LONG, JR., J H*; ROOT, R G; WATTS, P; Vassar College; Lafayette College; Applied Fluids Engineering: Is an undulating fish an oscillating wing?

Using 3D computational fluid dynamics, we previously found that pumpkinseed sunfish, Lepomis gibbosus, appear to swim steadily using the head and body as an oscillating, unsteady wing with bound vorticity, alternating camber, and a traveling stagnation point. If this is generally the case for body-caudal fin (BCF) swimmers, then theoretical simulations of simplified lifting bodies with fish-like properties should swim as well. To test this assertion, we chose as the basis of our simulation a special case often used as the starting point for lift mechanics: the Flettner rotor, a right-circular cylinder spinning about its long axis and moving normal to flow. Because of the unsteady fluid dynamics involved in undulatory BCF swimming, �Flettner fish� does not have a fixed body form as defined by closed streamlines of constant velocity potential. Instead, the body shape is a result of the fluid dynamic solution. For input, we gave Flettner fish alternating rotational motion, similar to that of the swimming sunfish�s head, and then performed a 2D optimization of swimming efficiency. For the most efficient swimming mode, the solved shape of the body is fish-like, suggesting that certain morphological features of BCF swimmers optimize lift-based propulsion. In addition, we documented, as a function of time, (1) the momentum fluxes approaching and leaving the Flettner fish and (2) the pressures on the solved, fish-shaped body. Compared with our previous 3D unsteady simulations of a swimming sunfish, Flettner fish displays a similar unsteady pressure pattern: a stagnation point that oscillates from operculum to operculum by way of the rostrum. In addition, approximately 10% of thrust is generated by leading edge suction, a condition also indicative of wing-like mechanical behavior.