PIV Experimental Research Of Coherent Structures In Wall-bounded Turbulent Flow And Drag Reduction Mechanism By Superhydrophobic Surfaces

The coherent structure is recognized as the most important structure in turbulenece.It has a significant influence on the maintenance,evolution and development of the turbulence and has an important value in engineering applications and academic research domain.The keystone of this dissertation is coherent structure in wall-bounded turbulent.Three related problems were studied by using the advanced fluid mechanics experimental techniques,including Stereo-PIV,Tomo-PIV and TR-2DPIV:?a?The real3D hairpin vortex structure was generated and then measured quantitatively;?b?the hydrodynamic law of the local flow field in the adjacent region of low/high-speed streaks was analyzed;?c?Drag reduction mechanism of wall-bounded turbulent by using the superhydrophobic?SH?surfaces was discussed from the perspective of coherent structure.During the progress,the spatial flow field distribution characteristics of hairpin vortex,low/high-speed streaks and ejection/sweep events were explored,as well as the development and revolution of spanwise vortex structure.Typical haipin vortex was generated in the boundary layer flow by synthetic jet and measured by phase-lock Stereo-PIV experiments.The 3D structure of hairpin vortex during the full cycle of synthetic jet was reconstructed successfully.By analyzing the real 3D flow field which contains the hairpin vortex,the finding extended our understanding on hairpin vortex:?a?A thin spanwise vorticity concentrated layer,a strong shear zone,was generated by the eject movement and external environment fluid;?b?High-speed streaks exist between the two legs from two adjacent hairpin vortices or the outer side of a single hairpin vortex leg and it corresponds to a sweep event;?c?The spanwise velocity distribution of a single hairpin vortex presents two layers on plane.The fluid which on the near wall side of two counter rotating vortex legs,is converging to the middle.While,the fluid flow is divergence on the other side.The former is dominat.Based on these new findings,the structural characteristics of typical hairpin vortices were summarized.Its significance is obvious.According to the topological information of target flow field,we can rebuild the flow structures,and then the relevant flow mechanism is expounded.The Tomo-PIV technique is used to investigate the coherent structures in the adjacent region of the low/high-speed streaks along the streamwise in the turbulent boundary layer.The locally-averaged-structure function,conditional extracting and averaing method were used to gain the topological coherent structures of conditional event.A quadrupole structure and a sextupole structure are detected for the first time by the experimental method.The topological information reveals the existence of a three pairs of counter-rotating hairpin vortex in the adjacent region of the low/high-speed streaks.According to the typical topological structure characteristics of the hairpin vortex,a“three hairpin vortex”model was precented and it can answer the connection among the low/high-speed streaks,the ejection/sweep event,and the hairpin vortex.The 3D model can be used to explain the physical mechanism of the arrangement rule of low/high-speed streaks in streamwise direction.From the perspective of coherent structures,drag reduction mechanism of wall-bounded turbulent over hierarchical structured SH surfaces were investigated based on the data in thex-yplane andx-z plane which were obtained by TRPIV.The drag reduction rate is about 11%.Turbulent statistics,topological information of coherent structure and the development and revolution of spanwise vortex structure were analyzed.In the near wall region(y?27?1.0?ory01???27?30),the mean velocity profile,the distribution of streamwise turbulence intensity and the Reynolds stress are all reflect the characteristics of drag reduction.The abnormal region of wall-normal turbulence intensity,where a bump was emerged aroundy^?10??28?20⁶0,gave an experimental proof for spanwise slip increases drag which obtained by direct numerical simulation?DNS?.The topological information about coherent structures onx-yandx-z planes and the development and revolution of spanwise vortex structure onx-yplane were analyzed.Combined with the typical topological structure characteristics of the hairpin vortex,the generation model of hairpin vortex and hairpin vortex packets model over SH surfaces were established.Then the drag reduction mechanism over SH surfaces can be given:Due to the influence of the SH surfaces,the structures of quasi-streamwise vortice and the single hairpin vortex in the turbulent boundary layer have a smaller inclination angle on the wall surface,the spanwise distance is larger,and the motion intensity is also weaker.Thus,the more ordered hairpin vortex packet structure produces a low-speed streak?low momentum region?with a shorter streawise scale and a wider spanwise distance.However,the propation velocity of the complete structure is faster and the streamwise velocity of low-speed fluid wrapped inside the vortex packet is also greater than normal condition.Furthemore,the motion of coherent structure over SH surfaces is not strong and causes the weaker of turbulence intensity and Reynolds stress.These hydrodynamic behaviors are conducive to the realization of turbulent drag reduction over SH surfaces.