The potential of using superhydrophobic surfaces on airfoils and hydrofoils: A numerical approach

Abstract

Fluids at their interface with ordinary solids are motionless. This condition is referred to as no-slip condition. On superhydrophobic surfaces, fluids have slip velocity which is quantified using Navier's slip length definition. On a superhydrophobic surface, slip velocity can be as large as 50% of the free-stream's velocity. We have studied the potential of using superhydrophobic surfaces to improve the performance of airfoils. For that, National Advisory Committee for Aeronautics (NACA) 4412, 4418, and 4424 were studied numerically. The chord-based Reynolds number was approximately 5,000. We found that increasing the slip from 0 to 50% results in up to 66% increase in the lift, and 45% decrease in the drag force when angle of attack is small (i.e., < 5°). For larger angle of attack values (i.e., > 5°), using superhydrophobic airfoil is still worthy, but its effectiveness becomes smaller. The less efficacy of superhydrophobic airfoils is explained by the laminar separation bubble phenomenon which can have an adverse effect on lift and drag. For small angle of attack values, by increasing the slip from 0 to 50%, the bubble length becomes smaller which is favourable and explains the well-behaviour of superhydrophobic airfoils at small angle of attacks. However, for larger angle of attack values, by increasing the slip, bubble's length grows which results in less efficacy of superhydrophobic airfoils at larger angle of attack values.