Flapping hydrodynamics and efficiency in a pteropod: A computational study of lift-based swimming

The flapping of wings is a common locomotion strategy for small organisms in both aerial and aquatic environments. Cuvierina atlantica, a marine pteropod, employs a distinctive “overlap-and-fling” motion to propel itself underwater. To examine the hydrodynamics underlying this behavior, we perform high-resolution fluid–structure interaction simulations using the lattice Boltzmann method coupled with the immersed boundary method. The body geometry and wing kinematics are accurately reconstructed from high-speed experimental videos and validated through direct overlay comparisons. The simulations show that C. atlantica relies primarily on lift-based propulsion, with both leading-edge and trailing-edge vortices contributing to thrust generation. The overlap-and-fling motion enhances circulation, promoting the development of strong vortices and sustained low-pressure regions along the wing edges, while wing–wing interaction increases vertical lift by more than 20%. Parametric analyses demonstrate that higher fluid viscosity elevates total hydrodynamic force but markedly reduces the lift-to-drag ratio and overall propulsion efficiency due to stronger viscous effects. Increasing flapping frequency boosts force production but leads to saturation in lift-to-drag and pressure-to-shear ratios, associated with increasingly chaotic wake structures. When interpreted using the Strouhal number, efficiency in the neutrally buoyant case exhibits an optimal value near 0.6. However, under gravity, this relationship breaks down, as weight-induced posture adjustments modify pitching dynamics and alter the phase relation between force generation and wake evolution. Across all conditions, higher chordwise Reynolds numbers generate numerous small vortices that enhance energy dissipation and reduce swimming efficiency.