An Experimental Investigation of the Impact of Anisotropic Slip Length on Turbulent Flow Over Superhydrophobic Surfaces Within an Open Channel

Abstract

This study investigates the sustainability and applicability of commercial superhydrophobic (SH) coatings for reducing skin friction drag. Three different SH surfaces were applied to flat plates using a spray coating technique, with static contact angles of 145 deg, 147 deg, and 155 deg, respectively. Turbulent flow measurements were conducted using a two-dimensional laser Doppler velocimetry (LDV) system in an open channel flow facility at a Reynolds number of 34200. The novelty of this work lies in characterizing drag reduction from the leading edge to the trailing edge of the fabricated surface in the streamwise direction rather than one measurement plane. Velocity measurements were performed in a spanwise direction at selected planes. The study also evaluated the correlation between slip velocity and slip length, showing that slip length becomes equivalent to the coating thickness as the plastron depletes. The fabricated SH surfaces increased turbulence intensity and Reynolds normal stress, primarily near the wall, with diminishing effects further away. This confirms the existence of an interference region of air/water near the wall induced by SH surfaces. Overall, the results demonstrated average drag reductions of 11%, 7%, and 18% for the tested surfaces. The study provides strong evidence for the effectiveness of SH surfaces in consistently reducing viscous drag across the entire plate span, from the leading edge to the trailing edge.