Vortex dynamics and performance of flexible and rigid plunging airfoils

The kinematic and aerodynamic behaviors of sinusoidally plunging, flexible airfoils were investigated over a parameter space broadly representative of biological or biomimetic flapping flight. PIV was used to characterize the velocity and vorticity fields, and the evolution of the flow structure is described. Comparisons with a rigid airfoil were performed in order to identify the effects of airfoil deformation on the evolution of the vorticity field. A modification in trailing-edge vortex evolution was observed at a reduced frequency of approximately 1.57, reflecting a change in its interaction with the leading-edge vortex. The transition to the higher-reduced-frequency flow structure correlates well with the propulsive efficiency leveling off to a broad peak. A scaling parameter was developed that provided good collapse of circulation values for the leading edge vortex within the parameter range investigated for each individual airfoil. Force and power measurements revealed that, while aerodynamic loads were strongly correlated with Strouhal number, the propulsive efficiency of the flexible airfoils was primarily dependent on reduced frequency k.