Numerical Investigation On The Effect Of Forced Oscillation Of Cylindrical Riser On Hydrodynamics

During the exploration of deep-sea oil and gas resources,the riser is the main link between the offshore platform and the subsea wellhead.The design of the riser is one of the key technologies in deep-sea technology.Such cylindrical risers will undergo vortex-induced vibration(VIV)which may cause fatigue damage to the structures,resulting in huge economic losses and ecological disasters.Hence,it is of great practical significance to carry out researches on this problem.The forced oscillation of the cylindrical risers can effectively reflect the effects of the structural oscillation on the resulting forces,the wake dynamics and the energy transfer between the structure and the fluid.By using numerical simulation,this paper focus on the effects of forced oscillation of the cylindrical risers on hydrodynamics.Both uniform circular and wavy cylinders undergoing forced oscillation normal to incoming uniform flow at Reynolds number of 150 were numerically investigated by using CFD software OpenFOAM.The effects of oscillation amplitude and frequency on hydrodynamic forces were examined.The lock-in regions were identified within which special attention was paid to the characteristics of lift force as well as vortex wake variations.The numerical simulations indicate that although the wavy cylinder can suppress Karman vortex shedding completely in fixed configuration,the variation of hydrodynamic forces with oscillation frequency of the wavy cylinder is similar to that of the uniform circular cylinder in forced oscillation.The reduction of the lift and drag forces by the wavy cylinder appears in low and high oscillation frequencies,respectively.The performance of lift coefficient changing with time depends on whether it is in lock-in state.It is found that in lock-in region,2S and C(2S)shedding patterns determined by oscillation frequency exist in the wake of the uniform circular cylinder,while the only one shedding pattern exists for the wavy cylinder.A three-dimensional investigation of flow past a uniform circular cylinder undergoing forced oscillation at Reynolds number of 500 was conducted by using CFD software Nektar++.The hydrodynamics were examined by gradually increasing or decreasing the oscillation frequency.The numerical simulations indicate that hysteretic phenomenon can be observed both in the variation of the lift and drag coefficients,energy transfer coefficient and phase difference.All sharp jumps occur at the upper critical frequency for frequency increasing manner,while for frequency decreasing manner,all occurs at the lower critical frequency.The lock-in region which is broader than that derived from the two-dimensional investigation is same for different frequency traverses.When the phase angle between the cross-flow motion and lift force falls into the range of 0—180 degrees,the energy transfer coefficient is positive,otherwise it is negative.