Experimental study of flow structures over rigid and flexible oscillating foils

Abstract

The flow structure formed behind the harmonically heaving and pitching airfoil in the water channel has been visualized using a hydrogen bubble visualization technique. The heaving and pitching motions of an airfoil are inspired from the fish locomotion, and the motions are actuated by a programmable motion actuator. Rigid and flexible foils are used to show the difference in flow structure when a compliant material is used as an undulating body. Flow visualization experiments were performed at Reynolds number of 1223 with variation in Strouhal number. Flow structures and wake patterns are strongly depend on the Strouhal number and material properties of a foil. The intensity of the leading edge vortex seem to be stronger when the flexible foil is used and clearer reverse Karman vortex street is shown in the wake of a flexible foil. Detailed illustration of the vortical structures and wake patterns are presented to highlight the difference in flow structure when a complaint foil is adopted instead of the conventional rigid foil.