Experimental Confirmation of an Aquatic Swimming Motion Theoretically of Very Low Drag and High Efficiency

Researchers used an anguilliform swimming robot to replicate an idealized “wakeless” swimming motion.

Office of Naval Research, Atlanta, Georgia

It has been established theoretically that self-propulsion of deformable bodies in ideal fluid can occur with a careful specification of the deformation mode shape. With the fluid assumed ideal, vortex shedding, rotational wake, and induced drag would not occur. The implication is that for a real fluid, provided the existence of a thin boundary layer, similarly configured bodies with the same deformation mode shape self-propel without vortex shedding, rotational wake, and induced drag. Only viscous drag effects, due to the existence of the thin boundary layer, are present and unavoidable. The motion mode in question is the little-exploited anguilliform mode exhibited in some aquatic animal swimming. The Anguilla includes the snake, eel, lamprey, and leach, among others.

An anguilliform swimming robot (Figure 1) was designed and built to replicate an idealized “wakeless” swimming motion. The idealized swimming motion is a reactive swimming technique that produces thrust by accelerations of the added mass in the vicinity of the body. The net circulation for the unsteady motion is theorized to be eliminated. Particle Image Velocimetry (PIV) equipment then measured the wake field velocities produced by the robot, and these results were compared to the predicted hydrodynamics. Figure 2 shows the final robot prototype, NEELBOT-1.1, swimming on the free surface of the Towing Tank of UNO (University of New Orleans) and a schematic showing the interested wake region behind it. Several assumptions are taken with both the theory and the experiment, but the evidence is still conclusive enough to draw comparisons between the two.