Effects of superhydrophobic-riblet surface on the uniform momentum zones in turbulent boundary layers

The effects of superhydrophobic-riblet (SR) surface on uniform momentum zones (UMZs) are investigated in a zero-pressure-gradient turbulent boundary layer at Reτ=uτδ/ν=667 (uτ is the wall friction velocity, δ is the nominal thickness of the boundary layer, and ν is the kinematic viscosity of water), based on water channel experiments. A superhydrophobic (SH) surface and a smooth surface (SM) are used as comparison groups. The instantaneous velocity fields covering the boundary layer region on the streamwise–wall-normal plane are obtained and analyzed via time-resolved particle image velocimetry. The results show that the SR surface achieves a drag reduction rate up to 17.06%, significantly exceeding the 10.68% for the SH surface. The turbulent/non-turbulent interface (TNTI) is identified using the local turbulent kinetic energy method, and the UMZs beneath the TNTI are identified based on the probability density function of the instantaneous streamwise velocity. The SR surface is found to reduce the mean number of UMZs owing to a reduction in hairpin vortex activity produced by a more organized boundary layer structure. Additionally, the increased modal velocities and thickness variations of UMZs in the near-wall region over the SR surface can be attributed to the enhanced streamwise flow slip close to the wall. Conditional averaged internal interfaces of UMZs carried out a significant attenuation of not only wall-normal velocity gradients, but streamwise fluctuations and spanwise vorticity on the SR surface as well.