Research On Large Eddy Simulation Of Flow Around A Riser And Vortex-induced Vibration Suppression Mechanism

In recent years,as available oil and gas resources on the land are becoming less and less,the exploitation of oil and gas resources in the ocean is getting more and more attention.As a channel connecting the offshore platform and subsea equipment,risers play a very key role in the process of offshore oil and gas production.In actual ocean engineering,due to the detachment of the vortex on both sides of the riser,the riser will be subjected to the lift in the cross-flow direction and the drag in the streamline direction,which will result in vortex-induced vibration.Sustained vortex-induced vibrations will cause fatigue damage to the riser,posing a threat to the safety of oil and gas production.Therefore,we must take certain measures to suppress vortex-induced vibration.The issue of flow past a riser is the basis for studying vortex-induced vibration of risers.Therefore,we need to study the flow around the riser more deeply and analyze the causes of vortex-induced vibration and the suppression mechanism of vortex-induced vibration suppression methods,in order to improve the existing vortex-induced vibration suppression methods and propose new vortex-induced vibration suppression methods.In this paper,we simulate the flow field at the subcritical Reynolds number before and after installing the fairing,using CFD software Fluent and LES method.Through the simulation,we analyze the flow field characteristics of the riser wake area systematically and study the effect of fairing on the flow field characteristics in the wake area and the fluid force of the riser.We relate drag of the riser to the flow field parameters in the wake region,and analyze the causes of the drag and the suppression mechanism of the fairing,using an average mechanical energy integral equation of flow around the riser.The results show that alternating detached vortices are formed on both sides of the riser,after the fluid flows through the riser,resulting in non-uniform pressure distribution around the riser,so that the riser is subjected to lateral lift and streamwise drag.After the fairing is installed,the interaction of the shear layer on both sides of the riser is suppressed,and the intensity of the shedding vortex decreases significantly,so the lift and the drag decrease significantly.Large scale vortexes are always concentrated within a certain range near the riser,and flow can be approximately considered as fully developed flow when it is far enough away from the riser.The energy loss of fluid flow through a blunt body mainly consumed in the average flow of viscous dissipation directly,coherent disturbance energy production and coherent pulse energy production.In these energies,coherent disturbance energy production accounts for a large proportion.If measures are taken to eliminate or reduce the unsteadiness of the trailing vortex in the wake of the blunt body,or reduce the wake area,it will have a good drag reduction effect.The fairing is mainly used to reduce the fluid force(drag)by controlling the motion of the coherent structure in the wake region.