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.

Figure 1

Snapshots of the C-start maneuver of a neutrally buoyant fish from the numerical simulation. The relative animation is reported in Movie S1.

Figure 2

Velocity components for the C-start maneuver of a neutrally buoyant fish.

Figure 3

Fluid impulses for C-start maneuver: (a) total forward impulse ${\mathcal{P}}_1$ and its vortical contribution ${P_v}_1$; (b) total angular impulse $\mathrm{\Pi }$ and its vortical contribution ${\mathrm{\Pi }}_v$.

Figure 4

Time history of (a) the added mass coefficient $m_{11}$ and of (b) the forward acceleration contributions for the C-start maneuver.

Figure 5

Time history of (a) the added mass coefficient $m_{33}$ and of (b) the angular acceleration contributions for the C-start maneuver.

Figure 6

Snapshots of the C-start maneuver combined with a wave undulation from the numerical simulation. The relative animation is reported in the Movie S2.

Figure 7

Comparison between forward and angular velocity components for the C-start maneuver with and without wave undulation.