Meeting Abstract

6.3  Thursday, Jan. 3  Resonance in undulatory swimming of a robotic-flapping fish LIM, J.L.**; LAUDER, G.V.; Harvard University, Cambridge, MA; Harvard University, Cambridge, MA lim@fas.harvard.edu

The undulating body of a swimming fish has the potential to display resonance behaviour. Resonance in fish swimming can be characterized by an increase in the ratio of tail-beat amplitude to rostral amplitude as the driving frequency (approximated by tail-beat frequency) approaches a natural frequency of the oscillating body. Previous work on the issue of resonance in fish swimming has lead to the hypothesis that fish actively alter their body stiffness so that their natural frequency matches their tail-beat frequency. This would presumably reduce the energetic costs of body bending. In beginning to answer this question directly, here we test the hypothesis that undulating fish display resonance phenomena by attempting to identify a natural frequency of a passive fish body. We used a freshly anaesthetized rainbow trout (Oncorhynchus mykiss) attached to a robotic flapping device to simulate a swimming fish, and oscillated the fish body at frequencies ranging from 0.25 to 4.50 Hz for given sets of heave and pitch values. The resultant tail-beat amplitude and midline kinematics of fixed-position and self-propelled fish were obtained from high-speed video. We predict that tail-beat amplitude as a function of driving frequency will have at least one resonance peak where driving frequency matches a natural frequency. We also compare the kinematics of live swimming fish to our robotic flapping fish to identify any differences in the relationship between tail-beat amplitude and beat frequency in actively and passively swimming fish. This current work will help determine whether live fish exploit resonance phenomena by modulating their body�s natural frequencies via changes in body stiffness or overall morphology and its associated hydrodynamic effects.