Numerical Investigation On The Vortex-induced Vibration Of A Flexible Slender Riser With Internal Oscillating Flow

As the main component which connected platform and submarine base,marine riser's importance is self-evident.When current passes by circular cylinders,risers for example,vortices shedding from each side of the structures alternatively,which generate periodic hydrodynamic forces acting on the risers and thus lead to vibration,and the vibration will in turn affect the shedding vortex,this phenomenon is called vortex-induced vibration(VIV").In allusion to practical engineering application,many researchers have launched a extensive and in-depth research on "VIV" of void pipe.But research on VIV of risers with inner flow changing with time is still limited.Based on the immersed boundary method(IBM),a parallel code,named as Cgles IBM which is used as a FSI solver,combining with an implicit dynamic structural solver,X-code,have successfully simulate the VIV of long flexible riser under the combined effects of uniform outflow and oscillatory internal flow.The contents and main conclusions achieved in this paper include:1.The simulations of 2DOF vibration of flexible risers with and without inner flow were carried out.It is found that the streamwise vibration of the risers always shows a beat phenomenon.While for the riser with internal flow,the beat phenomenon becomes even more intense.2.The simulations of 2DOF vibration of a low mass-ratio flexible riser with pure oscillating internal flow were carried out.It is found that the oscillating internal flow can excite the second order vibration mode in the streanmwise vibration of the riser.With the increase of the amplitude of the oscillating flow velocity,the second-order mode is more obvious,and the amplitude is obviously increased.On the contrary,it has little effects on cross-flow vibration of the riser.3.The simulations of 2DOF vibrations of a low mass ratio flexible riser with internal flow consisting of a constant flow and a oscillating flow were carried out.Through the comparison analysis of different conditions,it is found that the vibration of the flexible riser becomes very complex and without obvious regularity.