Fine-tuning a hydrofoil's interactions with a nearby solid boundary using asymmetric pitch kinematics

We discovered that asymmetric pitch kinematics can fine-tune a pitching hydrofoil’s interactions with a nearby solid boundary. In our experiment, a hydrofoil was suspended in a water channel and driven with prescribed asymmetric pitch motions. 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. Thrust and lift were measured by a six-axis force-torque sensor, hydrodynamic power was recorded using an encoder, and ground distance was measured by a laser distance sensor. Results show that 1) asymmetries affect lift and shift the equilibrium altitude, and 2) asymmetries provide thrust benefits with little to no penalty in efficiency. Our work reveals how spatial and temporal asymmetric pitch kinematics interact with a nearby solid boundary and how they can be applied to near-ground propulsion.