Numerical study of the natural transition in hydrofoil boundary layers with superhydrophobic surfaces

The natural transition of the boundary layer on a hydrofoil with superhydrophobic surfaces is investigated using numerical methods. A novel calculation approach featuring the adapted vertical mesh of basic laminar flows on hydrofoils is proposed and validated against experimental velocity profiles. The eN method, corroborated by experimental data, is employed to predict the transition locations of the boundary layers. It is observed that the superhydrophobic surface stabilizes the boundary layer, thereby delaying the natural transition of the hydrofoil. As the slip length increases, this delay effect of the superhydrophobic surfaces on the transition becomes stronger. For superhydrophobic surfaces, it consistently occurs that the transition location on the suction side of a hydrofoil takes place further downstream than that on its pressure side, aligning with established principles regarding ordinary surfaces. As a new passive flow control strategy, different superhydrophobic surfaces applied to the suction side and pressure side can be utilized to achieve symmetrical transition characteristics on asymmetric hydrofoils. Furthermore, the transitional behaviors of a superhydrophobic surface only applied to one side are discussed. It is found that the presence of a superhydrophobic surface on one side introduces distinct influences on the transition behavior of the opposing side that lacks such a surface, which may be attributed to a downstream move in the stagnation point located on the pressure side.