The role of tail resonance in thrust production and wake formation

A robotic lamprey with a flexible tail is used to investigate the influence of resonance on the wake structure and thrust production during anguilliform swimming, based on the silver lamprey, Ichthyomyzon unicuspis. A programmable microcomputer actuates 11 servomotors that produce a traveling wave along the length of the lamprey body. The shape of the tail is taken from a CT scan of the silver lamprey. The flexible tail is constructed of PVC gel. PIV measurements taken with the tail flapping at its resonant frequency and at twice this value show that resonance greatly enhances the coherence of the wake structures. Momentum balance calculations reveal that at resonance the tail is producing significantly more thrust-there is a 96% increase. In both cases, the main wake structure is a pair of vortices shed each half cycle (a 2P wake structure). At resonance these pairs are clearly defined and appear in a reproducible location. However, at twice the resonant frequency, the main vortices are composed of several smaller vortical structures. The vortices in the resonant wake retain coherence further downstream than the non-resonant wake structures.