Leading Edge Vortex in Flapping Fins
Bandyopadhyay, Promode R.; Hrubes, J. Dana
Insects and penguins flap their pectoral fins to produce forces. Flapping means simultaneous rolling and pitching oscillations with or without twist. Twisting is a differential pitching between the root and the tip of the fin, added to the normal pitching oscillation. The rolling and pitching oscillations are 90 deg apart. Flapping fins produce leading edge vortices which enhance lift forces and delay stall even at high angles of attack, and the mechanism is known as dynamic stall (ASME JFE v131 031801-29 2009...JEB v211 206-214 2008...IEEE JOE v33 59-68 2008.). In the fluid dynamics video we show dye-in-water flow visualization of the formation of the leading edge vortex (LEV) with and without twist. We also show the effects of increasing frequency of oscillation and roll angle on the LEV. In the absence of twist, the LEV is conically enlarging along span, with a spanwise flow away from the root. However, in the presence of twist, the LEV is more uniform along the span and this effect of twist becomes clearer as frequency of oscillation is increased; we explain that this is a result of the local and instantaneous angle of attack becoming more uniform along span due to twist.
fins; vortex; oscillation; fluid dynamics video