Experimental investigation on the modified coherent structures in the turbulent boundary layer by superhydrophobic surface

This work presents an experimental investigation on the modified coherent structures in the flat-plate turbulent boundary layer (TBL) by superhydrophobic (SHPo) surface. Variable interval spatial average technique is applied to detect the bursting events which are closely associated with the production of turbulent kinetic energy. The occurrence of bursting events becomes less frequent by SHPo surface, accompanied by the attenuation of their strength. Conditional averaged flow field based on the detection shows that the coherent structures are elongated in streamwise direction by the SHPo surface, which is consistent with the distribution of the spatial cross correlation coefficient. Quadrant analysis is performed and the contour lines of the joint probability density function (PDF) around the bursting events are more concentrated the small values for the TBL with SHPo surfaces, resulting in the reduced appearance of large velocity fluctuation. The weighted joint PDF shows that the contribution of ejection and sweep events to the total Reynolds shear stress is largely reduced by the SHPo surface. Swirling strength is calculated to detect both the prograde and retrograde vortices around the bursting events, both of which become significantly smaller for the TBL with SHPo surface. Statistical analysis of the swirling strength shows that both the mean and root mean square values are apparently suppressed by the SHPo surface.