The forces on a fish-inspired unsteady hydrofoil

Abstract

The flow around an unsteady hydrofoil, undergoing simultaneous heaving and pitching motions which mimic the tail motions of carangiform fish (such as tuna), was investigated using simultaneous particle image velocimetry (PIV) and force measurement. These investigations were performed at Reynolds numbers, based on the foil chord length, in the range of 1500 < Re < 12500. The Strouhal numbers, based on the foil heave amplitude, were in the range of 0.1 < Sth < 0.95. Hydrogen bubble visualisation was also performed on the foil at identical Sth, but at 500 < Re < 3500. Two force measurements were obtained. Firstly, instantaneous thrust and side forces were obtained by direct strain gauge measurements on the foil, which were then time-averaged. Secondly, time-averaged thrust forces were estimated using a momentum Integral around the time-averaged two-dimensional PIV velocity field around the foil. Both force measurements, which show excellent agreement for all foil parameters, indicate that the foil is able to produce very large thrust coefficients, Ct , in excess of 10. These large thrust coefficients typically occur when the non-dimensional foil heave amplitude is large, at conditions where hydrogen bubble visualisation indicates that large leading edge vortices are generated. This suggests that the unsteady foil may be able to use unsteady flow mechanisms to generate large forces.