Fine-tuning near-boundary swimming equilibria using asymmetric kinematics

When swimming near a solid planar boundary, bio-inspired propulsors can naturally equilibrate to certain distances from that boundary. We discovered that asymmetric pitch kinematics can fine-tune those equilibrium distances. We present a study of near-boundary pitching hydrofoils based on water channel experiments and potential flow simulations. By varying the bias angle (spatial asymmetry), stroke-speed ratio (temporal asymmetry), and normalized ground distance, we examined how near-ground thrust, lift, and efficiency were affected by asymmetric kinematics. We found that asymmetric pitch kinematics do affect near-boundary equilibria, resulting in the equilibria shifting either closer to or away from the planar boundary. The magnitude of the shift depends on whether the pitch kinematics have spatial asymmetry or temporal asymmetry. Swimming at stable equilibrium requires less active control, while shifting the equilibrium closer to the boundary can result in higher thrust with no measurable change in propulsive efficiency. Our work reveals how asymmetric kinematics could be used to fine-tune a hydrofoil's interaction with a nearby boundary, and it offers a starting point for understanding how fish and birds use asymmetries to swim near substrates, water surfaces, and sidewalls.