Study On Flow Around A Cylinder With The Dimpled Structure On Its Surface And Its Drag Reduction Characteristics

Offshore platform is the foundation of marine production,operation and life.At the same time,with the increasing of the operating depth of the offshore platform,the harsh working environment has put forward higher requirements for the overall performance of the offshore platform.The pile leg structure is an important structure to support the offshore platform and also an important index to test the overall safety performance of the offshore platform.The structure of pile leg will be affected by different operating depth,ocean current environment and working device.The periodic lift and drag of the pile leg in the ocean causes great damage to the pile leg.In this context,in view of the problem of reducing drag and vibration,the drag reduction effect of non-smooth dimpled surface on cylinder is discussed in this thesis,and the change of turbulent structure of dimpled surface is explained.Firstly,the numerical simulation is carried out on the smooth cylinder and the circular cylinder with the dimpled surface.The numerical simulation includes simplification of pile leg model,design of dimple parameters,selection of simulation method,selection of turbulence equation and meshing,etc.The parameter of the dimple is focused on the depth of dimple,and the numerical simulation is focused on the comparison of turbulence models and the meshing of the circular cylinder with the dimpled surface.After that,the simulation results are discussed.The numerical simulation focuses on the force coefficient,vortex shedding,time-averaged velocity field and the recirculation zone of the smooth cylinder and the circular cylinder with the dimpled surface.The results show that when the Reynolds number is 1×10~5,the dimpled structure can effectively reduce the drag of the cylinder.The maximum drag reduction rate reaches 19.00%when four-column dimples are distributed on the surface of the cylinder.The total drag of the circular cylinder with the dimpled surface is affected by the combined action of pressure drag and skin friction drag.On the other hand,the vortex shedding strength of the circular cylinder with the dimpled surface decreases,but the shedding frequency increases slightly.Through the distribution of the time-averaged velocity,it is found that the relative slip caused by the vortex cushion effect increases the velocity gradient of the boundary layer and the turbulent flow changes in advance,thus improving the flow field properties.Besides,the time-averaged vortex structure is analyzed,and it is found that the time-averaged vortex scale of the circular cylinder with the dimpled surface is larger than that of the smooth cylinder,which indicates that small scale vortices or secondary vortices are generated in the location of the dimpled structure.Thirdly,the particle image velocimetry(PIV)technique was used to observe the wake flow fields of the smooth cylinder and the circular cylinder with the dimpled surface.A total of 12 numerical examples with three flow rates and four cylindrical surfaces are planned.After data extraction,the velocity and recirculation zone were used to verify the numerical simulation.Then,a preliminary analysis of the fluctuation velocity shows that the energy dissipation of the circular cylinder with the dimpled surface is slower than that of the smooth cylinder.Finally,the proper orthogonal decomposition method(POD)is used to discuss the drag reduction mechanism in terms of POD mode energy distribution,POD mode coefficients,the correlation based on POD mode coefficients,and the coherent structure of POD modes.The results show that the energy of the dominant mode for the circular cylinder with the dimpled surface is lower than that for the smooth cylinder.At the same time,the fluctuation strength of the mode coefficient of the circular cylinder with the dimpled surface is lower than that of the smooth cylinder.The correlation of the mode coefficients also loses the interference law due to the dimpled structure.The coherent structure also shows that the inherent flow mode of wake flow is broken by the dimpled structure.On the other hand,based on the POD mode coefficient,the time-frequency analysis using the continuous wavelet transform(CWT)shows that compared with the smooth cylinder,the energy distribution of the wake of the circular cylinder with the dimpled surface is more dispersed.