The menace of momentum: Dynamic forces on flexible organisms

It has been proposed that the mechanical flexibility of many wave-swept organisms reduces the hydrodynamic forces imposed on these plants and animals. For example, reorientation of the organism can render it more streamlined, and by “going with the flow” a flexible organism can reduce the relative velocity between itself and the surrounding water, thereby reducing drag and lift. Motion of the body allows the organism to gain momentum, however, and this momentum can apply an inertial force when the organism’s motion is slowed by the deformation of the body’s supporting structures. Through a series of mathematical models we show that the inertial forces imposed on flexible plants and animals can be large enough to increase the overall force on the organism, more than offsetting the advantages of moving with the flow. A dimensionless index, the jerk number, is proposed as a tool for predicting when inertial forces will be important, and the utility of this index is explored through an examination of the forces applied to kelps and mussels. The tendency for inertial loading to peak at certain frequencies raises the possibility that the structure of organisms can be tuned (either by evolution or physiological response) to avoid potentially damaging loads.