Assessment of Clauser chart method for determination of drag on superhydrophobic surfaces*

Superhydrophobic surfaces (SHS) are patterned hydrophobic surfaces that reduce skin friction drag via the entrapment of air pockets when immersed in water. Experiments of patterned SHS have shown that pressure control can sustain wall-attached air films in turbulent flow. Additionally, the dynamic modulation of air film height can further enhance drag reduction, as measured by the Clauser chart method. In this study, we numerically simulate the impact of air film oscillation on drag reduction in a turbulent channel. We quantify how the Clauser chart method, which assumes a log layer region characteristic of smooth walls, performs on patterned SHS, as compared to direct measurement of wall shear stress. We present potential modifications to the Clauser chart method for SHS, motivated by the modified Clauser chart method proposed for rough walls (Perry and Li, J. Fluid Mech. 2006), and discuss relevant findings on the flow structure over both flat and pressure-modulated superhydrophobic surfaces.