Flow visualization and three-dimensional characteristics of the leading edge vortex on a hovering model hummingbird wing

The unsteady low Reynolds number aerodynamics of flapping motion is investigated experimentally through suspended particle imagery. The flapping kinematics and wing scaling are modeled after a hummingbird during hovering flight. At flapping frequencies greater than 3 Hz, visualization indicated a stable leading edge vortex that remained attached to the wing throughout each downstroke, and increased in size from base to tip. However, analysis of the flow structure at the middle of each downstroke indicated that size of the separation bubble is not measurably different through a range of flapping frequencies. This supports the conclusion that a standard dynamic stall condition does not exist and vorticity is removed from the system by three-dimensional effects. Introduction of various barriers perpendicular to the wingspan revealed that leading edge vorticity becomes very unstable when spanwise flow is inhibited. This implies that LEV stability requires axial flow from the base to the wing tip to bleed building vortex strength in higher Reynolds number regimes.