Swimming performance and unique wake topology of the sea hare (Aplysia)

The Aplysia, commonly referred to as the “sea hare,” is a marine mollusc that swims using large-amplitude flapping of its wide, winglike parapodia. In this study, flow simulations with a relatively simple kinematical model are used to gain insights into the vortex dynamics, thrust generation, and energetics of locomotion for this animal. A unique vortex pattern characterized by three distinct trains of vortex ringlike structures is observed in the wake of this animal. These vortex rings are associated with a positive momentum flux in the wake that counteracts the drag generated by the body. Simulations indicate propulsive efficiencies of up to 24% and terminal swimming speeds of about 0.9 body length per cycle. Swimming speeds are found to increase with increasing parapodial flapping amplitude as well as wavelength of undulation.

Figure 1

(a) External features of free-swimming sea hare (A. fasciata) in a streamlined posture; (b) the planform used in this study adopted from a dissected A. brasiliana with the dorsal visceral mass removed [5].

Figure 2

Comparison of the reconstructed motion of the Aplysia with screenshots of a freely swimming A. fasciata [13]. (a) Fully closed; (b) beginning of opening phase; (c) fully open; (d) beginning of closing phase. The thick black arrow denotes the direction of swimming.

Figure 3

Vortex topology for the baseline case $\lambda /L=2.0$: (a) perspective view; (b) dorsal view; (c) side view. The visualized vortex structures corresponding to the isosurface of $Q=2.0$ are colored by vorticity in the spanwise direction. The thick arrow denotes the direction of swimming and the other arrows depict the direction of rotation of these vortices.

Figure 4

Correlation between vortex structures, induced velocity and momentum flux in the wake for the baseline case $\lambda /L=2.0$ at the terminal condition: (a) top-view slice intersecting the paired vortex rings $(R_p$ and $R_s)$; (b) slice intersecting the top set of vortex rings $\left(R_d\right)$, the colored contours are the streamwise momentum flux $\left(u\right|u\left|\right)$; (c) perspective view of isosurfaces of $\left(u\right|u\left|\right)$ superposed on vortex structures; (d) ventral view of (c); (e) spanwise vorticity on the symmetry plane, red dashed lines indicate the sliced location of (a) and (b).

Figure 5

Cycle-averaged streamwise force coefficient per unit area on the body for the baseline case $\lambda /L=2.0$, higher curvature in the right parapodium leads to higher thrust.

Figure 6

Representative kinematic models at the closing phase with wavelength $\lambda /L=2.0$: (a) Baseline case, ${\psi }_{0,l}=97.5^{\circ }$ and ${\psi }_{0,r}={165}^{\circ }$; (b) C3, ${\psi }_{0,l}={\psi }_{0,r}={90}^{\circ }$; (c) C5, ${\psi }_{0,l}={\psi }_{0,r}={75}^{\circ }$. Arrow denotes the swimming direction.

Figure 7

Comparison of the time history of (a) swimming speed and (b) surge force coefficients for the kinematic parameters investigated in the current study.

Figure 8

Temporal profile of surge force coefficients for four grid sizes of an animal swimming in a uniform inflow with the same prescribed kinematics of the case C5.