Mitigating tip vortex cavitation by a flexible trailing thread

Tip Vortex Cavitation (TVC) is a major issue in design and operation of axial hydraulic machines. We investigate the capacity of a flexible trailing thread in alleviating TVC by analyzing the flow-induced motion. For this purpose, a nylon thread with three diameters is cut in various lengths and attached to the tip of an elliptical hydrofoil. The selected threads are flexible enough to become unstable and start to flutter under almost all the tested flow conditions. Due to the vortical flow, an oscillating thread is forced to spiral around the vortex axis. The resulting rotational motion is shown to decelerate the axial velocity in and around the vortex core via two possible mechanisms: first by exerting a local drag and taking energy from the flow and second by increasing the flow fluctuations and turbulent mixing. Our results reveal that a thread becomes more effective in TVC suppression when it is comparable in size with the viscous core of the tip vortex. In fact, a sufficiently thick thread may be sucked into the vortex core under the effect of the pressure field. This results in the hereby-called "whipping" motion that consists of the quasiperiodic coincidence of a part of the thread and the tip vortex axis close to the root. Compared with the rotational motion, the whipping motion is found superior in mitigating TVC. We propose a model that predicts that whipping motion, in contrast to rotational motion, could lead to viscous core thickening, which is validated by the velocity measurements.