Experimental simulation of fish-inspired unsteady vortex dynamics on a rigid cylinder

The unsteady hydrodynamics of the tail flapping and head oscillation of a f ish, and their phased interaction, are considered in a laboratory simulatio n. Two experiments are described where the motion of a pair of rigid flappi ng foils in the tail and the swaying of the forebody are simulated on a rig id cylinder. Two modes of tail flapping are considered: waving and clapping . Waving is similar to the motion of ht caudal fin of a fish. The clapping motion of wings is a common mechanism for the production of lift and thrust in the insect world, particularly in butterflies and moths. Measurements c arried out include dynamic forces and moments on the entire cylinder-contro l surface model, phase-matched laser Doppler velocimetry maps of vorticity- velocity vectors in the axial and cross-stream planes of the near-wake, as well as dye flow visualization. The mechanism of flapping foil propulsion a nd maneuvering is much richer than reported before. They can be classified as natural or forced. This work is of the latter type where discrete vortic es are forced to form at the trailing edge of flapping foils via salient ed ge separation. The transverse wake vortices that are shed, follow a path th at is wider than that given by the tangents to the flapping foils. The unst eady flap-tip axial vortex decays rapidly. Significant higher order effects appear when Strouhal number (St) of tail flapping foils to the power input to the actuators, reaches a peak below the St range of 0.25-0.35. Understa nding of two-dimensional flapping foils and fish reaching their peak effici ency in that range is clarified. Strouhal number of tail flapping does emer ge as an important parameter governing the production of net axial force an d efficiency, although it is by no means the only one. The importance of an other Strouhal number based on body length and its natural frequency is als o indicated. The relationship between body length and tail flapping frequen cy is shown to be present in dolphin swimming data. The implication is that , for aquatic animals, the longitudinal structural modes of the body and th e head/tail vortex shedding process are coupled. The phase variation of a s imulated and minute head swaying, can modulate axial thrust produced by the tail motion, within a narrow range of +/-5 percent. The general conclusion is that, the mechanism of discrete and deterministic vortex shedding from oscillating control surfaces has the property of large amplitude unsteady f orcing and an exquisite phase dependence, which make sit inherently amenabl e to active control for precision maneuvering. [S0098-2202(00)00102-4].