| The motion of coherent structures（CSs）is the prominent reason of high friction in turbulent boundary layer（TBL）.The researches on the time evolution characteristics of CSs,the relationship between different types of CSs,and the relationship between CSs and wall friction drag can provide important theoretical support for flow drag reduction（DR）control,so it has great academic and application value.The study of the mechanism in DR control will improve the theoretical development of the dynamic characteristics of CSs in TBL.In the present Ph D dissertation,a moving time resolved particle image velocimetry（MTR-PIV）system was designed to track the time evolution characteristics of CSs in TBL.The time filtering method is developed to remove the mechanical vibration noise in the flow fields.The time evolution characteristics of hairpin vortices,hairpin packets,very-large-scale motions（VLSMs）and uniform momentum zones（UMZs）are analyzed.The relationship between hairpin packet and UMZs is revealed.Then,the time-resolved particle image velocimetry（TR-PIV）is used to experimentally research the statistical characteristics of UMZs and its influence on the friction drag.The analysis is based on classificational method.The friction drag is measured by the single-pixel ensemble correlation（SPEC）method.Finally,the synthetic jet actuator is used to perform the active control in TBL.The TR-PIV is used to measure the flow field in the normal plane and spanwise plane.The single-row cross correlation（SRCC）method is used to calculate the instantaneous flow fields at the near wall regions.The influence of synthetic jet on the statistical characteristics of flow fields,CSs and the corresponding drag reduction mechanism are studied.The experimental results of MTR-PIV show that,the collision of large-scale ejection and large-scale sweep makes large-scale high and low speed coherent fluids break into small-scale structures.The generation of hairpin packets requires more strict conditions than hairpin vortices.The small-scale low momentum fluids increase its spatial scale by merging with other small-scale low-speed fluids,and then form a large-scale low-speed fluid mass to the make the independent hairpin vortices to be organized into a coherent hairpin packet.The length of the continuous shear layer in the hairpin packet is a key factor for the off-spring generation of hairpin vortices.The generation of very-large-scale low-speed bands is similar to the large-scale low-speed fluid mass in hairpin packets.The evolution of UMZs below the half boundary layer thickness is quasi periodic.The hairpin packets impact the lower UMZs by inducing large-scale low-speed fluid mass to modulate the probability density function（PDF）distribution of instantaneous streamwise velocity.The evolution of UMZs with velocity less than0.5∞is the result of the interaction of large-and small-scale streamwise fluctuation velocity.The statistical characteristics of UMZs show that UMZs with height less than0.51contain low momentum fluids,UMZs with height greater than 0.51contain high momentum fluids.The increase of（the number of the UMZs in the instantaneous field）will increase the intensity of the large-scale fluctuation velocity.The fields with lowhave relatively high friction drag,while the fields with highhave relatively low friction drag.Under 0.51,fields with lowpresent large-scale Q4 events,fields with highpresent large-scale Q2 events,the increase ofwill transform the Q4 events into the Q2 events in the fields.The Q4events in the fields with lowwill raise the friction drag by enhancing the small-scale turbulent energy and Reynolds shear stress at the near wall region.The Q2 events in the fields with highwill reduce the friction drag by suppressing the small-scale turbulent energy and Reynolds shear stress at the near wall region.Increasing the proportion of velocity fields with highmay be a direction to reduce the skin-friction drag within the TBL.The experimental results of the active control show that the synthetic jet generates a long low-speed trip downstream of the outlet,and increases the thickness of the boundary layer and viscous sublayer.The enhancement of turbulence fluctuation and Reynolds shear stress by the synthetic jet is mainly concentrated in the parabolic strip region in the outer region.The streamwise fluctuation and Reynolds shear stress are suppressed,while the wall-normal fluctuation is enhanced in the near wall region with+>300.The induced effect of the hairpin packet generated by the synthetic jet is the reason for the low-speed region downstream of the outlet.In the near wall region with+>300,the large-scale Q2 and Q4 events are suppressed,the large-scale Q1 and Q3events,and small-scale Q events are enhanced.Active control of the synthetic jet transform the large-scale streamwise fluctuation into the large-scale wall-normal fluctuation and small-scale fluctuation,and reduces the average velocity gradient of the viscous sublayer,so as to reduce the friction drag. |
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