| In recent years,the consumption of petroleum resources in my country has increased rapidly.In order to meet the increasing domestic demand for petroleum,the state has issued a number of policies to encourage the development of deep-sea engineering.In the development of deep-sea resources,the Spar platform is more and more favored by the ocean engineering community due to its simple structure.However,because the main structure of the Spar platform is a deep draft cylinder,when the ocean current passes,the vortex shedding of its wake will cause fluid pulsation,which will cause the vortex-induced motion of the Spar-type ocean platform.At a certain ocean current speed,the vortex-induced motion will have a "frequency lock-in" phenomenon,resulting in a large horizontal displacement,causing fatigue damage to the mooring line and riser,resulting in a reduction in the overall fatigue life of the platform and increasing the maintenance cost of the platform in the later period.This paper combines the National Natural Science Foundation of China "Research on the mechanism of flow around chords with complex rack structure and vortexinduced vibration",through numerical simulation methods,from the change law of vortex-induced characteristics such as motion response amplitude,motion frequency rate,force,etc.Start with an in-depth study of the vortex-induced motion characteristics of the Spar platform.In this paper,the numerical simulation method is used to study the flow around and forced motion of a two-dimensional cylinder under different incoming flow velocities.(1)Through testing and calculation,the grid density and time step are determined comprehensively considering the calculation accuracy and speed,and compared with the experimental results of Bai Zhining and Song Hong,which proves the credibility of the simulation results.(2)Perform numerical simulations at 12 different flow rates,analyze the frequency of the flow around the cylinder,and find that the lift coefficient is more than twice the drag coefficient,and the pulsation value is large.As the flow rate increases,the pulsation value will continue to decrease.With drag coefficient as the abscissa and lift coefficient as the ordinate,the lift-drag curve is drawn as a "8" shape.(3)Analyzed the two-dimensional forced motion vortex leakage mode,set different motion amplitudes and motion cycles,found that the vortex leakage mode was converted from 2S to 2P or P+S mode,and compared with the classic experiment,reproduced the experimental results.(4)Analyzed the influence of forced motion on the flow field,and found that the greater the average force of the resistance on the cylinder,the stronger the resistance coefficient curve and the amplitude.When the motion amplitude is the same,the resistance coefficient has a certain negative value.In order to realize the fluid-solid coupling characteristics of vortex-induced motion,the motion system is simplified to a mass-spring-damping system,the fourth-order Runge-Kutta method is used to solve the differential equations of motion,and UDF is programmed and embedded in the Fluent software,combined with dynamic mesh technology In order to realize the numerical simulation of vortex-induced motion characteristics.(1)Analyze the force of unidirectional vortex-induced motion and compare it with the characteristics of the flow around a cylinder.It is found that the average value and mean square error of the drag coefficient increase first and then decrease.When the reduced speed is less than 1.92,it will be compared with the flow around the cylinder.The difference is small;the frequency-contrast flow around the cylinder increases with the increase of the reduced speed,and the one-way vortexinduced motion locks in at the reduced speed of 5.53-16.05.(2)Research on the amplitude of the two-way vortex-induced motion,the simulation results are in good agreement with the classical experimental results,and can reflect the true experimental conclusions;the comparison of the one-way and two-way vortex-induced motion,the difference in drag coefficients is small,and the lift coefficient data exists Large fluctuations.(3)The effects of high and low damping ratios on the characteristics of vortex-induced motion are studied.First,in the low damping ratio,there is a local peak in the downstream amplitude,while the high damping ratio does not have a peak.The reason is the frequency lock of the vortex-induced motion.Secondly,in the force study of different damping ratios,as the damping ratio increases,the phase difference between lift force and cross-flow displacement decreases,while the range continues to narrow;drag force and downstream displacement are transformed from the same phase to the opposite direction.The area becomes wider,and the phenomenon of "beating" is found in the low damping ratio.(4)Compare the vortex leakage modes under the flow around a cylinder,forced motion,one-way vortex-induced motion,and two-way vortex-induced motion,and study the evolution of the vortex leakage mode.In summary,this article has conducted an in-depth study on the vortex-induced motion of the Spar platform through numerical simulation,and analyzed in detail the effects of damping ratio,reduced speed,motion amplitude,vortex leakage mode and other factors on the vortex-induced motion of the Spar platform.A more reliable conclusion was obtained,and the mechanism of the vortex-induced motion of the Spar platform was initially revealed,providing a reference for the development and design of the offshore platform. |
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