| In offshore engineering,the construction of structures such as offshore wind power,subsea oil and gas pipelines and offshore bridges requires a large number of pile legs to support them,and when water flows through the pile legs,it generates resistance to bypass flow,which easily leads to tilting,swinging or destabilization of the structures,thus adversely affecting the safety and stability of the structures.In order to allow the water to flow more smoothly through the pile legs and reduce the resistance of the water to bypass the pile legs,thus improving the safety and stability of the offshore structures.The study of pile leg bypass resistance reduction is simplified to the study of cylindrical bypass resistance reduction,and the asymmetric groove structure is reconstructed by imitating the bird feather structure based on the previous studies,and the surface cylindrical bypass resistance reduction characteristics of the asymmetric groove structure are investigated by combining numerical and experimental studies.Firstly,numerical simulations of smooth cylinders and asymmetric grooved cylinders are carried out at Reynolds number Re=6×10~3 in this thesis.The single-factor variable method is used to compare the average drag coefficients of asymmetric grooved cylinders with different depths and different numbers.The maximum drag reduction was 6.11%by varying only the depth of the asymmetric grooves.The maximum drag reduction rate was 18.04%by changing only the number of asymmetric grooves.The results show that:the number of grooves has a greater effect on the resistance reduction effect of cylindrical winding;the depth of grooves is too deep or the number of grooves is too much will have the effect of increasing resistance.Therefore,this thesis selects the asymmetric groove cylinder with drag reduction effect and can reflect the trend of drag reduction rate with depth and number of grooves according to the average drag coefficient.The main contents of the numerical simulation are spectral characteristics,time-averaged flow line,separation point prediction,time-averaged pressure,time-averaged friction coefficient and trailing vortex intensity.The results show that:the dominant frequency of vortex shedding is lower in the asymmetric grooved cylinder compared with the smooth cylinder;the recirculation zone is reduced;the separation point is closer to the downstream;the front-to-back pressure difference is smaller;the surface frictional resistance obtained from the undercut stress integral is smaller;and the peak of trailing vortex intensity is smaller.Then,in this thesis,the particle image velocimetry(PIV)technique is used to experimentally study a cylinder with Reynolds number Re=6×10~3 to compare the wake structures of different numbers of asymmetric grooved and smooth cylinders.The time-averaged streamlines,time-averaged velocities,root mean square velocities,Reynolds stresses and turbulent kinetic energies were investigated and found that:the recirculation zone of the asymmetric grooved cylinder was reduced compared with that of the smooth cylinder;the velocity gradient was smaller at the same profile location and at each location;the peak values of the flow direction velocity fluctuation intensity and lateral velocity fluctuation intensity were reduced and the locations were delayed;and the Reynolds stresses and turbulent kinetic energies were reduced.Finally,the mechanism of drag reduction in asymmetric groove structure is discussed from the perspectives of POD mode energy distribution,POD mode coefficients and correlation of POD mode coefficients using the intrinsic orthogonal decomposition(POD)method.The results show that:the energy of the dominant mode of the asymmetric groove cylinder is lower compared with that of the smooth cylinder;the vortex shedding intensity and vortex shedding frequency of the mode coefficients are lower;the correlation of the mode coefficients is not related to the quasi-circle,and the coupling effect of the POD modes is weakened. |
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