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UBC Theses and Dissertations

How to make a tuna burst : the role of angle of attack in the production of thrust Whale, James Callum Andrew


Tuna—along with whales and lamnid sharks—utilise thunniform locomotion, a mode of swimming that optimises efficiency at high speed and isolates thrust production to the caudal fin. Thunniform performance is controlled by adjustments in the way the caudal fin interacts with fluid flow which, in turn, determines thrust and efficiency. The effect of tail motion on performance provides insight into the link between locomotor muscle biomechanics and hydrodynamics; these insights can be used to mimic and optimise animal motion in a robotic context. This study focuses on how the maximum angle of attack (α_max), contributes to tuna cruising and bursting, and the corresponding effects on fluid flow. I hypothesise that cruising tuna do not adjust α_max to modulate thrust but instead vary amplitude via Strouhal number. I also hypothesise that α_max affects thrust by changes in vorticity shed by the tail. To study these phenomena, I constructed a tuna tail model 3-D printed from CT scan data of a tuna tail. I then oscillated this model in a water tunnel across a range of biologically relevant motions. I calculated thrust and efficiency from direct measurements of force and torque and then used ink-flow visualisation and particle image velocimetry to reveal the resulting flow structures. The results indicate that the efficiency optimum of α_max peaks around 15° with the thrust optimum beyond 30°. Mechanistically, an increase of α_max increases the magnitude of the resultant force but angles it to the side, increasing the amount of wasted lateral energy. Increasing α_max increases the size and strength of shed vortices eventually causing shedding of an additional leading edge vortex at midstroke. These results, paired with red muscle work loop data, suggest that during cruise the α_max undergoes minimal variation, and suggest that in order to take advantage of the additional thrust that high values of α_max provide, white burst muscles need to advance peak force timing. In addition to contributing to a better understanding of the hydrodynamics of swimming and the associated musculature, these results also offer insight into the field of biomimetics and the construction of fish-mimicking robots such as AUVs.

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