Hydrodynamics of scallop locomotion: unsteady fluid forces on clapping shells

Abstract

A potential flow model has been formulated for scallop swimming. Under the smalldisturbance approximation, the problem of the unsteady flow past the wing-like configuration of a scallop is separated into two linear sub-problems: the steady lifting problem and the unsteady symmetric thickness problem. The latter is associated with the expansion and contraction of the boundary surface of the scallop due to the shell opening and closing. A quasi-two-dimensional analytical solution of the thickness problem was obtained to give the time-dependent fluid forces acting on the outer surfaces of the shells. In addition to the added-mass effect, which has been widely accepted in the hydrodynamics of aquatic locomotion, there are two other mechanisms in the fluid reaction: flow-induced pseudo-elasticity and pseudo-viscosity. The pseudoelasticity provides a force proportional to the gape angle displacement, and will assist shell opening but resist shell closing. The pseudo-viscosity force is proportional to the angular velocity of the gape, and benefits both shell opening and closing. Their roles are discussed through comparison with those of shell inertia, hinge ligament elasticity and hinge damping. At 10 °C the hinge damping in the scallop was found to be almost compensated by the flow pseudo-viscosity. The unsteady fluid reaction may have a significant effect on the operation of the dynamic swimming system of scallops.