Abstract
Large eddy simulations were performed to characterize the flow and mass transport mechanisms in the interior of two Pocillopora coral colonies with different geometries, one with a relatively loosely branched morphology (P. eydouxi), and the other with a relatively densely branched structure (P. meandrina). Detailed velocity vector and streamline fields were obtained inside both corals for the same unidirectional oncoming flow, and significant differences were found between their flow profiles and mass transport mechanisms. For the densely branched P. meandrina colony, a significant number of vortices were shed from individual branches, which passively stirred the water column and enhanced the mass transport rate inside the colony. In contrast, vortices were mostly absent within the more loosely branched P. eydouxi colony. To further understand the impact of the branch density on internal mass transport processes, the non-dimensional Stanton number for mass transfer, St, was calculated based on the local flow time scale and compared between the colonies. The results showed up to a 219% increase in St when the mean vortex diameter was used to calculate St, compared to calculations based on the mean branch diameter. Turbulent flow statistics, including the fluctuating velocity components, the mean Reynolds stress, and the variance of the velocity components were calculated and compared along the height of the flow domain. The comparison of turbulent flow statistics showed similar Reynolds stress profiles for both corals, but higher velocity variations, in the interior of the densely branched coral, P. meandrina.
Highlights
Determining hydrodynamic properties in the interior of coral colonies is essential for understanding growth and mass transport process in branching corals [1,2]
The authors determined the mean velocity profile and Reynolds stresses from the velocity measurements at the same measurement points, both inside and above the model reef [20]. These analyses provide an excellent picture of the character of turbulent flow fields throughout a coral reef, but complete, detailed, three-dimensional flow fields inside an entire coral colony remain exceedingly difficult to obtain
When the mean branch diameter was used along the length of coral, the results showed a nearly constant Stanton number throughout the colony in the flow direction, which is the upper bound of mass transfer for the whole colony
Summary
Determining hydrodynamic properties in the interior of coral colonies is essential for understanding growth and mass transport process in branching corals [1,2]. Because of each colony’s unique geometric structure, distinct flow fields evolve in different individual colonies, both in the interior and at the exterior of the colony. The mean flow profiles inside the colony influence the exchange of solutes across the coral tissue-water interface, control the nutrient transport, and determine the dissolved oxygen concentration near the coral surface [3]. In an early flume experiment, Chamberline et al visualized the flow around different coral morphologies and concluded that loosely branched colonies likely have a relatively higher probability of obtaining suspended food particles inside the colony. Tightly branched colonies must contend with the reduced access to food inside the coral due to their complex branching structures that impede the flow [4].
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