Drag coefficients for single coral colonies and related spherical objects

Flows over and through coral canopies and isolated coral colonies are sometimes approximated using arrays of spheres and cylinders, which may not adequately represent the effects of their complex geometry. We report the results of a series of lab experiments measuring drag on branching coral colonies and other similar objects. The results expressed in terms of a drag coefficient (C_D), based on the exposed frontal area, are compared with those of several spherical objects, including perforated, hollow spheres. Our results show that a sphere is not an accurate model of an individual branching coral in that C_D can be much larger than that of a sphere, especially at low Reynolds numbers where the fluid in the interior of the colony is nearly stagnant. Using the measured drag coefficients, we show that drag is likely to dominate inertial forces for wavy conditions commonly found on coral reefs.