Three Dimensional Numerical Simulation Of Vortex Induced Vibration Of A Cylinder Near Plane Boundary

Under the influence of the sea-bed,the vibration response and fluid structure interaction mechanism of ocean engineering structures are quite different from those in isolated structures,so is the failure and fatigue.Therefore,there is significant to study the vortex induced vibration of ocean engineering structures near plane boundary.This paper will conduct three dimensional numerical simulation on the vortex induced vibration of a cylinder near plane boundary.To investigate the interaction mechanism between “Flow-Cylinder-Plane” in the vortex induced vibration near plane boundary,the simulation are carried out for reduced velocity from 2 to 12 and Reynold numbers from 2000 to 12000 at different gap ratio G/D?= 2,1,0.75,0.5,0.25 and 0.005?using larger eddy simulation.The research focus on the vibration response and fluid structure interaction mechanism,the results are summarized as follows:Vortex induced vibration of a cylinder near plane boundary can be divided into three regimes according to the vibration characteristics: Regime I: G/D ? [0.75,2],the vortex induced vibration is mainly affected by the plane boundary,and there are complete three branches: initial branch,upper branch and lower branch.Regime II: G/D ? [0.25,0.5],the cylinder will collide with the plane at some cases,part of cylinder is immersed in the boundary layer of the plane.Vibration of cylinder results the separation of the boundary layer from the plane,and produces a clockwise vortex which suppresses the formation and development of the bottom vortex shedding from the cylinder.With the increasing free-stream velocity,the boundary layer thickness along the plane get thinner,and the vortex developed near the plane boundary is attached near the plane surface and cannot develop along downstream.Regime III: G/D = 0.005,where the reduced velocity U_r corresponding to the maximum normalized amplitude A*_max will shift to the right compared with other case at others G/D.This is due to reattachment of the upstream plane boundary layer on the cylinder as the increasing velocity and plane boundary.In some cases of near plane boundary,nonlinear response occurs,which is reflected in the power spectrum of the drag coefficient C_D.There are multiple integer multiples of the frequency based on the vortex shedding frequency.The “second harmonic” and “super harmonic” phenomena appear in sequence.The phenomenon of “second harmonic” is caused by the fact that the development of the bottom vortex shedding of the cylinder is affected by plane,and the dissipation of the vortex shedding is faster than the top of the cylinder.The phenomenon of "super harmonic" is the result of further reduction of the G/D,where the cylinder vibration is not only affected by the plane,but also interact with the boundary layer of the plane.The vortex at the bottom of the cylinder induces a clockwise separation vortex from boundary layer of the plane as the same frequency as the vortex frequency behind the cylinder.Simultaneous multiple shear layers shedding at the same frequency close to the cylinder and the cylinder tail result in multiple integer peak frequencies at power spectrum of C_D.As the gap ratio G/D decreases,the flow field downstream of the cylinder change from double-rowed vortex street to single-rowed vortex street,the vortex mode in the flow field is no longer a regular 2S or 2P mode,that is,the vortex mode changes from 2S to S or 2P to P;Isolated cylinder vibrate on the upper branch and upper branch,the vortex mode is 2P.At a small gap ratio G/D,the plane and boundary layer of the plan limit the development of the vortex shedding and development at the cylinder wake flow,it cannot form a counterclockwise small vortex pair with clockwise votex at the downstream of the cylinder,the vortex mode change from P to S;In Regime II,the shear layer length of cylinder will be stretched under the influence of the plane boundary when the cylinder amplitude reduced to smaller value.