Observations on Tip Leakage Cavitation in Ducted Marine Propellers

In ducted marine propellers, the first site of cavitation inception is often associated with the tip leakage flow that rolls up to form the tip leakage vortex (TLV). This presentation discusses results of ongoing experiments aimed at characterizing the primary sites of inception based on high-speed imaging of events occurring around the tip of a transparent (acrylic), 30.06 cm diameter, two-bladed ducted propeller that has a tip gap of 2.12 mm. The experiments are performed in the JHU refractive index-matched test facility, where the working fluid, an aqueous solution of sodium iodide, has the same refractive index as acrylic, providing unobstructed optical access for flow visualization and velocity measurements. Systematic observations at varying advance ratios and ambient pressure (cavitation index) show the location, frequency, and appearance of cavitation. At inception, cavitation appears as intermittent short-lived events occurring in the in the TLV, away from the blade but upstream of the trailing edge. As the pressure is reduced, the cavitation expands to a large fraction of the TLV, allowing us to visualize its origin, trajectory, and interactions with secondary flow structures associated with it. In addition, the cavitation also expands into the tip gap. The observed trends will further be compared to results of PIV measurements.