An experimental investigation into the dynamics of propeller tip vortices and the associated cavitation noise

An experimental investigation into the tip leakage vortex (TLV) flow from a 3-bladed ducted marine propulsor was performed to assist in understanding the dynamics of ‘limited event-rate cavitation’ (LEC) inception in the vortex core. The study involved particle image velocimetry (PIV) measurements of the liquid phase TLV synchronized with the propeller angular position. PIV measurements of the tip vortex from the propeller without the duct were also acquired. Additionally, the dynamics of cavitation nuclei capture by a concentrated vortex were investigated experimentally through observations of laser induced bubbles placed near the core of a simple line vortex, and numerically using a particle tracking model for a similar flow. The acoustic noise from cavitation bubbles in a simple line vortex was also studied experimentally.; Velocity-inferred vortex core pressures performed here and combined with cavitation inception observations by Chesnakas & Jessup (ASME-FEDSM 2003) indicated that LEC occurred at a location downstream from the blade far from the average location of minimum vortex pressure, and at a higher cavitation inception number than expected. Detailed analysis of the instantaneous PIV fields revealed that the TLV was comprised of a system of multiple vortices. An identification procedure was developed to measure the multiple vortices. Elevated velocity fluctuations coincided with the location along the vortex core where LEC occurred. This suggests that LEC is associated with fine vorticity filaments that are possibly being stretched and undergoing transient pressure dips sufficient for cavitation. This finding contradicts the classical scaling law of McCormick (J. Basic Eng. 1962).; The noise produced by collapsing cavitation bubbles inserted optically into the core of a line vortex was examined experimentally, and the produced acoustic pulses upon collapse were found to be broadband (>100 kHz). The noise due to bubble growth is negligible compared to the collapse pulse. The efficiency of vortex cavitation bubbles at transforming their stored mechanical energy into radiated acoustical energy decreased with decreasing cavitation number.