Search for: All records

A classic lift decomposition is conducted on potential flow simulations of a near-ground pitching hydrofoil. It is discovered that previously observed stable and unstable equilibrium altitudes are generated by a balance between positive wake-induced lift and negative quasi-steady lift while the added mass lift does not play a role. Using both simulations and experiments, detailed analyses of each lift component's near-ground behaviour provide further physical insights. When applied to three-dimensional pitching hydrofoils the lift decomposition reveals that the disappearance of equilibrium altitudes for AR < 1.5 occurs due to the magnitude of the quasi-steady lift outweighing the magnitude of the wake-induced lift at all ground distances. Scaling laws for the quasi-steady lift, wake-induced lift and the stable equilibrium altitude are discovered. A simple scaling law for the lift of a steady foil in ground effect is derived. This scaling shows that both circulation enhancement and the velocity induced at a foil's leading edge by the bound vortex of its ground image foil are the essential physics to understand steady ground effect. The scaling laws for unsteady pitching foils can predict the equilibrium altitude to within 20% of its value when St < 0.45. For St > 0.45, there is a wake instability effect, not accounted for in the scaling relations, that significantly alters the wake-induced lift. These results not only provide key physical insights and scaling laws for steady and unsteady ground effects, but also for two schooling hydrofoils in a side-by-side formation with an out-of-phase synchronization. 

Abstract When swimming near a solid planar boundary, bio-inspired propulsors can naturally equilibrate to certain distances from that boundary. How these equilibria are affected by asymmetric swimming kinematics is unknown. We present here a study of near-boundary pitching hydrofoils based on water channel experiments and potential flow simulations. 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 (e.g. a bias angle, θ 0 ) or temporal asymmetry (e.g. a stroke-speed ratio, τ ). 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. 

Undulatory fin motions in fish-like robots are typically created using intricate arrays of servo motors. Motor arrays offer impressive versatility in terms of kinematics, but their complexity leads to constraints on size, hydrodynamic force production, and power consumption, particularly when studying propulsive performance at high-frequencies. Here we present an alternative design that uses a single motor and a tunable rotary cam-train system to achieve a spectrum of fin motions running from oscillation (wavenumber < 1) to undulation (wavenumber > 1). Our platform enables thrust, lift, power, and wake measurements at prescribed pitch amplitudes, frequencies, and wavenumbers. We demonstrated the platform’s oscillating and undulating capabilities via force and wake measurements in a water tank. Studies of fin wavenumber offer design insights for fish-like underwater robots, particularly those with stingray-inspired designs. 

Abstract One of the emerging themes of fish-inspired robotics is flexibility. Adding flexibility to the body, joints, or fins of fish-inspired robots can significantly improve thrust and/or efficiency during locomotion. However, the optimal stiffness depends on variables such as swimming speed, so there is no one ‘best’ stiffness that maximizes efficiency in all conditions. Fish are thought to solve this problem by using muscular activity to tune their body and fin stiffness in real-time. Inspired by fish, some recent robots sport polymer actuators, adjustable leaf springs, or artificial tendons that tune stiffness mechanically. Models and water channel tests are providing a theoretical framework for stiffness-tuning strategies that devices can implement. The strategies can be thought of as analogous to car transmissions, which allow users to improve efficiency by tuning gear ratio with driving speed. We provide an overview of the latest discoveries about (1) the propulsive benefits of flexibility, particularly tunable flexibility, and (2) the mechanisms and strategies that fish and fish-inspired robots use to tune stiffness while swimming. 

Fish maintain high swimming efficiencies over a wide range of speeds. A key to this achievement is their flexibility, yet even flexible robotic fish trail real fish in terms of performance. Here, we explore how fish leverage tunable flexibility by using their muscles to modulate the stiffness of their tails to achieve efficient swimming. We derived a model that explains how and why tuning stiffness affects performance. We show that to maximize efficiency, muscle tension should scale with swimming speed squared, offering a simple tuning strategy for fish-like robots. Tuning stiffness can double swimming efficiency at tuna-like frequencies and speeds (0 to 6 hertz; 0 to 2 body lengths per second). Energy savings increase with frequency, suggesting that high-frequency fish-like robots have the most to gain from tuning stiffness.