Numerical exploration of the role of appendage flexibility in drag-based rowing at intermediate Reynolds numbers

Ctenophores, a phylum of gelatinous marine zooplankton, employ millimeter-scale flexible appendages (ctenes) composed of bundled cilia (hair-like oscillating structures) to generate flow by beating sequentially in rows (i.e., metachronal coordination, a strategy used by many aquatic invertebrates). They are the largest animals that employ cilia for locomotion, making them an ideal subject for studying the hydrodynamic scaling of ciliary flows from low to intermediate Reynolds numbers. Here, we focus on investigating the fluid-structure interaction (FSI) of these flexible appendages by implementing a freely available two-dimensional solver (IB2D) that uses the immersed boundary method (IBM). We simulate a single appendage and compare different cases of varying rigidity by hybridizing prescribed kinematics (as measured in freely swimming ctenophores) with two-way coupling (in which the appendage interacts freely with the surrounding fluid). We prescribe the kinematics from the root of the appendage to a fixed point, leaving the remainder of the appendage free. We then explore the effects of changing the flexibility-rigidness ratio (from fully prescribed to fully flexible) on the generated drag and thrust forces during the power and recovery strokes. Our results provide insight into both fundamental biology via key functional behaviors such as swimming as well as the potential role of flexibility in future bioinspired technology development.