Force Transmission in Non-Uniform Fluid Flow by Controlling Vortices

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.