Unexpected trends of lift for hydrofoils with superhydrophobic coating

Abstract

In this paper, the physics of external flow past a superhydrophobic SD7003 hydrofoil is studied at Reynolds number of 105. We used Navier’s slip length to model the superhydrophobic surfaces, and investigated the effect of different slip lengths on hydrofoil performance by means of RANS (Reynolds-averaged Navier–Stokes) simulations. The slip length boundary condition is imposed on upper, or/and lower side(s), to study the effect of each superhydrophobic region on lift coefficient. By increasing the slip length (ls), it was expected to see lift enhancement and drag reduction. However, the lift enhancement trend had some exceptions. The unexpected trends were explained by studying the location of laminar separation and reattachment for different angles of attack (AOA) and slip lengths. For example, the lift coefficient of a full superhydrophobic hydrofoil at AOA = 2° does not change monotonically against slip length. By increasing the slip length, separation and reattachment points are shifted towards the trailing edge and at slip length 200 μm, there is separation with no reattachment which results in lift reduction. By further increasing the slip length, the separation is shifted further down and eventually there would be no separation n\\and flow becomes fully attached.