Effects of superhydrophobic surfaces on the flow around an NACA0012 hydrofoil at low Reynolds numbers

Abstract

In the present study, the effects of superhydrophobic surface on the flow around an NACA0012 hydrofoil are experimentally investigated at low Reynolds number range of 0.2–1.0 $$\times 10^{4}$$ . The velocity fields were measured using two-dimensional digital particle image velocimetry in a water tunnel while varying the angle of attack from $$0^\circ$$ to $$20^\circ$$ . The spray-coating of hydrophobic nanoparticles was used to create superhydrophobic surfaces. Depending on the Reynolds number and angle of attack, we found that the effects of superhydrophobic surface show up differently, which is determined by the relative strength of turbulence caused by both the surface slip (influence of trapped air pockets) and roughness, compared to that of background (i.e., uncontrolled) flow. In general, the superhydrophobic surface imposes a little influence on the wake behind a hydrofoil when the angle of attack is very low (attached flow) or high (fully separated flow). At intermediate angles of attack (flow separates between the leading and trailing edges), however, it is measured that the flow over superhydrophobic surface has a stronger turbulence, and thus, the enhanced shear-layer instability forces the early vortex rollup in the wake and reduction of vortex formation length. Interestingly, there is a transitional range of angle of attack, in which this effect is reversed, and thus, the vortex rollup is slightly delayed. This trend can be explained based on the changes in the uncontrolled flow structures, and finally, we classify the effects of superhydrophobic surface in terms of Reynolds number and angle of attack, in the considered ranges of both.