The fish ability to accelerate and suddenly turn in fast maneuvers

Abstract

Velocity burst and quick turning are performed by fish during fast maneuvers which might be essential to their survival along pray–predator encounters. The parameters to evaluate these truly unsteady motions are totally different from the ones for cruising gaits 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 requests. Such impressive performances, still poorly understood, are not common to other living beings and are clearly related to the interaction with the aquatic environment. Hence, we focus our attention on the water set in motion by the body, giving rise to the relevant added mass and the associated phenomena in transient conditions, which may unveil the secret of the great maneuverability observed in nature. Many previous studies were almost exclusively concentrated on the vortical wake, whose account, certainly dominant at steady state, is not sufficient to explain the entangled transient phenomena. A simple two-dimensional impulse 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 recoil motions.

How Free Swimming Fosters the Locomotion of a Purely Oscillating Fish-like Body

“…For a quick insight, the swimming styles of the two fishes together with the released vortical wakes are represented in comparison in Figure 6 and related animation (see Supplementary Material Movie S4 ) where a sort of swimming race clearly illustrates, in a synthetic way, their overall performance. The significant difference between the two vortical wakes gives an immediate feeling of the energy consumption of the two gaits even though the footprints corresponding to the vortex agglomertion are not clearly representing the complex phenomena along the swimming maneuvers [ 41 , 42 , 43 ].…”

Locomotion performance for oscillatory swimming in free mode

“…In fact, the free self-propulsion reveals the importance of the recoil rigid motions, given by the fluid interaction, which are essential to guarantee the overall equilibrium and may drastically modify the kinematics of the caudal fin with respect to the prescribed one, to finally obtain a better swimming performance. The recoil reaction and the locomotion speed are obtained here by a simple impulse model able to highlight the added mass and the released vorticity contributions, together with their coupling terms which are especially important in transient conditions [45]. This model allows to understand the capability of the potential terms to attenuate the recoil reaction continuously forced by the vortex shedding which is directly related to the wasted energy.…”