Research On Flow-induced Vibration Of Two Circular Cylinders Based On Overset Grid

Marine riser is an indispensable and irreplaceable device in ocean engineering,which connects offshore working platform and seafloor equipment.With the gradual advance of ocean engineering to the deep sea,the aspect ratio of the marine riser is also increasing,making it more“soft”.Under the action of seawater,the marine riser is more easily to deformation in the free stream direction,which even leads to vortex-induced vibration of the riser.When the vibration frequency of the riser is close to the vortex shedding frequency,the amplitude of the riser will increase significantly,resulting in fatigue damage of the marine riser,threatening the stability and even safety of the marine engineering.Therefore,it is of great theoretical significance and application value to make an in-depth study on the inducing factors and action mechanism of vortex-induced vibration of marine riser,and then put forward effective measures to restrain the vortex-induced vibration of marine riser.In this paper,numerical simulation of the flow-induced vibration phenomena of two cylinders of two-degree-freedom in parallel,tandem and staggered arrangement are carried out by using overset grid,and the measure to suppress the vibration of two cylinders by rigid connection of two cylinders is put forward.And the flow-induced vibration responses of rigid connected two cylinders in two-degree-freedom in parallel,tandem and staggered arrangements are obtained,and the results are compared with the flow-induced motion responses of two cylinders without suppression measures,and the following conclusions are obtained.The motion tracks of parallel two cylinders are symmetrically distributed,the time history curves of lift and drag and wake vortex shedding modes of two cylinders almost change periodically,showing an obvious“flip-flopping”phenomenon,and the wake vortex structure of two cylinders is more complex.Most of the motion tracks of the two cylinders in tandem are in the shape of“8”,and the amplitude of the transverse flow direction of the cylinder 2 increases with the increase of reduced velocity.When the reduced velocity U~*?7,the time history curves of the lift and drag of the two cylinders are approximately sinusoidal.And with the increase of the reduced velocity,the time history curves of lift and drag of the two cylinders show multi-frequency vibration;and the wake structure of the two cylinders is simple.The shear layer separated on both sides of cylinder1 will attach to the surface of cylinder 2 and will not fall off alone.With the increase of the reduced velocity,the motion tracks of the staggered two cylinders will change from oval and“crescent”shape to“crescent”shape and“water drop”shape respectively.The time history curve of lift and drag of the two cylinders will also change from approximate sinusoidal vibration to multi-main frequency and multi-period vibration,and the wake vortex structure of cylinder 1 is relatively stable.Rigid connection can reduce the vibration of two cylinders to a great extent,and the good suppression effect can be obtained under most working conditions.When the reduced velocity U~*?6,the amplitude,lift coefficient and drag coefficient of parallel two cylinders in rigid connection are relatively small,and with the increase of reduced velocity,the amplitude,lift coefficient and drag coefficient of the two cylinders increase greatly,the amplitude of the two cylinders on transverse flow direction is reach 1.19D,and the wake vortex structure of the two cylinders is relatively stable.The tandem two cylinders in rigid connection show the best suppression effect,and the amplitude of the two cylinders decreases significantly,the lift coefficient and drag coefficient fluctuate slightly,and the wake vortex structure is very stable.When the reduced velocity U~*?7,the tandem two cylinders have almost no wake vortex.The amplitude of the two cylinders with staggered rigid connections decreases obviously when the reduced velocity U~*?8;when the reduced velocity U~*=9 and 10,the amplitude of the two cylinders is between the amplitudes of staggered two cylinders;the lift coefficient and drag coefficient of staggered two cylinders in rigid connection do not fluctuate significantly;the wake region of cylinder 1 is relatively narrow and the wake structure is stable.