PUBLICATIONS

Tunable stiffness enables fast and efficient swimming in fish-like robots

Fish are thought to adjust their tail stiffness to swim efficiently over a wide range of speeds, but how they tune stiffness has been a mystery. We derived a model that combines fluid dynamics and bio-mechanics to reveal that muscle tension should scale with swimming speed squared. By applying our strategy to a fish-like robot, we were able to nearly double its efficiency.

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Science Robotics

Aspect Ratio Affects the Equilibrium Altitude of Near-Ground Swimmers

Animals and bio-inspired robots can swim/fly faster near solid surfaces, with little to no loss in efficiency. How these benefits change with propulsor aspect ratio is unknown. Here we show that lowering aspect ratio weakens unsteady ground effect: thrust enhancements become less noticeable, equilibrium altitudes shift lower and become less stable, and wake asymmetries become less pronounced. Water channel experiments and potential flow simulations reveal that these effects are consistent with known unsteady aerodynamic scalings. We also discovered a second equilibrium altitude even closer to the wall (< 0.35 chord lengths). This second equilibrium is unstable, particularly for high-aspect-ratio foils. Active control may therefore be required for high-aspect-ratio swimmers hoping to get the full benefit of near-ground swimming. The fact that aspect ratio alters near-ground propulsion suggests that it may be a key design parameter for animals and robots that swim/fly near a seafloor or surface of a lake.

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Journal of Fluid Mechanics

Scaling Laws for the Propulsive Performance of a Purely Pitching Foil in Ground Effect

Scaling laws for the thrust production and power consumption of a purely pitching hydrofoil in ground effect are presented. For the first time, ground effect scaling laws based on physical insights capture the propulsive performance over a wide range of biologically-relevant Strouhal numbers, dimensionless amplitudes, and dimensionless ground distances. This is achieved by advancing previous scaling laws (Moored & Quinn 2018) with physics-driven modifications to the added mass and circulatory forces to account for ground distance variations. The key physics introduced are the increase in the added mass of a foil near the ground and the reduction in the influence of a wake vortex system due to the influence of its image system. The scaling laws are found to be in good agreement with new inviscid simulations and viscous experiments, and can be used to accelerate the design of bio-inspired hydrofoils that oscillate near a ground plane or two out-of-phase foils in a side-by-side arrangement.

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Journal of Fluid Mechanics

How Dorsal Fins Make Fish Faster and More Efficient

The dorsal and anal fins of fish interact with the tail fins to produce higher thrust and efficiency. We focused on thin elongated dorsal fins, like those of jackfish. We discovered that dorsal fins can act like the wing strakes of fighter jets, promoting flow attachment on a main lifting surface (wing/tail) by inducing spanwise flow and reducing the effective angle of attack. Beyond a critical sharpness, the effect is more costly than beneficial, meaning dorsal fins may be optimal when they are slightly blunted rather than razor sharp. See a video of the results here.  (This work was done in collaboration with the Flow Simulation Research Group at the University of Virginia.)

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Journal of Fluid Mechanics

Modeling Lateral Station-Keeping in Fish

Fish flap their tails asymmetrically to maneuver around obstacles. In contrast, classic fish tail models assume symmetric motions in a uniform flow. We tested how well these classic models work for maneuvering tails. In some cases, the models work well: even 2D wakeless models were able to predict the phase of high frequency lateral displacements. As for predicting overshoot and settling time, only a semi-empirical model was accurate to within 10%.

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Biomimetics

Stable Equilibria Exist for Near-Surface Swimmers and Fliers

Fish and birds experience different forces when they swim/fly near a flat surface (e.g. seabed, solid ground, still lake). We discovered that the vertical forces they feel switch from negative (downward) to positive (upward) at a particular distance from the surface. In other words, there’s a stable equilibrium altitude where they are neither pushed down nor up. Animals and bio-inspired robots should factor this altitude into their control schemes; ignoring it could lead to high energy costs when swimming/flying near a flat surface. (This work was done in collaboration with the Biofluids Research Lab at Lehigh University.)

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Journal of Fluid Mechanics

Scaling Laws for 3D Pitching Hydrofoils

Building on our previous work on 2-D pitching airfoils, we explored how forces and torques scale for 3-D pitching airfoils. The terms we added to existing theories were inspired by the 3-D elliptical ring shapes of wake vortices. We validated the new terms by comparing our predictions with water channel experiments over a range of frequencies, amplitudes, and aspect ratios. The modified scalings offer guidance for theories about fish morphology and design strategies for bio-inspired robots. (This work was done in collaboration with the Biofluids Research Lab at Lehigh University.)

Journal of Fluid Mechanics

Passive Pitching of a Flapping Wing in Turning Flight

Building on my previous work on passive pitching wing,  we explored the effects of  torsional wing flexibility, inertial and rotation velocity on maneuverability.  We found the body rotations change the relationship between aerodynamic and elastic torque exerted about the wing pitching axes. Results of this study reveal that torsional flexible on wing could enhance maneuverability of micro air vehicle (MAV).

AIAA Journal

A Nonlinear Dynamics-Based Estimator for Functional Electrical Stimulation: PreliminaryResults from Lower-Leg Extension Experiments

To precisely predict body motion with wearable IMU sensor and non-linear limb dynamic model , we proposed a new class of nonlinear state estimation technique named state-dependent coefficient (SDC) estimation to dealing with non-linearity of limb dynamic model. The experiment results showed our SDC estimator result in  smaller estimation error compare to an extended kalman filter (EKF) and require less information but higher accuracy than rotation matrix method.

IEEE Transactions on Neural Systems and Rehabilitation Engineering