Mass Transport and Turbulent Statistics within Two Branching Coral Colonies

• Main Authors: Md Monir Hossain, Anne E. Staples
• Format: Article
• Language: English
• Published: MDPI AG 2020-09-01
• Series: Fluids
• Subjects: coral hydrodynamics; immersed boundary method; large eddy simulation; turbulent stresses; mass transfer
• Online Access: https://www.mdpi.com/2311-5521/5/3/153

Large eddy simulations were performed to characterize the flow and mass transport mechanisms in the interior of two <i>Pocillopora</i> coral colonies with different geometries, one with a relatively loosely branched morphology (<i>P. eydouxi</i>), and the other with a relatively densely branched structure (<i>P. meandrina</i>). 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 <i>P. meandrina</i> 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 <i>P. eydouxi</i> 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 <inline-formula><math display="inline"><semantics><mrow><mn>219</mn><mo>%</mo></mrow></semantics></math></inline-formula> 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, <i>P. meandrina</i>.