Effect of the laminar separation bubble induced transition on the hydrodynamic performance of a hydrofoil

The present study deals with the effect of the laminar separation bubble (LSB) induced transition on the lift, drag and moment coefficients of a hydrofoil. A 2D numerical study, based on the SST γ-Reθ transition model of ANSYS-CFX®, is conducted on a NACA66 hydrofoil. Angles of attack range from −4°to 14°and the chord-based Reynolds number is Re=7.5×105. An experimental investigation is carried out in the French naval academy research institute's hydrodynamic tunnel based on the measurements of lift, drag and moment. Experiments on a smooth, mirror finished, hydrofoil enable comparison with RANS calculations using the transition model. Experiments with a roughness added on the leading edge enable comparison with RANS calculations using the SST fully turbulent model. For angles of attack below 6°, the LSB triggered laminar to turbulent transition of the boundary layers of the suction and pressure sides is located near the trailing edge of the smooth NACA66. As the angle of attack reaches 6°, the LSB suddenly moves to the leading edge on the suction side while transition is located at the trailing edge on the pressure side. The smooth hydrofoil shows higher CL and CM and lower CD than the rough leading edge one from −4°to 6°. Both experiments lead to the same coefficients from 6°to 14°. The calculations show that both models are in good agreement with their corresponding experiments. Velocity profiles in the vicinity of the LSB at an angle of attack of 2°and pressure coefficients of the calculations using the transition model are compared with published experimental studies and show very good agreement. The SST γ-Reθ transition model proves to be a relevant, even essential, prediction tool for lifting bodies operating at a moderate Reynolds number.