Flow over hydrofoils with varying hydrophobicity

The effect of the shape of a hydrofoil on its force characteristics is well documented; however, the role of the surface on the ensuing flow is not yet understood. Here, we report the effect of hydrophobicity on the flow dynamics of NACA0015 hydrofoil based on more than 100 particle image velocimetry based experiments. Acrylic, Cellophane tape, Teflon coated, and superhydrophobic painted surfaces are studied for Reynolds number range of 6500-30,800 at five angles of attack (0-20 degrees) in this work. The static contact angle varies from 70 degrees to 152 degrees for these surfaces. The superhydrophobicity helps to reduce the range of Reynolds number where the flow separation occurs. We observe up to 40% reduction in drag at 15 degrees angle of attack for Re = 30,800, and superior hydrodynamic performance for the superhydrophobic painted surface as compared to the bare Acrylic surface. The superhydrophobic painted surface is also found to delay flow separation and stall angle (by up to 5 degrees based on the gliding ratio), which is attributed to a reduction in the separation bubble size (by up to 56%). The Teflon coated surface behaves similar to the superhydrophobic painted surface until an angle of attack of 5 degrees. The vortex strength is reduced for the superhydrophobic painted surface at 15 degrees angle of attack compared to the Acrylic surface. The results of this study show that the hydrodynamic efficiency can be increased and also sustained over a relatively large range of angle of attack by employing superhydrophobic surfaces.