Session A02: Focus Session: Hydrodynamics of Benthic Marine Life

Between the branches: Recent results from large eddy simulations of local coral colony flow fields

Local hydrodynamics play a central role in physiological processes like respiration and nutrient uptake in coral reefs. Despite the importance of reefs as hosts to a quarter of all marine

life, and the pervasive threats facing corals, characterizing the hydrodynamics between the branches of scleractinian corals has remained a significant challenge. Here, we review recent results from large eddy immersed boundary simulations of the flow through and around Pocillopora meandrina, Pocillopora eydouxi, and Montipora capitata coral colonies. The studies suggest that colonies with higher branch densities have more complex mean velocity profiles with three different characteristic regions, while more loosely branched colonies have approximately constant mean velocity profiles along the flow direction; that surface roughness can counterintuitively increase the Reynolds stresses above the colony and hence enhance vertical transport; that passive geometric features of branching colonies produce highly vortical internal flows that enhance mass transfer at the interior of the colony, compensating almost exactly for flows speed reductions there of up to 64% so that the advection time scale remains roughly constant throughout the colony; and that the mean vortex diameter, rather than the mean branch diameter, may be the correct length scale to use to calculate the mass transfer Stanton number for intracolonial coral flows.   

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.   

Multiscale flow between the branches and polyps of gorgonians

Gorgonians, or sea fans, are soft corals that are well known for their elaborate branching structure and they way that they sway in the ocean. This branching structure can create feeding flows that are optimized for a particular range of velocities, and presumably for a particular size of prey. As water moves through the elaborate branches, it is slowed, and recirculation zones can form downstream of the fan. At the smaller scale, individual polyps that emerge from the branches expand their tentacles, further slowing the flow. At the smallest scale, the tentacles are covered in tiny bristles where prey capture and exchange occur. In this paper, we quantify the gap to diameter ratios for a variety of sea fans at the scale of the branches and the bristles on the polyp tentacles. We then use computational fluid dynamics to determine the flow patterns at both levels of branching. We quantify the leakiness between the branches and bristles over the biologically relevant range of Reynolds numbers and gap to diameter ratios. We find that the branches and bristles can either act as a leaky rake or solid plate depending on these dimensionless parameters. The results of the study have implications for how sea fans can generate efficient feeding flows while also reducing drag during storms.   

Numerical Simulations of Pulsing Soft Corals and the Photosynthesis of their Symbiotic Algae

Sessile octocorals in the family Xeniidae actively and almost constantly pulse their tentacles; this behavior is unusual as the movement is not for locomotion or to facilitate feeding. A prior experimental study suggested that the pulsing mixes the fluid and enhances the photosynthesis of their symbiotic algae by removing the photosynthesis byproduct, oxygen. It is hypothesized that the coral’s main energy source is from this symbiotic photosynthesis. To investigate this phenomenon, mathematical modeling and numerical methods are used. The immersed boundary method is used to simulate the fluid-structure interaction of the tentacle pulsing and resulting fluid motion. The fluid simulations are coupled to a photosynthesis model, which tracks the oxygen concentration in the fluid. In these simulations, physical parameters are varied to gain insight into the role of fluid inertia and diffusivity on mixing in the fluid and the resulting photosynthesis dynamics.   

Linking substrate topography and coral larval settlement in oscillatory flow

Marine larvae undergo passive transport in the water column and physical interactions with the benthic surface that influence recruitment and settlement. Using experiments and simulations, we discuss results from a 2D agent based model that was developed to investigate the effect of substrate topography and boundary layer flow on larval settlement. To model various benthic environments, we used a ridged substrate as a model topography and systematically varied the ridge height and ridge spacing from the micron to the millimeter scale. We found that substrate topography directly influenced settlement by modifying the boundary layer flow structures and the local fluid speed near the settlement surface. The effects of larval shape and larval swimming speed on settlement were also investigated. Our findings highlight how the materials used for underwater construction and habitat restoration can be designed intentionally with surface features to either promote or inhibit the settlement of marine invertebrate larvae.   

Feeding and exchange currents of epibenthic upside-down jellyfish

Through periodic bell pulsations, epibenthic Cassiopea jellyfish drive jets into the water column for suspension feeding, excretion and exchange of nutrients and gases. Hydrodynamic interactions of medusan jets with ambient low-speed flow can facilitate prey capture and enhance benthic nutrient fluxes. We conducted two-dimensional, two component (2D-2C) particle image velocimetry (PIV) measurements on Cassiopea individuals placed in a recirculating flow tank under continuous background flow speeds from 0.5 cm/s to 2 cm/s. Interaction of the background flow with the branches of the oral arms resulted in shedding small-scale vortices, with a larger spatial region where suspended particles can be brought closer to medusan feeding structures. With an advection-driven particle transport model that used PIV velocity fields as inputs, we observed that low background flow of 0.5 cm/s (characteristic of Cassiopea habitats) decreased escape efficiency and increased feeding efficiency of massless particles. Increasing background flow further to 2 cm/s increased escape efficiency and decreased feeding efficiency. Collectively, these findings suggest that suspension feeding is promoted by the interaction of unsteady medusan jets with low-speed, continuous background flows.   

Benthic jellyfish dominate water mixing in mangrove ecosystems

Water mixing is a critical mechanism in marine habitats that governs many important processes, including nutrient transport. Physical mechanisms, such as winds or tides, are primarily responsible for mixing effects in shallow coastal systems, but the sheltered habitats adjacent to mangroves experience very low turbulence and vertical mixing. The significance of biogenic mixing in pelagic habitats has been investigated but remains unclear. In this study, we show that the upside-down jellyfish Cassiopea sp. plays a significant role with respect to biogenic contributions to water column mixing within its shallow natural habitat (<2 m deep). The mixing contribution was determined by high-resolution flow velocimetry methods in both the laboratory and the natural environment. We demonstrate that Cassiopea sp. continuously pump water from the benthos upward in a vertical jet with flow velocities on the scale of centimeters per second. The volumetric flow rate was calculated to be 212 L h^-1 for average-sized animals (8.6 cm bell diameter), which translates to turnover of the entire water column every 15 min for a median population density (29 animals per m²). In addition, we found Cassiopea sp. are capable of releasing porewater into the water column at an average rate of 2.64 mL h⁻¹ per individual. The release of nutrient-rich benthic porewater combined with strong contributions to water column mixing suggests a role for Cassiopea sp. as an ecosystem engineer in mangrove habitats.   

Slowing the Flow: the Hydrodynamics of Benthic Protrusions and their Impact on European Eel (Anguilla anguilla) Kinematics

Eels are often unable to navigate fast flows caused by anthropogenic modifications (e.g. tidal gates, hydrokinetic turbines) due to their limited swimming performance and this is a contributing factor in the dramatic population decline of the European eel (Anguilla anguilla). A novel solution is the use of ‘eel tiles’ to create more favourable conditions allowing eels to navigate upstream. These tiles, composed of two strips of protrusions, one side with smaller diameter protrusions and the other with larger diameter protrusions, were placed on the bed of a laboratory flume. The hydrodynamics of the tiles were recorded using Particle Image Velocimetry (PIV) under conditions of varying bulk velocity and flow depth with diameter based Reynolds number in the range 925 - 7642. A reduction in mean velocity in the shear layer was observed immediately above the protrusion layer, which could aid eel passage during fast flows, and a periodic upwards shedding was also identified composed of a large scale turbulent structure among the small scale shedding from the individual wake protrusions, resulting in a highly turbulent and complex surface flow layer. We then observed the kinematic behaviour of European eels swimming in the flume over the different sections of the tiles and related the kinematic parameters of different swimming modes back to the hydrodynamic conditions of the flow. These pilot experiments show promising results for the widespread application of this technology and investigate the swimming kinematics of eels in turbulent flows, for which a knowledge gap exists   

Not Participating

The swimming kinematics of juvenile rainbow trout (Oncorhynchus mykiss) were experimentally evaluated in the flow field around a three-bladed vertical axis turbine (VAT). Fish were tested under two discharge conditions, with stationary VAT or rotating at optimum tip speed ratio. PIV measurements were used to measure the hydrodynamics, and the fish kinematics including centre of mass, head and tail position and orientation, and swimming speed were studied using an open-source motion tracking algorithm.

The flow field was characterized by flow acceleration on either side of the turbine, shear layers interfacing the free stream flow and the wake, and a low momentum region that was asymmetric relative to the rotor’s centerline. Fish reaction to the flow changes induced by the turbine blockage and rotation did not significantly vary among the test cases, where fish swam in the vicinity of the turbine, navigated the flow past the turbine from wake to upstream of the turbine, but remained predominantly in the low momentum wake for increased energy conservation. Future work will relate the fish motion to the local flow properties upstream and in the turbine wake.