Vortical patterns generated by flapping foils of variable ratio chord-to-thickness

Depending upon the flapping amplitude and frequency, different types of vortex streets are shed downstream of a pitching foil. In the present experimental work, the foil aspect ratio chord-to-thickness is varied (\(L/D=5;\,4;\,3\) and 2) additionally to the frequency, while the amplitude is kept constant. Dye visualizations allow to find that the number of vortices shed by oscillation cycle slightly depends upon the foil aspect ratio. Moreover, whereas the von Kármán street can reverse for the longest two foils, this change is not observed for the shortest two foils. Finally, when the frequency is high enough, the formation mechanism of the asymmetric reverse Kármán street, which is significative of a positive thrust, is shown to be the same as the \(P+S\) pattern that develops for an oscillating cylinder. Detailed visualizations show that a jet is produced for the longest two foils because the foil tip vorticity is maximum just when a counter-rotating vortex is ejected.