| The oil and gas resources contained in the ocean play an inseparable role in human production and life.As a significant equipment in the progress of deep-sea resources,the marine riser,due to its large aspect ratio,produces vortex-induced vibration under the influence of ocean currents,and thus exhibits multi-modal vibration,causing structural damage and fatigue damage to the riser.Vortex-induced vibration(VIV)refers to the phenomenon that the structure in the ocean has alternating vortexes in the wake field due to the action of the incoming flow,which generates periodic pulsating lift for the structure,which causes a large-scale lateral movement.Exploring the mechanism of vortex-induced motion and how to effectively suppress the damage of vortex-induced vibration to structures has always been a concern in the engineering field and academia.The discrete vortex method(DVM)used in this paper expresses the flow field as discrete vortex elements.Each time step will generate a new vortex element and move to the wake field under the action of the incoming flow.The vorticity-current function equation is used to solve the vortex intensity of the newly generated vortex elements on the surface,and the induced velocity between the vortex elements is obtained by combining with the Biot-Savart law.Because the transfer of physical quantities in the discrete vortex method relies on the vortex elements in the flow field,instead of dividing the grid in the flow field.Therefore,the calculation efficiency is very high,and it is suitable for the numerical simulation calculation of the flow around the bluff body and the vortex-induced vibration with large Reynolds number.In this paper,the discrete vortex method is used to numerically simulate the two-dimensional flow around a cylinder,and the vortex-induced vibration of a fixed cylinder and an elastically supported cylinder are calculated separately.Comparing the parameters such as the force on the cylinder,the vibration amplitude,and the Stouhal number with the experimental data,the "locking" phase and the descending section of the cylinder in a certain speed range are captured.These results confirm the accuracy of the discrete vortex model for vortex induced vibration problem of two-dimensional cylindrical computing.Subsequently,the discrete vortex method,based on the slice method theory and the finite volume method,building a numerical calculation model for the vortex-induced vibration of a quasi-three-dimensional long flexible riser.The discrete vortex method is used to calculate the hydrodynamic force on the tangent plane where the control point is located,and the numerical results of each tangent plane are combined with the structural dynamics algorithm for coupling calculation.Compared with the full-scale three-dimensional calculation of the CFD method,the calculation efficiency is greatly improved.The comparison with the experimental and numerical results proves the validity and accuracy of the numerical model for calculating the vortex-induced vibration response of a three-dimensional flexible riser.On this basis,the effect of the fairing on the suppression of the vortex-induced vibration of the three-dimensional flexible riser is studied,and it is discovered that the fairing can successfully lower the vibration mode of the riser and reduce the amplitude of the lateral vibration response at different flow rates,which makes the wake vortex shedding mode of the riser shows a more stable 2S state. |
