A limpet shell shape that reduces drag: laboratory demonstration of a hydrodynamic mechanism and an exploration of its effectiveness in nature

Abstract

As the velocity of flow increases, smooth, symmetrical objects such as spheres and cylinders exhibit an abrupt transition from a laminar to a turbulent boundary layer. As a consequence, these shapes experience a substantial reduction in fluid-dynamic drag at velocities above the transition. The possibility was explored that this form of drag reduction operates in benthic marine organisms, and a single individual limpet has been found that exhibits the phenomenon in a laboratory flume. When the limpet's anterior end is oriented upstream, the shell shows a sudden 40% reduction in drag at a water velocity of 1.6 m/s, a velocity that is commonly encountered on wave-swept shores. It is unlikely, however, that this drag-reduction mechanism operates effectively under field conditions because flow is often from a direction inappropriate for drag reduction and the presence of upstream objects can abolish the effect. Furthermore, drag is much less likely to act as an agent of disturbance than is lift, so any reduction in drag is unlikely to enhance survivorship. The likelihood that drag reduction via an abrupt boundary-layer transition is ineffective under natural conditions may help to explain why many benthic organisms do not have "typical" low-drag shapes.