Experimental Study On Vortex-induced Vibration Of Stepped Cylinders In Uniform Flow

In marine riser,buoyancy blocks and other appendages are often installed to improve mechanical properties,and the shape is a stepped cylinder.The recent researches on the vortex-induced vibration characteristics of stepped cylinders are insufficient.In this research,an experimental study on the vortex-induced vibration(VIV)of stepped cylinders in uniform flow was performed,where the Reynolds number is Re = 450 ～ 6000.In the case of isolated cylinder,the effects of coverage ratio R,diameter ratio D/d and aspect ratio L/D are considered.In the case of two cylinders in tandem arrangement,the effects of spacing ratio s/D and the phase of configuration are considered.In the Synchronization region,the oscillation frequency of the stepped cylinder sequentially locks onto two frequencies with the increase of the reduced velocity,which is termed as the “dual lock-in”.The phenomenon of "dual lock-in" is caused by the frequency of vortex-leakage in different segments locking onto the oscillation frequency of cylinder with the increase of the reduced velocity.A "transition branch" appears in the vibration responses of the stepped cylinder when R = 25% ～ 75%.For the stepped cylinders in tandem configuration(the upstream cylinder is fixed),the variation of the spacing ratio s/D will change the flow pattern,and affect the oscillation characteristics of the downstream cylinder.According to the result,three spacing-ratio regions are defined:(ⅰ)region of small spacing-ratio;(ⅱ)region of the critical spacing ratio;(ⅲ)region of large spacing ratio.When the different segments of the upstream cylinder are arranged in reverse phase,the phenomenon of wake-induced vibration(WIV)occurs.The WIV of the downstream cylinder is excited by the reattachment point of the upstream shear layer switching from one side of the downstream cylinder to the other.