. The full Navier-Stokes equations are solved using an initial value problem approach after two-dimensional perturbations of varying amplitude are introduced into the basic flow. A second-order-accurate finite difference scheme is used in the normal-to-the-wall direction, while a pseudospectral approach is employed in the other directions; temporally, an implicit Crank-Nicolson scheme is used. Extensive use of the efficient fast Fourier transform (FFT) algorithm has been made, resulting in substantial savings in computing cost. Results for the two-dimensional linear regime of perturbations are in very good agreement with past numerical and theoretical investigations, without the need for specific assumptions used by the latter, thus establishing the generality of our method.     

Numerical and experimental study of a micro-blast wave generated by pulsed-laser beam focusing

This paper describes a numerical and experimental study of a micro-blast wave which is produced from the source of several tens microns in dia. and propagates in the length scale of a few centimeter in diameter. The micro-blast wave was generated by focusing a Nd:Glass pulsed-laser beam in ambient air. Its propagation and reflection were visualized by using double exposure holographic interferometry and simulated numerically using the dispersion-controlled scheme to solve the Euler and Navier-Stokes equations with initial conditions of a point-source explosion specified with the Taylor similarity law. Good agreement was obtained between numerical solutions and experimental results, and this spherical micro-blast wave was shown to be a handy model of blast waves created in large scale explosions. Received 28 October 1997 / Accepted 30 April 1998     

Simulation of non-selfadjoint control of thin elastic plates in the presence of conservative in-plane forces

Summary Based on the analysis presented in [1], a closed-form solution for the response of a controlled plate to a transversely applied dynamic loading is presented. Both the applied loading and the control are considered to be continuously distributed. The results and the corresponding parametric study are presented for two common boundary conditions of SSSS and SFSF in the plates. Received 11 March 1997; accepted for publication 18 March 1998     

A study on the effect of electronic engine speed regulator on agricultural tractor ride vibration behavior

In this study, the effect of electronic speed adjustment on tractor ride vibration levels is examined. With normal pedal operation the engine rotational speed drops with an increasing load. The electronic regulator provides a constant speed mode of operation independent of the load. Vibration levels were measured under different operating conditions and surfaces. As a first series of tests, the tractor was driven on a conglomerate bituminous track at speeds of 20, 25 and 28 km/h. Vibration was measured upon the surface of the operator seat simultaneously in the x, y and z directions. The reference axis system was that defined by the ISO 2631-1 [1]. The weighted r.m.s. acceleration was found to be between 8% and 8.6% higher for the case where operation with electronic speed adjustment had been selected. Secondly, cultivating was chosen as the field task and the vibration was measured while the tractor was traversing a rough farm track at speeds of 6, 7.5 and 9 km/h. In this case, the vibration levels with automatic speed adjustment were between 4.3% and 8.6% lower than when driving with normal foot pedal operation. From the above results, we may infer that electronic speed regulation should not be used in transportation on asphalt country roads. On the contrary, it seems that electronic regulation has an advantage when used in typical field tasks such as cultivating.     

Numerical study of the lift force influence on two-phase shock tube boundary layer characteristics

The importance of the lift force acting on the dispersed phase in the boundary layer of a laminar gas-particle dilute mixture flow generated by a shock wave is investigated numerically. The particle phase is supposed to form a continuum and is described by an Eulerian approach. The ability of the Eulerian model to simulate particle flows and the importance of the two-way coupling are proven by comparison with experimental data as well as with the numerical results from schemes based on a Lagrangian approach. The models used for the lift force are discussed through comparisons between numerical and experimental results found in the literature. Some results about the formation of a dust cloud are numerically reproduced and show the major role of the lift force. Simulations of two-dimensional two-phase shock tube flows are also performed including the lift force effects. Although the wave propagation is weakly influenced by the lift force, the force modifies substantially the dynamics of the flow near the wall. Received 17 February 2000 / Accepted 30 November 2000     

Numerical study on micro-fracture mechanism of rock dynamic failure induced by abrupt unloading under high in-situ stresses

Rock burst is a kind of severe engineering disaster resulted from dynamic fracture process of rocks.The macro failure behaviors of rocks are primarily formed after experiencing the initiation,propagation,and coalescence of micro-cracks.In this paper,the grain-based discretized virtual internal bond model is employed to investigate the fracturing process of unloaded rock under high in-situ stresses from the micro-fracture perspective.The simulated micro-fracturing process reveals that the longitudinal stress waves induced by unloading lead to the visible unloading effect.The influences of in-situ stresses,mineral grain sizes,and grain heterogeneity on rock macro and micro fracture are investigated.Micro-crack areas of tensile and shear cracks and micro-crack angles are statistically analyzed to reveal the rock micro-fracture characteristics.The simulated results indicate that the combined effect of the stress state transition and the unloading effect dominates the rock unloading failure.The vertical and horizontal in-situ stresses determine the stress state of surrounding rock after unloading and the unloading effect,respectively.As the vertical stress increases,the stress level after unloading is higher,and the shear failure characteristics become more obvious.As the horizontal stress increases,the unloading effect increases,leading to the intensification of tensile failure.The mineral grain size and grain heterogeneity also have nonnegligible influences on rock unloading failure.The micro-fracture perspective provides further insight into the unloading failure mechanism of deep rock excavation.     

Numerical study of the axially symmetric motion of an incompressible viscous fluid in an annulus between two concentric rotating spheres

The research reported herein involved the study of the transient motion of a system consisting of an incompressible Newtonian fluid in an annulus between two concentric, rotating, rigid spheres. The primary purpose of the research was to study the use of a numerical method for analysing the transient motion that results from the interaction between the fluid in the annulus and the spheres which are started suddenly by the action of prescribed torques. The problems considered in this research included cases where: (a) one or both spheres rotate with prescribed constant angular velocities and (b) one sphere rotates due to the action of an applied constant or impulsive t?orque. In this research the coupled solid and fluid equations were solved numerically by employing the finite difference technique. With the approach adopted in this research, only the derivatives with respect to spatial variables were approximated with the use of the finite difference formulae. The steady state problem was also solved as a separate problem (for verification purposes), and the results were compared with those obtained from the solution of the transient problem. Newton's algorithm was employed to solve the algebraic equations which resulted from the steady state problem, and the Adams fourth-order predictor–corrector method was employed to solve the ordinary differential equations for the transient problem. Results were obtained for the streamfunction, circumferential function, angular velocity of the spheres and viscous torques acting on the spheres as a function of time for various values of the system dimensionless parameters.