Drag Reduction Characteristics And Mechanism Of Pufferfish Skin And Biomimetic Spine-covered Protrusion Surface Based On Circulating Water Tunnel

The coherent structure generated within the turbulent boundary layer(TBL)is closely related to the wall frictional drag and is an important object of study for turbulence drag reduction.The surface structure of organisms in nature has evolved to possess powerful drag reduction capability,which can provide an important bionic basis for improving energy efficiency in ship submarine and aerospace.In this thesis,inspired by the characteristics of the pufferfish skin,we use particle image velocimetry(PIV)to analyze the drag reduction characteristics of the body surface of pufferfish,and designed biomimetic spine-covered protrusion(BSCPs)samples to investigate how these samples affected the coherent structure and analyze the drag reduction mechanism within TBL.The specific research of this thesis is as follows.(1)Fresh skin of pufferfish was obtained,and four epidermal samples with three kinds of drag reduction characteristics(roughness,flexibility and mucus)were made for PIV.The results show that the “roughness” feature has the greatest effect on the coherent structure near the wall and produces the least Reynolds shear stress within the log-law layer.Decomposition of the turbulent kinetic energy in different directions shows that the effect of the rough feature on the flow direction turbulent pulsation < ' > is weak,while the improvement of the normal turbulent pulsation < ' > is the most obvious among the three features,which effectively suppresses the generation of the near-wall coherent structure.The “flexibility” and “mucus” features mainly affect the turbulent structure in the outer region of the boundary layer,both of which have basically no difference in the flow and normal correlation coefficient extension length , in the log-law layer compared with the smooth skin,and the effect on the near-wall coherent structure is very weak.(2)In the case of Reynolds number 0)= 2.7?4.7 × 10,force and PIV velocimetry experiments were carried out on three rough forms(hydraulically smooth,transitional rough,fully rough)and two types of array(aligned,staggered),a total of six different BSCPs samples and smooth sample.The results showed that only the hydraulically smooth and staggered sample produced the drag reduction effect,with the maximum drag reduction rate of 6.59%,corresponding to the upward shift of the velocity profile compared with that of the smooth surface.However,as the Reynolds number 0)increases,the drag reduction effect will gradually become worse.The Reynolds shear stress,turbulent kinetic energy,and turbulence intensity of the hydraulically smooth sample are still the best among various samples,while the other two rough forms produce serious perturbations to the boundary layer structure.In addition,the staggered arrangement of smaller , affects the size of the coherent structure in the normal and spreading directions,while the aligned array has only a weak effect on the flow direction turbulence.(3)Using various coherent structure identification criteria,we analyze the generation of coherent structures near the walls of two hydraulically smooth samples to further reveal the drag reduction mechanism of BSCPs samples.As the roughness height of BSCPs decreases,the vorticity near wall decreases,and the proportion of vorticity decreases significantly within the log-law layer,and the staggered array is significantly better than the aligned one.The peak vorticity of the hydraulically smooth sample is reduced by 30-50%compared with other samples.At the same time,the vortex intensity and energy burst intensity of the staggered array are lower near wall compared with the smooth plate,which effectively inhibits the generation and development of turbulent coherent structures.