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

Abstract

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-Re θ transition model of ANSYS-CFX R , is conducted on a NACA66 hydrofoil. Angles of attack range from −4°to 14°and the chord-based Reynolds number is Re = 7.5 × 10 5. 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 C L and C M and lower C D 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-Re θ transition model proves to be a relevant, even essential, prediction tool for lifting bodies operating at a moderate Reynolds number.

Discontinuity of lift on a hydrofoil in reversed flow for tidal turbine application

“…Between 6°and 8°, it is observed that the lift coefficient deviates from the theoretical linear curve 2π α. This bump in the curve was also observed in simulations with a transition model [17,12], where it could be associated to the position of transition points on the hydrofoil. The inflexion occurs when the pressure side transition point reaches the trailing edge and the transition is close to the leading edge on the suction side.…”

“…The numerical model was defined according to standard recommendations and to a detailed verification process (mesh and time step convergence particularly) achieved in former works in a similar configuration [17,12,32]. The resulting mesh is illustrated in Fig.…”

“…This numerical configuration derives from a sensitivity study, regarding mesh size and time-step value and adapted from methods presented by Delafin et al [34] and Ducoin [32] and developed in the laboratory. A detailed verification procedure is out of the scope of this paper but extensive verification and validation have been made in a similar configuration, see [12,17].…”

“…A large amount of work has been done on Laminar Separation Bubbles (LSB) on classical airfoils with rounded nose (see e.g. [12,13] and references therein). The effects of the LSB on the suction side boundary layer transition and then on the airfoil performance have been well documented.…”

Comparison of low-order aerodynamic models and RANS CFD for full scale 3D vertical axis wind turbines

“…Additionally, the γ − Re θ transition model [37] coupled with the k-ω SST turbulence model (referred to as SST-TM henceforth) has been used at the lowest tip speed ratio. Taking the laminarturbulent transition into account in RANS calculations improves the accuracy of the lift and drag coefficients predictions in presence of transition effects [38,39] and improves the modeling of the dynamic stall [40]. It has also been shown to improve the results of VAWT calculations at low tip speed ratios [16], where blades experience dynamic stall.…”