The fish ability to accelerate and suddenly turn in fast maneuvers
Velocity burst and quick turning are performed by fish during fast maneuvers which are essential to their survival along pray-predator encounters. These truly unsteady motions are totally different from cruising gaits, aimed at low expended energy, since a very large acceleration, up to several times the gravity, and an extreme turning capability, in less than one body length, are now the primary goals. Such impressive performances are not common to other living beings and are clearly related to the interaction with the aquatic environment, since no external actions are applied. Hence, we focus our attention on the water set in motion by the body, giving rise to the relevant added mass and to its variation in transient conditions, which are able to explain, together with the vortex shedding, the great maneuverability observed in nature. A simple two-dimensional model with concentrated vorticity is used for the self-propulsion of a deformable body in an unbounded fluid domain, to single out the potential and the vortical impulses and to highlight their interplay induced by the unavoidable recoil reactions.
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