Effects of waves and surface roughness on turbulence-mediated heat and mass transfer in corals*

We present measurements of local flow fields over wall-mounted hemispheric corals with varying surface morphologies obtained in the field and in a recirculating water tunnel using in-situ and laboratory particle image velocimetry (PIV). We also performed Large Eddy Simulations (LES) coupled with a convective thermal energy transport model to simulate heat and mass flux for unidirectional and oscillatory flow conditions over idealized coral surfaces. The field and laboratory PIV measurements show that increased surface roughness—as measured by the ratio of roughness spacing to roughness height—corresponds to greater intensities of near-surface and downstream Reynolds shear stresses and turbulent kinetic energy, delays in surface flow separation, and smaller downstream recirculation areas. The LES-heat transfer model shows large roughness ratios generate up to a 2x enhancement in heat and mass transfer, while oscillatory flow enhances transfer coefficients between 1.2 – 2.0x. The model also demonstrates that for high Reynolds number oscillatory flow, small roughness ratio morphologies produce greater total fluxes due to their greater surface area, despite exhibiting smaller mean transfer coefficients—an important consideration for colonial organisms that can share resources internally. We find that fast, wavy flows shift the dominant transfer mechanisms from drag-induced turbulent convection to local inertial accelerations driven by steep pressure gradients. We conclude that across a range of flow conditions, trade-offs to maximize heat and mass transfer exist between larger roughness ratios that enhance turbulence production and smaller ratios that enhance available surface area. Such trade-offs suggest critical differences in survivorship during heat-induced coral bleaching and selective pressure for specific morphologies adapted to local flow conditions.