Drag and Flexibility in Sessile Organisms

Most large, sessile organisms when exposed to rapid flows of air or water are markedly deformed as a consequence of their structural flexibility. Responses to air and water movement are similar, although both extreme and typical forces generated by water flows are greater, and erect organisms are commonly shorter in water than in air. A useful way of viewing data on the scaling of drag with flow speed is with a graph of speed-specific drag (drag divided by the square of speed) against speed. Since an ordinary solid body usually gives a horizontal line on such a plot, deviations from the ordinary are immediately evident. The slopes of the double logarithmic version of these graphs provide useful numerical comparisons. All of the cases considered here—trees, macroalgae, sea pens, etc.—give negative slopes at high flow rates, indicating that speed-specific drag drops with increasing flow. Such results may be taken as evidence that the flexible response commonly constitutes an adaptively useful reconfiguration as opposed to a mere incidental consequence of the material economy afforded by flexibility.