Between the branches: Recent results from large eddy simulations of local coral colony flow fields*

Local hydrodynamics play a central role in physiological processes like respiration and nutrient uptake in coral reefs. Despite the importance of reefs as hosts to a quarter of all marine

life, and the pervasive threats facing corals, characterizing the hydrodynamics between the branches of scleractinian corals has remained a significant challenge. Here, we review recent results from large eddy immersed boundary simulations of the flow through and around Pocillopora meandrina, Pocillopora eydouxi, and Montipora capitata coral colonies. The studies suggest that colonies with higher branch densities have more complex mean velocity profiles with three different characteristic regions, while more loosely branched colonies have approximately constant mean velocity profiles along the flow direction; that surface roughness can counterintuitively increase the Reynolds stresses above the colony and hence enhance vertical transport; that passive geometric features of branching colonies produce highly vortical internal flows that enhance mass transfer at the interior of the colony, compensating almost exactly for flows speed reductions there of up to 64% so that the advection time scale remains roughly constant throughout the colony; and that the mean vortex diameter, rather than the mean branch diameter, may be the correct length scale to use to calculate the mass transfer Stanton number for intracolonial coral flows.