Mixing, Photosynthesis, and Muscle Models

Xeniid soft corals are unusual in that they are sessile animals that rhythmically pulse their tentacles. The pulsing behavior is characterized by an active contraction followed by a passive expansion and resting period. There is evidence that these corals do minimal feeding. Instead, it is hypothesized that this pulsing behavior mixes the surrounding fluid to facilitate the photosynthesis of their symbiotic algae, their main energy source. This work focuses on the interaction of the pulsing behavior, the fluid mixing, and the resulting photosynthesis of these organisms. The immersed boundary method, developed by Charles Peskin, is used to model the fluid-structure interaction. This presentation will cover two-dimensional and three-dimensional simulations of these soft corals. The two-dimensional work uses the classical immersed boundary method. It is a common way to simplify coupled elastic-structure fluid interactions by assuming the elastic structure is infinitely thin and massless with a prescribed motion. The two-dimensional simulations include mixing analysis using dynamical systems approaches and a photosynthesis model coupled to the fluid-structure interaction. The more recent three-dimensional work will include using an updated immersed boundary approach called the immersed boundary finite element method developed by Boyce Griffith. This work allows for a fully three-dimensional elastic structure in the fluid governed by fundamental solid mechanics. Here the motion is not prescribed. Instead, the pulsing motion is driven by muscle contraction modeled as active tension allowing for observing emergent behavior.