Experimental Investigation of Propeller Wake Evolution by Means of LDV and Flow Visualizations

An experimental investigation of the propeller wake has been performed in a cavitation tunnel using LDV and flow visualizations. The objective is the hydrodynamic and geometrical characterization of the wake flow field and its downstream evolution features. Implications of the physical aspects for wake modeling are also highlighted. The viscous blade wake, originating in the boundary layer on the blade surfaces, the trailing vortex sheets, due to the radial gradient of the bound circulation, as well as the turbulence distribution are identified at the trailing edge and followed. The near-wake geometry is quantitatively determined describing the progressive bending of the blade wake sheets, the slipstream contraction and the tip vortex trajectory. Furthermore, the effects of turbulent diffusion and viscous dissipation, which cause a rapid space-broadening of the velocity gradients in the trailing-edge wake, are examined. Insights into the viscous interaction between blade flow and roll-up process in the tip region are also proposed. Finally, the onset and development of the slipstream instability leading to the breakdown of the vortices system in the far wake are studied also by means of visualizations in incipient cavitating flow conditions.