BOLLER, M.L.*; CARRINGTON, E; University of Rhode Island: A new model for drag generation in flexible organisms

Rocky intertidal organisms experience large hydrodynamic forces due to extreme water velocities created by large waves. Flexible organisms, like algae, often experience lower drag than rigid bodied organisms because their shape and area projected into the flow change as velocity is increased. This phenomenon, known as reconfiguration, has been previously quantified by modification of the exponent of velocity in the drag equation (Vogel�s E). This quantification is inadequate because it adds units to the unitless drag coefficient (a shape-dependent index of streamlining) and cannot be used to extrapolate drag to higher water velocities. We propose a new model of drag generation that preserves the squared relationship between drag and fluid velocity by using new projected area and drag coefficient functions to predict drag. To develop the model, drag, area projected into the flow, and shape were simultaneously measured in the lab on individual algae (Chondrus cripsus) from 0 to ~2.5 m/s. Reconfiguration was due to whole plant reorientation (deflection at the stipe base) and a change in area projected into the fluid (compression of the crown). Statistical analyses suggest that there is a critical velocity at which the mechanism of reconfiguration changes. Shape and area projected into the flow decrease more rapidly at low velocities (< 1 m/s) than high. Drag coefficients are high and variable at low velocities but converge and asymptote at velocities > 1 m/s. These results suggest that drag may be easily predicted at higher velocities if measurements are taken above the critical velocity and that extrapolation of hydrodynamic data from low velocity flow tanks (< 0.5 m/s) to high field velocities (>> 1 m/s) may be inappropriate.