Modification of the uniform momentum zones in the turbulent boundary layer by superhydrophobic surface

Experimental investigation is carried out in a water tunnel to study the influence of the superhydrophobic (SHPo) surface on the coherent structures, especially the uniform momentum zones (UMZs) and their edges, in a zero-pressure gradient turbulent boundary layer (TBL) at the friction Reynolds number of 650. Particle image velocimetry (PIV) technique is applied to capture the instantaneous velocity field in the streamwise-wall-normal plane. The UMZs are detected based on the probability density function of the PIV-measured instantaneous streamwise velocity. The mean value of the UMZ number is reduced by the SHPo surface, indicating the more organized coherent structures. The modal velocity of the UMZs under SHPo surface is always higher than that with smooth wall at the same wall-normal location, which may directly result from the velocity slip on the wall. The internal interfaces between the neighboring UMZs are further examined. The area fraction of the internal interfaces relative to the total TBL is reduced by the SHPo surface, consistent with the smaller number of the detected UMZs. Conditional average is carried out based on the internal interfaces and the statistical characteristics of the velocity gradient at the interface are compared. Both the mean and root mean square values of the velocity gradient are reduced for the SHPo surface, resulting from the weakened spanwise vortices at the internal interfaces.