Numerical Investigations On Flow Around And Vortex Induced Vibration Of A Dual-step Circular Cylinder In Shear Flow

The marine riser needs to apply sufficient top tension on its top to maintain stability and strength.However,the strength of the joint and the riser itself are limited,so the top tension it can withstand is very limited.Therefore,it is necessary to set buoyancy module along the riser to provide distributed buoyancy,so as to improve the safety performance and stability of the riser.On the other hand,the incoming flow in actual engineering is rarely uniform,and shear flow is a more common incoming flow condition.However,at present,there have been few studies on the marine riser with buoyancy module.Further,related research in complex inflow conditions,such as shear flow,is more limited.Therefore,in order to make up for the research gaps in related fields,the flow around and the vortex-induced vibration of dual-step cylinders with different diameters ratio at various shear rates are numerically simulated by the iterative and high precision immersed boundary method in this paper.By analyzing the vortex shedding frequency,the velocity fluctuation in the flow field,the topology change of the wake vortex,the vibration characteristics and the fluid force,the related characteristics of the flow around and the vortex-induced vibration of the dual-step cylinder under distinct conditions are investigated.In the study of the flow around a dual-step cylinder in shear flow,it is found that when the shear rate is small(?=0.067 and?=0.1),similar to the case in uniform flow,distinct vortex cells appear along the cylinder span.(that is the S_l-,L-and S_h-cells).When the shear rate increases to?=0.12,there exists a large number of streamwise vortices and no obvious vortex cell appears,indicating a transition from two-dimensional to three-dimensional in the flow filed.When decreasing the diameter ratio,the strength of the"down wash"at the step is weakened,and the vortex shedding exhibits better continuity along the cylinder span.The lock-in has an important influence on the vortex-induced vibration of the dual step cylinder in shear flow.From the numerical simulation results of this paper,it can be concluded that the vibration displacement,vibration stability and wake vortex are all related to lock-in.When the shear rate is increased or the diameter ratio is decreased,the lock-in range along the cylinder is increased,and the degree of lock-in is enhanced,which causes the vibration displacement of the cylinder to increase and to be more stable.At the same time,the wake vortex field will also change from a complex long-period change to a more stable vortex shedding mode.On the other hand,the relative strength of the vibration frequency also significantly affects the vibration characteristics and wake vortex.In the in-line direction,as the shear rate increases or the diameter ratio decreases,the intensity of the main vibration frequency will increase and its difference from the"beat"frequency will be reduced.Therefore,the lock-in is less weakened,and the vibration displacement increases.And the complexity of long-cycle wake vorticity changes will also decrease.