The velocity-vorticity correlation structure in turbulent boundary layer on superhydrophobic surface*

Effects of superhydrophobic surface (SHS) on skin-friction drag were investigated through experiments of a turbulent boundary layer (TBL), with spray coating of hydrophobic naonparticles to make a superhydrophobic coating on a flat plate, in comparison with an uncoated aluminum plate. The instantaneous velocity fields near the wall were measured through particle image velocimetry (PIV) capable of resolving the flow down to the viscous sublayer. A significant drag reduction was observed on the superhydrophobic plate up to 10% in the fully turbulent region. We applied the velocity-vorticity correlation structure (VVCS) to capture the coherent structures in the near-wall region, and measured the scale and the wall distance of VVCS, which were found consistent with the postulation of 'slip velocity'. By the analysis of structure ensemble dynamics (SED), the mixing lengths for the superhydrophobic and the uncoated surfaces was obtained, and show that the near-wall turbulence for both surfaces has the same multilayer structure but different scales. The same forms of VVCS and the mixing length functions for both flows display the similar features of the TBLs on SHS and the smooth surface.