Emergent Kinematics and Flow Structure of Tension Driven Pulsing Xeniid Corals

This work presents a three-dimensional fully-coupled fluid-structure interaction (FSI) model of a pulsing soft coral polyp where the movement of the tentacles is driven by a prescribed active tension during contraction with a passive expansion due to the elastic behavior of the tentacles. The resulting motion of the tentacles is emergent rather than prescribed. This approach allows one to determine how the coral's underlying morphology, mechanics, and neural activation affect its kinematics and the resulting fluid motion, which has implications for soft robotic design. More specifically, one can easily vary the maximum tension exerted by the coral, the elasticity of the model coral body, and the pulsation frequency to understand how altering neuromechanical parameters affects the flux above the coral and the energy required to pulse actively. When the parameters are tuned such that the emergent motion is similar to that measured for live coral, a large amount of upward flux is generated for a relatively low energy expenditure. Additionally, a circulation analysis reveals the generation of stopping and starting vortices with each pulse cycle, as seen in other Cnidarians such as jellyfish. We find that the relationship between kinematics, upward flux, circulation, and the polyp's active and passive material properties is highly complex. Our results suggest that the corals operate at or near an energetically favorable regime. This work further increases our understanding of how and when sessile organisms should expend energy to actively pulse to enhance nutrient exchange.