Meeting Abstract
Many fishes use body-caudal undulation to navigate their environments. Some fish, traditionally called anguilliform swimmers, use more than two thirds of their bodies to produce thrust. The hydrodynamics of these anguilliform swimmers has been well characterized and modeled by biologists, mathematicians, and engineers, but primarily focusing on flows in the horizontal plane. Some of the more recent characterizations have used conventional particle image velocimetry (PIV) methods to characterize the wake, showing that elongate fishes such as eels shed vortices which produce lateral jets along the length of the body and in the wake. Though these studies are comprehensive in 2D, we have not yet looked at the 3D motion of the body and tail. We know that as a 2D lateral bending wave passes down the body, a 3D torsional wave follows. In this study, we characterize flow around the body that results from that torsional wave in the American eel (Anguilla rostrata). We use a transverse PIV setup, in which the light sheet is perpendicular to the swimming direction. We find that as the tail beats and twists back and forth, two vortices are produced, one from the dorsal surface and one from the ventral surface. These vortices are likely part of the vortex loops shed as a result of the bending wave, and produce an upwards jet against the tail. The jet could indicate a lift force on the body, which may to cause the figure-eight pattern seen when looking at the tail from behind. It may also serve to lift the negatively buoyant fish off the ground, reducing fraction with the sediment and allowing the fish to move more easily through the water. This mechanism may be important in other negatively buoyant fishes as well, especially those with reduced pectoral fins.
