PIV measurements of the asymmetric wake of a two dimensional heaving hydrofoil

Particle image velocimetry is used to examine the flow behind a two-dimensional heaving hydrofoil of NACA 0012 cross section, operating with heave amplitude to chord ratio of 0.215 at Strouhal numbers between 0.174 and 0.781 and a Reynolds number of 2,700. The measurements show that for Strouhal numbers larger than 0.434, the wake becomes deflected such that the average velocity profile is asymmetric about the mean heave position of the hydrofoil. The deflection angle of the wake, which is related to the average lift and drag on the hydrofoil, is found to lie between 13° and 18°. An examination of the swirl strength of the vortices generated by the hydrofoil motion reveal that the strongest vortices, which are created at the higher Strouhal numbers, dissipate most rapidly. This research article was submitted for the special issue on Animal locomotion: The hydrodynamics of swimming (Vol. 43, No. 5).