Research On Vortex-induced Vibration Suppression Of Underwater Mooring Platform With The Ridged Structure

The underwater mooring platform is a type of underwater equipment that is placed in the ocean and directly connected to the sea floor by means of an anchor chain.It can be equipped with sensors such as pressure,water temperature,cameras,underwater acoustic monitoring,underwater acoustic communication,and water quality monitoring to achieve different the task,which has broad application prospects in both military and civilian areas.Aiming at the working stability of the underwater mooring platform,this paper studies the hydrodynamic characteristics and the vortex-induced vibration suppression mechanism of the underwater mooring platform in combination with the drag reduction theory of ridge surface structure.The main research results are as follows:（1）In order to study the effect of ridge structure surface on the suppression of vortex-induced vibration of the underwater moored platform,a numerical prediction model of an underwater moored platform on vortex induced vibration was established.First,the relevant mathematical models of vortex-induced vibration and underwater mooring cables need to be understood.Then a spring-damping-vibrator motion model of the underwater mooring platform on vortex induced vibration was established,and the Newmark-βmethod was used to solve the fluid-solid coupling response.Finally,the feasibility of the numerical model was verified through classical experiments.（2）Although the active control method can effectively control the separation of the cylindrical boundary layer,it requires the input of external energy,so it is costly and difficult to operate.This paper proposes a passive control method based on the ridge surface structure.Thedistributionpositionsoftheridgestructuresare30°≤≤60°,120°≤≤150°,-60°≤≤-30°and-150°≤≤-120°.With the increase of the ridge structure parameterε,C_d and C_L decrease first and then increase,and S_tcontinuously decreases.And it shows that the vortex shedding of the cylinder is controlled,and the ridge surface structure is conducive to suppressing the vortex shedding.The optimal parameter isε=0.05（h=2mm）,correspondingly,DR=29.87%and DL=65.39%.Whenθ=15°,there is the best drag reduction effect（DR=52.29%and DL=72.7%）.Whenθ=10°,the ridge structure has the most obvious control effect on the flow around the cylinder,and St shows a minimum value of 0.172,correspondingly,DR=25.07%and DL=52.36%.（3）By constructing cone-shaped and wave-shaped ridge surface structures to control the separation of the boundary layer,the effect of vortex-induced vibration suppression is achieved.The surface is made into a continuous sawtooth-like cone-shaped structure,and a total of 9 types of different protrusion structures with different height ratios are numerically calculated（T1～T9）.The vortex-induced vibration suppression effect of T4（ε=0.04）is the best.The cone-shaped ridge structure has a certain drag reduction effect.The peak value of the drag coefficient can be reduced by 40%and the peak value of the lift coefficient can be reduced by 69%.Whenε=0.04,the vibration amplitude is 0.055m,compared with T1（vibration amplitude 0.09m）,which is reduced by 38.9%.The tapered grooved band does not change the structure frequency significantly.In order to study the vortex-induced vibration suppression of the cylindrical structure with the surface of the traveling wave structure,three sets of simulations were performed.The first group was ordinary surfaces,and the second and third groups were surfaces with different distribution densities of the traveling wave structures.The structure is only distributed on the semicircle in the direction of the incoming flow.Compared with the ordinary surface cylinder,the traveling wave structure surface has a more obvious vibration suppression effect,and the amplitude is reduced by about 92.97%.（4）A vortex-induced vibration test system for the underwater moored platform was established to study the vortex-induced vibration suppression.The effects of ridge structures with different distribution locations and different height ratios on the vortex-induced vibration suppression were studied.When the initial anglesθ_min of the distribution are 60°,74°,and 90°,the ridge structure has the worst vibration suppression effect.Especially for the high-reduction velocity range,the vortex-induced vibration is strengthened to a certain extent,and galloping may occur.When the initial anglesθ_min of the distribution are 30°,120°,and150°,the vibration suppression effect of the ridge structure is the best.Especially whenθ_min=120°,the distribution position of the ridge structure is optimal,and the amplitude ratio is reduced to 0.4,whcih is reduced by about 60%（U_r=4.5）.When the dimensionless dimensionsεof the ridge structure are 0.08,0.10,and 0.12,the vibration suppression effect of the ridge structure is the worst.Especially for the high-reduction velocity range,the vortex-induced vibration is strengthened to a certain extent,and galloping may occur.When the dimensionless dimensionsεof the ridge structure are 0.02,0.04,and 0.06,the vibration suppression effect of the ridge structure is the best.Especially whenε=0.04 and 0.06,the vibration suppression effect of the ridge structure is optimal,and the amplitude ratio is reduced to 0.4,which is reduced by about 60%（U_r=4.5）.