Tip Vortex Cavitation Inhibition by Drag-Reducing Polymer Solutions

This paper presents previous and recent results obtained by the authors concerning the modification of the tip vortex cavitation of a finite span hydrofoil by drag-reducing polymer solutions. Experiments were conducted with homogeneous solutions and with semidilute solutions ejected at the tip of the wing. Measurements of the onset cavitation number for tip vortex cavitation, hydrodynamic forces on the hydrofoil and tangential velocities in the tip vortex have been conducted. The results show that tip vortex cavitation is inhibited in all cases but for different reasons. In homogeneous polymer solutions the lift of the hydrofoil, and hence the circulation, is considerably reduced leading to a less intense vortex as shown by tangential velocity measurements. With semidilute polymer solution ejections there is no noticeable change of the hydrodynamic forces but a significant modification of the tangential velocities in the core region. The mechanism for tip vortex cavitation inhibition is thus completely different in these two situations.