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Biomechanics

The quirks of jerks

Nature volume 396, pages 621–623 (1998)Cite this article

The mechanical flexibility of organisms attached to the substratum can reduce the fluid-dynamic forces that they must withstand by several mechanisms. Flexible organisms — such as trees in the wind and sea anemones in tidal currents — experience a considerable reduction in the drag force that pushes them downstream as they are reconfigured by the moving fluid into more streamlined shapes4,5. Flexibility can also sometimes protect the attachment organs of bottom-dwelling creatures from bearing large forces if the creatures are subjected to the back-and-forth water motion of ocean waves. Wave-swept organisms are subjected to forces that depend on the instantaneous velocity (drag and lift) and acceleration (added mass force and virtual buoyancy) of the water relative to them4,6,7. The sum of these forces on a rigid organism in waves varies with time as the water flows back and forth. In contrast, a flexible organism that can move with the water in waves can avoid being pulled by these hydrodynamic forces until it is strung out in the direction of flow and the water moves past it. The longer the organism relative to the distance the water moves before it flows back the other way, the more likely the organism is to avoid flow forces at times when accelerations and velocities are high8.

Denny et al.3 now point out that such a reduction in hydrodynamic force may not protect the structure that attaches the organism to the shore from potentially damaging loads if the inertial force on the organism is high when it reaches the end of its tether and suddenly stops moving with the flow. They used mathematical models to illustrate how the mass and size of an organism, the stiffness of its tether, and the period and peak velocity of the waves affect the inertial force on the organism when it reaches the end of its rope.

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Figure 1: Flexible organisms like these kelp attached to wave-swept shores can avoid bearing hydrodynamic forces by moving with the water.

M. KOEHL

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  1. The Department of Integrative Biology, University of California, Berkeley, 94720-3140, California, USA

    M. A. R. Koehl

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Koehl, M. The quirks of jerks. Nature 396, 621–623 (1998). https://doi.org/10.1038/25231

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