On the Mechanics of Flow-Induced Vibration of Soft Corals and Particle Interception

RÉSUMÉ Les coraux mous sont des espèces marines flexibles se déformant à l’effet des écoulements d’eau. Au passage d’une vague, alors que le tronc principal vacille, un mouvement particulier est observé: les branches se mettent à vibrer rapidement, avec de petits déplacements, et transversalement à la direction de l’écoulement. Dans ce mémoire, nous expliquons l’origine de ces vibrations et cherchons leur impact sur les coraux mous. Le critère de Glauert-den Hartog étant invalidé pour une section de branche de corail idéalisée, et les fluctuations de l’écoulement d’eau de mer ayant une fréquence de pic assez petite, nous avançons que les vibrations induites par vortex (VIV) sont la cause la plus plausible du mouvement rapide des branches. Par ailleurs, le fait que les coraux mous soient des espèces se nourrissant en filtrant l’eau de ses particules comestibles, nous faisons l’hypothèse que ces vibrations peuvent influencer leur taux d’alimentation. À l’aide d’un code maison d’éléments finis d’interaction fluide-structure, en plus de scripts codés en Python, nous avons simulé les trajectoires de particules sphériques autour d’un cylindre circulaire, puis calculé le taux de capture. Nous avons trouvé que, lors de la synchronisation de fréquence, les cylindres vibrants capturent jusqu’à 40% plus de particules que ceux fixés. Ainsi, les VIV augmenteraient probablement le taux d’alimentation des coraux mous et leur offriraient une meilleure nutrition.----------ABSTRACT Soft corals are flexible marine species that deform when exposed to a flow of water. Under the action of a wave surge, while the stem sways back and forth at the low frequency of the wave, a yet unreported motion takes place: the branches vibrate at high frequency, with small amplitude, and transverse to the water flow. The goal of this thesis is twofold: to explain the origin of these vibrations, and to find their impact on soft corals. Because the Glauert-den Hartog criterion is unfulfilled for an idealised coral branch, and since the peak frequencies of the seawater disturbance are too small, we consider vortex-induced vibrations (VIV) the only remaining probable cause of the observed rapid branch motion. Given that soft corals are sessile passive filter feeders that catch particles brought by currents, we hypothesise that these vibrations may affect their feeding rate. Using an in-house monolithic fluid-structure interaction (FSI) finite element solver along with a Python code, we simulated trajectories of spherical particles around a circular cylinder and calculated the capture rate. We found that vibrating cylinders capture up to 40% more particles than fixed ones at lock-in. Thence, VIV plausibly increase the rate of food capture and offer soft corals better nutrition

Vortex-induced vibrations: a soft coral feeding strategy?

Soft corals, such as the bipinnate sea plume Antillogorgia bipinnata, are colony building animals that feed by catching food particles brought by currents. Because of their flexible skeleton, they bend and sway back and forth with the wave swell. In addition to this low-frequency sway of the whole colony, branches of A. bipinnata vibrate at high frequency with small amplitude and transverse to the flow as the wave flow speed peaks. In this paper, we investigate the origin of these yet unexplained vibrations and consider their effect on soft corals. Estimation of dynamical variables along with finite element implementation of the wake-oscillator model favour vortex-induced vibrations (VIVs) as the most probable origin of the observed rapid dynamics. To assess the impact of the dynamics on filter feeding, we simulated particles advected by the flow around a circular cylinder and calculated the capture rate with an in-house monolithic fluid-structure interaction (FSI) finite element solver and Python code. We observe that vibrating cylinders can capture up to 40% more particles than fixed ones at frequency lock-in. Therefore, VIVs plausibly offer soft corals a better food capture.Comment: 20 page

Direct interception or inertial impaction? A theoretical derivation of the efficiency power law for a simple and practical definition of capture modes

We study the capture of particles advected by flows around a fixed cylinder. We derive theoretically the power law of the capture efficiency, usually obtained from data fitting only. Simulations of particle trajectories reveal that captured particles following the power law are smaller than the boundary layer of the cylinder and experience direct interception, whereas the ones diverging from it are larger and observe inertial impaction. We show that a simple comparison between the particle size and boundary layer thickness splits accurately numerical results into their dominant capture mode. This criterion is practical in experiments and simulations, and would lift the controversy on the scaling of the capture efficiency