Numerical evaluation of the clearance geometries effect on the flow field and performance of a hydrofoil

The blade tip leakage flow with efficiency losses and cavitation phenomena is a concern for the low-head tidal power units. A simplified case of NACA0009 hydrofoil in a water tunnel is used to investigate the effects of tip clearance geometries including the foil tip shape and gap width on the flow features and foil performance. Steady non-cavitating simulations are implemented for a round tip foil and a sharp tip foil with two incidence angles (alpha = 10 degrees and 5 degrees) and different normalized gap width (tau). The minimum pressure is used to reflect the normalized vortex intensity (Gamma*) and cavitation characteristics. The Gamma*-tau curves at different streamwise positions show that the sharp tip foil generates relatively weaker tip leakage vortex with more flat curves, but its higher Gamma* of tip separation vortex in wider gaps increases the risk of clearance cavitation. The flow features on a cross section inside the gap suggest that the sharp tip reduces the leakage flow losses and increases the velocity gradient due to the boundary layer separation. The lift coefficient is a little higher for the sharp tip foil than the round tip foil, with small differences for alpha = 5 degrees but noticeable deviations for alpha = 10 degrees especially within 0.3<tau < 1. (C) 2016 Elsevier Ltd. All rights reserved.