Animal mechanics

A robotic ribbon fin with twelve independent fin rays, elastic fin membrane, and a body
of adjustable height was developed for this thesis specifically to test the 1990 theory put forth
by Lighthill and Blake that a multiplicative propulsive enhancement exists for Gymnotiform and
Balisiform swimmers based on the ratio of body and fin heights. Until now, the theory has not
been experimentally tested. Proof of such a momentum enhancement could have a profound effect
on unmanned underwater vehicle design and shed light on the evolutionary advantage to body-fin
ratios found in nature, shown as optimal for momentum enhancement in Lighthill and Blake’s theory.
Thrust tests for various body heights were conducted in a recirculating flow tank at different flow
speeds and fin flapping frequencies. When comparing different body heights at different frequencies
to a ’no-body’ thrust test case at each frequency no momentum enhancement factor was found. Data
in this thesis indicate there is no momentum enhancement factor due to the presence of a body on
top of an undulating fin.