The menace of momentum: Dynamic forces on flexible organisms

Abstract

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.-

Size, strength and allometry of joints in the articulated corallineCalliarthron

“…Furthermore, the time that flexible fronds spend reorienting and reconfiguring may exceed the duration of brief hydrodynamic loads, such as the wave impingement force, potentially allowing them to evade these maximal forces (Gaylord, 2000;Gaylord et al, 2001). Flexibility has its limitations, as particularly massive macroalgae sometimes develop momentum as they reorient (Gaylord and Denny, 1997;Denny et al, 1998) and, under some circumstances, experience a harmful whiplash effect (Friedland and Denny, 1995). However, in general, flexibility is thought to be beneficial to marine macroalgae and flexible reconfiguration has been described as a 'prerequisite for survival' in unstable flow conditions (Harder et al, 2004).…”

Subtidal and intertidal mussel beds (Mytilus edulis L.) in the Wadden Sea: diversity differences of associated epifauna

“…By creating structurally complex reefs, soft-bottom mussel beds modify the physical environment which in turn shapes the associated community by providing habitat space, outcompeting other surface dwellers and creating a sedimentary environment high in organic material and low in oxygen, thus favouring some species and eliminating others (Jacobi 1987;Seed and Suchanek 1992;Denny et al 1998;Commito and Rusignuolo 2000). In particular, mussel beds may alter water flow, which can influence the recruitment of macrofauna including the settlement of larvae as well as redistribution of settled individuals (Commito and Rusignuolo 2000;Snelgrove and Butman 1994).…”

Shallow Flows Over a Permeable Medium: The Hydrodynamics of Submerged Aquatic Canopies

“…Presumably to reduce their drag, submerged canopies tend to be sufficiently flexible that the canopy height is a strong function of flow velocity (see, e.g., Vogel (1984); Denny et al (1998); Stephan and Gutknecht (2002)). While the mean deflection of submerged vegetation in a current can be easily accommodated in numerical models, very little is known about the hydrodynamic impact of a pronounced and coherent waving, the monami.…”