Hydrodynamic Performance of a Flexible Fish Pectoral Fin*

Numerical simulations have been used to examine in detail the hydrodynamic performance of a pectoral fin of a bluegill sunfish. The pectoral fin of this fish is highly flexible and undergoes significant shape and area change during its flapping cycle. The numerical simulations employ a 3D immersed boundary solver that allows us to examine in detail the hydrodynamics of the fin. Simulations reveal that the fish uses the fin flexibility to produce a highly complex and asymmetric stroke that does not fit any of the classic notions of ``paddling'' or ``flapping.'' The numerical simulations clearly reveal the distinct vortex structured produced by the fin and the connection between the vortex structures and hydrodynamic performance is examined. Finally, comparison between a flexible fish fin and a rigid flapping foil allows us to assess the benefits of flexibility on the hydrodynamic performance.