| With the depletion of onshore oil and gas resources,the exploitation of offshore oil and gas resources is imperative.As an important facility for offshore oil and gas resources development,the riser will be subject to alternative vortex shedding in the wake field when encountered with currents,resulting in fluctuating lifting force.When the flow velocity is in a certain range,and the ratio of the vortex shedding frequency of the riser to its natural frequency is close to 1,the riser will have vortex-induced vibration,namely large periodic transverse vibration,which will lead to fatigue damage and shorten the fatigue life of the riser.It will affect the operation and even cause leakage or sudden fracture of the riser,resulting in huge economic loss and severe environmental pollution.The effective suppression of vortex-induced vibration(VIV)of underwater slender structures,such as marine risers,is a challenge for marine engineering.In this paper,an improved numerical discrete vortex method and model based on it are introduced.The strength of new vortex is solved based on the vorticity-stream function formulation.The stochastic rigid vortex model is used to solve the induction between vortices.The modified Dirac kernel function is introduced to eliminate the numerical singularity.A vortex merging device is used to improve the calculation efficiency and the appropriate method is applied to revise the vortices moving into the body surface.In addition,the treatment of solving the flow around multi-body structures is proposed.This method is a mesh-free method,which is simple and easy to programme,fast to solve,with high resolution in the concentration of vortices,as well as the applicability of large time step.It is very suitable for solving two-dimensional problems such as the flow around blunt body,vortex-induced vibration(VIV)of riser and vortex-induced motion(VIM)of platform under high Reynolds number.In this paper,this model is used to solve the problem of flow around a fixed cylinder.By comparing the hydrodynamic loads,the frequency of vortex shedding and the pressure distribution on the surface of the cylinder with the test results,the applicability of the model for solving such problems under high Reynolds number is validated.At the same time,the vortex induced vibration of the elastic supporting cylinder is simulated,and the transverse displacement response of the cylinder with single degree of freedom is compared with the test.Through the simulation of the 2-DOF cylinder,the transverse vibration frequency and the vortex shedding frequency are obtained,and the "lock-in" effect of the vortex induced vibration is successfully restored.In addition,the model is used to effectively predict the vortex shedding frequency of the two-dimensional hydrofoil and the "lock-in" effect of pitching motion of the elastic supported hydrofoil is also reproduced.The feasibility and accuracy of this method in solving the problem of flow around blunt bodies under high Reynolds number are validated again.In order to study the suppression effect of the control device on the flow around the riser,the discrete vortex model is used to simulate the flow around the two-dimensional rigid riser with a fairing.Through the analysis of the lift and drag coefficients,the frequency of the shedding vortex and the characteristics of wake field,the flow control effects of different tail patterns and top shape angles of the fairing on the flow around the riser are compared and summarized,and better fairing configuration is then concluded.At the same time,the flow around the riser with small attached cylinders is simulated,and the effects of the number and diameter of attached cylinders,as well as the distance between boundaries of attached cylinders and of the main riser on the lift and drag coefficients of the riser are explored under a certain Reynolds number range.A better arrangement of attached cylinders is obtained from the aspect of hydrodynamic force control. |
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