Experimental Investigation Of Drag-Reduction Mechanism For Superhydrophobic Surfaces In Wall-Bounded Turbulent Flows With Time-Resolved Particle Image Velocimetry

As a passive control method for turbulent drag reduction,the turbulent boundary layer flow with superhydrohobic surfaces(SH)was investigated by TRPIV experiments and the drag-reduction mechanism was studied as while.The article put forward the concept of multi-scale spatial locally-averaged structure function and quasi-vorticity on the basis of velocity vector fields measurand by the time-resolved particle image velocimetry(TRPIV).The spatial topologies of the coherent structures in turbulent boundary layer(TBL)over hydrophilic surfaces(PH)and superhydrophobic surfaces,which obtained through the conditional sampling and phase average methods to discuss the influence of coherent structures.Drag-reduction rate for SH surfaces is 5.8%,which is calculated through the analysis of mean velocity profile atRe_??13500.The mean velocity profiles,turbulence intensity and Reynolds stress along wall-normal direction straightforward showed drag reduction characteristics of SH.The ejection and sweep events at different normal positions were detected over PH surfaces and SH surfaces.Results indicate that the SH surfaces can weake the burst strength.Based on the multi-scale spatial locally averaged structure function,we proposed new concept and detection method of quasi-vorticity to obtain quasi-vorticity spatial distribution maps,which provide a basis for extracting coherent structures.Then,the multi-scale spatial locally-averaged vorticity were utilized to extract the spatial topologies of the clockwise and retrograde vortex.In this study,the angle?,which is the angle between shear line and the horizontal line,was compared between SH surfaces and PH surfaces to research the drag-reduction mechanism of SH surfaces.And we found the influnence produced by retrograde vortex on clockwise vortex.Finally,we studied the effect of retrograde vortex on drag-reduction mechanism.For one thing,the distributions of streamlines around clockwise and retrograde vortex visualized vortex structure and their relative position relation.For another,through the comparative research on the number of retrograde vortex,for SH surfaces,the occurrence probabilities of retrograde vortex seems to be bigger than SH surfaces.To draw a conclusion:retrograde vortex can inhibt the development of the corresponding clockwise vortex and restrain its strong ejection and sweep events.Accelerating the fluid in the near-wall region is also the direct cause for the drag-reduction effect of SH surfaces.These results show that the drag-reduction effect of SH surfaces is related to the differences of retrograde vortex.