Turbulence Hydrodynamics and Scalar Transport Processes in Boundary Layer Flow Past Corals

Coral reefs provide an extensive range of ecosystem services to society, playing a critical role in shoreline protection, fishery habitat, tourism, and cultural/historic ties for local and indigenous communities. In addition to the inherent environmental significance of corals, these ecosystem services contribute billions of dollars annually in global economic value. Given the importance of corals, it is vital to understand the biological, ecological, and physical conditions that determine coral survival, especially in light of recent and projected future climate impacts that threaten these fragile organisms. Perhaps the most important bio-physical driver of coral health is the fine-scale turbulent boundary layer, which acts as a “gatekeeper” to the coral surface through its hydrodynamic control of momentum, mass, and thermal transport processes. 

The main objective of this dissertation is to investigate three key modulators of coral turbulent boundary layer dynamics: 1) wave-driven oscillatory flow, 2) topographic surface roughness, and 3) algal-canopy cover. To address these questions, an interdisciplinary research approach was undertaken combining in situ field measurements over algae-covered corals in a fringing reef in Bocas del Toro, Panama, laboratory experiments of rib roughness on a wall-mounted hemisphere using a recirculating water tunnel, and numerical modeling of hydrodynamics and heat transfer from hemispheres using Large Eddy Simulations (LES).

Acoustic Doppler velocimetry (ADV) and particle image velocimetry (PIV) were used to obtain measurements of combined wave-current flow over healthy and algae-covered Siderastrea siderea corals in the reef in Panama. The results demonstrate that the turbulence characteristics of the boundary layer shift in the presence of an algal canopy, from a traditional wall-bounded shear layer to a plane mixing layer. As a consequence of this transition, the algal canopy increases turbulent kinetic energy within the roughness sublayer by ~2.5x compared to the healthy coral, while simultaneously reducing bed shear stress by nearly an order of magnitude. Lower surface stresses suggest a corresponding reduction in mass transfer at the coral-water interface, leading to negative impacts on a coral's ability to obtain resources and limiting its metabolic productivity.

The impact of coral surface roughness was investigated by collecting PIV measurements and dye visualizations in a recirculating water tunnel for unidirectional flow past a range of 3D-printed coral models with rib-type roughness and a model of an actual coral skeleton obtained by computed tomography scanning. The hydrodynamics of the boundary layer and wake structure are shown to depend on Reynolds number and the roughness element spacing, such that densely packed ‘d-type’ roughness generates flow properties--in particular, mean velocities, streamlines, eddy shedding frequency, and shear stresses--that are similar to a smooth hemisphere model. Widely spaced ‘k-type’ roughness, in contrast, displays strong vortex ejection from leading edge roughness cavities, resulting in enhanced shear stress (between 50-100%) and turbulent mixing (~50%) in the wake flow.

Lastly, LES models of flow dynamics and heat transfer--used as an analogue for mass transfer--were implemented for unidirectional and oscillatory flows past the same coral geometries used in the laboratory experiments. The numerical results show that under unidirectional forcing, drag forces dominate the physical mechanisms that drive turbulence characteristics and heat transfer; while under oscillatory forcing, inertial forces play a larger role. In general, increased surface roughness and oscillatory flow conditions each enhance heat transfer between 1.2-2.1x. However, in specific flow and roughness combinations--e.g., oscillatory, high Reynolds number flow--the increased surface area resulting from ‘d-type’ roughness outperforms the greater turbulence produced by ‘k-type’ roughness with respect to heat transfer enhancement. The complex interaction of flow speed, steadiness of flow, and nature of surface roughness determines the physical mechanisms that optimize trade-offs from surface morphology and creates profound consequences for the bio-physical processes responsible for coral resilience and sustainability.