Effect of superhydrophobicity on the wake of a pitching foil across various Strouhal numbers

Abstract

Particle image velocimetry-based measurements have been performed to study the flow over a pitching superhydrophobic NACA—National Advisory Committee for Aeronautics—0012 foil (SphNACA0012 foil) and to compare the same with NACA0012 foil. This is perhaps the first experimental study on a pitching SphNACA0012 foil. The objective is to understand the effects of superhydrophobicity on NACA0012 foil pitching about its leading edge at various Strouhal numbers, St = 0.08–0.65, with a constant pitching amplitude, θ0 = 4°, and Reynolds number, Re = 2400. Superhydrophobicity is found to significantly affect three different flow regimes: 2S von-Ka´rma´n wake, symmetric wake, and asymmetric wake regimes at smaller, intermediate, and larger values of St. In the 2S von-Ka´rma´n wake regime, although the wake structures of both the foils are quite comparable, a reduction in the shear layer strength and early rolling-up of vortices are observed for SphNACA0012 foil due to a partial slip effect. With a further increase in St, a transition to the symmetric wake regime is observed for both the foils and another transition that leads to a symmetry breaking of the vortex street is found to be earlier for SphNACA0012 foil, which results in the onset of lift generation at lower St than that of NACA0012 foil. The shear layer strength is weaker for the SphNACA0012 foil except at St = 0.24, which is a critical St for the jet deflection point and results in a 13% larger thrust coefficient for SphNACA0012 foils as compared to NACA0012 foils. These results on the larger lift as well as thrust coefficient at low St suggest improved maneuverability with high propulsive efficiency when superhydrophobicity is employed on the propulsor of underwater vehicles.