TRPIV EXPERIMENTAL INVESTIGATION OF THE EFFECT OF RETROGRADE VORTEX ON DRAG-REDUCTION MECHANISM OVER SUPERHYDROPHOBIC SURFACES

The discovery of coherent structures in turbulent in the last century suggests affinew direction for turbulent controy, especially the vortex in the near-wall region, which has a great contribution to the skin friction. In this study, the spanwise vortex characteristics, compared between superhydrophobic (SH) surfaces and hydrophilic (PH) surfaces in the turbulent boundary layer, were investigated by time-resolved particle image velocimetry (TRPIV). Firstly, the multi-scale spatial locally-averaged vortices were utilized to extract the spatial topologies of the spanwise vortex head (clockwise vortex) and retrograde vortex, and this method accurately identified the vortex core and excluded the interference of the minor scales vortex. Secondly, the distributions of streamline around clockwise and retrograde vortex visualized vortex structure properly to study relative position relation between the two kinds of vortex. Finally, 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. There is a conclusion that retrograde vortex can inhibit 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 show that the drag-reduction effect (approximately 5.8% when Re_δ≈13 500) of SH surfaces is related to the differences of retrograde vortex.