|M.Sc Student||Malul Dror|
|Subject||Dancing out-of-phase: Coral Tentacle Stiffness Evokes|
Out-of-Phase Motion That Improves Mass Transfer
|Department||Department of Civil and Environmental Engineering||Supervisors||Professor Uri Shavit|
|Dr. Roi Holzman|
|Full Thesis text|
Sessile organisms spend their life interacting with the fluid around them. Corals and sea-anemones have flexible tentacles that extend to the flow, swaying and bending under the torques applied by the waves and currents. These animals rely on the ambient flow for nutrient supply, waste removal and gas exchange. The tentacle motion of three species of Hexacorallia was recorded in-situ and in a standing wave laboratory flume using high-speed cameras. The flow around the tentacles was non-intrusively measured using a Particle Image Velocimetry (PIV) system. In all of the experiments, tentacles exhibited an out-of-phase motion: the velocity of the tentacle oscillated with the same frequency as the waves, but preceded the velocity of the water by around ~1/4 of the wave period (a -90∘ phase difference). These measurements (N>120) led to three research questions: (1) is the out-of-phase motion general among other tethered organisms? (2) What are the mechanisms that generate this out-of-phase motion? (3) Is there a benefit in this motion, in terms of gas exchange with the water?
A dynamic model, in which the tentacle was represented as a torsional spring-damper system, was tested on the tentacles of the coral Dipsastraea favus. It was found that the stiffness of the tentacle, represented by a spring coefficient κ, is the main contributor to the phase difference. An order-of-magnitude analysis revealed that the spring torque applied by the tentacle was comparable in magnitude to the drag torque exerted by the flow. An analytical solution of the model suggests that the out-of-phase motion is general across tethered elastic organisms, and that the model is useful in describing their dynamics in the flow.
Corals experience sever hypoxic conditions during the night, due to intense respiration of the coral and its symbionts. Numerical simulations were conducted to study the effect of the phase difference on the mass flow rate of oxygen to the tentacle during that time. Measured values of the water velocity and the tentacles motion in the laboratory were imposed on a 2D model, representing the tentacle and its interaction with the water in its vicinity. For each of 10 combinations of water velocity and tentacle motion, five simulations were conducted in which phase differences of -180, -135, -90, -45 and 0 were imposed. These simulations show that moving out-of-phase enhances the mass flow of dissolved oxygen to the tentacle during a single period of oscillation by up to 25%, compared to an in-phase motion. The enhancement is achieved due to the increase in relative velocity between the tentacle and the water caused by the out-of-phase motion. The potential enhancement via this mechanism is greatest when the wave velocity amplitude is lower than ~5 cm s-1. Such conditions are frequent in many reef habitats, indicating that this improvement may represent an adaptive advantage to the coral in the competitive reef environment. The same principals and inferences may apply to other sessile organisms that oscillate, either due to wave flow regime, vortex shedding or turbulence caused by instabilities in unidirectional flow.