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This paper investigates the use of imposed rotations of an underwater cylinder reversing direction at a desired frequency in order to transmit vortices in a flow and enable a new method of underwater force transmission. A hydrofoil interacts with controlled vortices, which modulates the forces on the hydrofoil. The motivation is to assist and resist users walking on an underwater treadmill in a continuous-flow aquatic therapy pool used for gait rehabilitation, utilizing buoyancy to reduce apparent limb weight and impact force while walking. Previously, we have shown this concept on a small scale with a passive double pendulum when the incoming fluid flow is highly uniform. This paper shows that force transmission is also possible in such a harsh environment (a continuous-flow aquatic therapy pool) where the incoming flow is highly non-uniform and at a much larger scale. By measuring forces acting on a downstream hydrofoil, we show that the frequency of the vortices generated upstream can be perceived by the downstream hydrofoil.more » « lessFree, publicly-accessible full text available July 15, 2025
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Abstract The goal of this work is to present a method based on fluid–structure interactions to enforce a desired trajectory on a passive double pendulum. In our experiments, the passive double pendulum represents human thigh and shank segments, and the interaction between the fluid and the structure comes from a hydrofoil attached to the double pendulum and interacting with the vortices that are shed from a cylinder placed upstream. When a cylinder is placed in flow, vortices are shed in the wake of the cylinder. When the cylinder is forced to rotate periodically, the frequency of the vortices that are shed in its wake can be controlled by controlling the frequency of cylinder’s rotation. These vortices exert periodic forces on any structure placed in the wake of this cylinder. In our system, we place a double pendulum fitted with a hydrofoil at its distal end in the wake of a rotating cylinder. The vortices exert periodic forces on this hydrofoil which then forces the double pendulum to oscillate. We control the cylinder to rotate periodically, and measure the displacement of the double pendulum. By comparing the joint positions of the double pendulum with those of human hip, knee and ankle joint positions during walking, we show how the system is able to generate a human walking gait cycle on the double pendulum only using the interactions between the vortices and the hydrofoil.more » « less
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null (Ed.)Walking requires control of where and when to step for stable interlimb coordination. Motorized split-belt treadmills which constrain each leg to move at different speeds lead to adaptive changes to limb coordination that result in after-effects (e.g. gait asymmetry) on return to normal treadmill walking. These after-effects indicate an underlying neural adaptation. Here, we assessed the transfer of motorized split-belt treadmill adaptations with a custom non-motorized split-belt treadmill where each belt can be self-propelled at different speeds. Transfer was indicated by the presence of after-effects in step length, foot placement and step timing differences. Ten healthy participants adapted on a motorized split-belt treadmill (2 : 1 speed ratio) and were then assessed for after-effects during subsequent non-motorized treadmill and motorized tied-belt treadmill walking. We found that after-effects in step length difference during transfer to non-motorized split-belt walking were primarily associated with step time differences. Conversely, residual after-effects during motorized tied-belt walking following transfer were associated with foot placement differences. Our data demonstrate decoupling of adapted spatial and temporal locomotor control during transfer to a novel context, suggesting that foot placement and step timing control can be independently modulated during walking.more » « less