The Investigation Of Oscillation Suppression And Drag Reduction For Two-degree-of-freedom Vortex-induced Vibration With Electro-magnetic Force

The electro-magnetic forces generated by electromagnetic field take control of the flow in the electrolyte solution With the application of electro-magnetic force to the flow past a bluff body and vortex-induced vibration,the vortex shedding can be eliminated and the fluctuation of lift/drag forces can be suppressed,which will further lead to the vibration suppression of the bluff body.In this paper,the mechanism of two-degree-of-freedom vortex-induced vibration controlled by electro-magnetic forces is investigated numerically and experimentally.The experiment is conducted in the rotating tank filled with weak electrolyte,where the cylinder coverd by electro-magnetic actuators is placed in the tank and dye markers are injected for the visualization of the flow field.For the calculations,with the coordinate on the moving cylinder,the stream function-vorticity equations,the initial and boundary conditions together with the distributions of hydrodynamic forces are deduced in the exponential-polar coordinate.The equation of vorticity transport is solved by the Alternative-Direction Implicit(ADI)algorithm.The equation of stream function is integrated by means of a Fast Fourier Transform(FFT)algorithm The cylinder motion is calculated using the Runge-Kutta method.The flow field,pressure,lift/drag and cylinder displacement,which are interacted with each other along the transverse and streamwise directions,are discussed.1)The derivation shows that the vibration displacement in one direction,whose effects on the flow field influence the vortex-induced forces in both directions,yet affects the inertial force only in the corresponding direction and is independent with that in the other directiona2)The vortex-induced vibration is affected by two factors,i.e.the vortex shedding and the cylinder shift.Both of the two factors have influences on the shear layers in the transverse direction and the secondary vortex in the streamwise direction,which further leads to the variations of lift/drag and the cylinder motion.Notably,the effect of cylinder shift is dominant and opposite to that of the shedding vortex so that the former is dominated in the total effects.3)The flow separation and vortex shedding are suppressed as the fluid of boundary layer is accelerated with the effect of electro-magnetic forces.Meanwhile,the vibration displacements decrease gradually along both the transverse and streamwise directions,which also suppresses the effects of pressure/suction sides.Therefore,the vibration is suppressed and the cylinder tends steady rapidly.The experimental results agree well with the numerical Cd=0 results.Moreover,based on Cd= 0,the value of electro-magnetic force is dependent on the flow field and cylinder motion in the closed-loop control Meanwhile,the cylinder eventually returns to the initial position as the trust generated by the wall electro-magnetic force counteracts the drag induced by the field electro-magnetic force.