Drag and inertia forces on a branched coral colony of Acropora palmata

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Some branched corals are capable of surviving in wave/current exposed areas with moderate to high energy environments, where their branches, their geometry and the global and rigid structure of their colonies play an important role for wave damping due to bottom friction and wave energy dissipation due to turbulence. Constant frictional coefficients are commonly used within wave propagation models to consider reef roughness but without accounting for the type of coral species, its topological characteristics, form and distribution over the reef surface. In this sense, this study aims to improve the understanding of near-bed hydrodynamics in coral reefs by examining the drag and inertia coefficients of branched coral colonies of Acropora palmata. Laboratory tests were carried out considering steady and oscillatory flow conditions. In-line forces, flow velocities and water surface elevations over 3-D models of Acropora palmata were recorded for both a single coral colony and a group of corals. Additionally, the validated open source CFD (Computational Fluid Dynamics) toolbox OpenFOAM® was used to simulate the hydrodynamic performance over the coral structures for a wide range of wave heights and periods. The results show that under steady flow conditions, the drag coefficient (CD) can be well represented as a function of the Reynolds number (Re) by means of a power law equation as CD= aReb+c. Also, under oscillatory flow conditions, it was found that the inertia force (FM) dominates over the drag component (FD), explaining more than 84% of the total force exerted on the coral structure. Prediction formulas were developed and validated with the laboratory test to predict the resistance coefficients (drag and inertia) as function of Keulegan and Carpenter number (KC) for the CD (R2=0.89) and CM (R2=0.92). The CFD modeling showed to be an alternative for the modeling of the hydrodynamic forces exerted over complex coral shapes and to analyze the fluid–structure interaction around natural structures.