Flow visualisation of a pitching and heaving hydrofoil

Abstract

A fish-inspired heaving and pitching foil was investigated in a water tunnel using dye visualisation, at Reynolds numbers between 870 and 3480 (based on the chord length) and Strouhal numbers between 0.06 and 1.37. The variation of the wake pattern with respect to the Strouhal number was investigated. Three different wake regimes were observed – a drag wake, a transition wake and a thrust wake. The drag wake consisted of a combination of a regular Karman street and an array of ‘primary’ stop-start vortices, whereas the thrust wake consisted of a reverse Karman vortex street, commonly observed in swimming fish. The transition wake regime, which occurs at approximately 0.2 < ST < 0.3, is interpreted as a momentum balanced wake, where the thrust developed by the foil approximately balances its produced drag. This wake was observed to either consist of an in-line vortex street, or a paired vortex pattern.