Propeller tip vortex analysis by means of PIV

A propeller wake investigation is carried out in a cavitation tunnel using PIV. The analysis is focused on the geometrical and hydrodynamic features of the downstream tip vortex which evolves undisturbed for many propeller diameters. The contributions of the viscous wake, originated by the boundary layers on the blades, of the trailing vortex sheets, as well as of the vorticity distribution, are furthermore resolved and discussed. The near wake flow is characterized by strong deformations, due to the bending of the blade wake sheets, to the contraction of the slipstream and to the trajectory of the tip vortex. Furthermore, the turbulent diffusion and the viscous dissipation rapidly lead to broadening and decay of the trailing edge wake. The roll-up and viscous effects in the spiraling process are also pointed out. Finally, the slipstream instability leading to the breakdown of the hub and tip vortices system are outlined.