Measurement of Unsteady Flow and Pressure in the Tip Region of a Ducted Marine Propeller Using Tomographic Particle Tracking Velocimetry

Near design conditions, cavitation inception in the tip region of ducted propellers often occurs within secondary vortices surrounding the primary tip leakage vortex (TLV). Recent visualizations and measurements around a refractive index-matched acrylic propeller, with a tip diameter of 300.6 mm and a tip gap of 2.1 mm, show that quasi-axial vortices, the most likely sites of cavitation inception, originate from the blade suction side and are stretched axially by several circumferentially aligned vortices, including the TLV. The current effort characterizes the time evolution of flow and pressure within these vortices by applying tomographic particle tracking velocimetry. The dataset consists of over 10,000 instantaneous particle tracks recorded at 14 kHz and covers 20×22×8 mm³ sample volumes around the blade tip and in the near wake. The velocity and acceleration fields are subsequently interpolated onto structured grids, and the material acceleration is spatially integrated to obtain the pressure distributions. Comparisons with the prior stereo-PIV data show that all the prominent flow features are reconstructed, and confirm that axial stretching of quasi-axial vortices by circumferential vortices indeed creates the lowest instantaneous pressure. Ongoing analysis examines the location, frequency, and mechanisms affecting the pressure statistics in the cavitation inception region.