Mechanism of multiscale cavitation induced pressure pulses on a propeller

This study presents a model scale experimental investigation on multiscale characteristics of cavity dynamics and induced pressure pulses around a highly skewed propeller using synchronous measurement and digital holographic measurement technology. The experimental studies reveal three distinct cavitation patterns, namely, the tip vortex and sheet cavitation (TVSC), the vortex cavitation (VC), and no cavitation (NC) under different advance coefficients and cavitation numbers. Propeller cavitation has significant effects on induced pressure pulses. In the low-frequency range, a predominant frequency component of blade passing frequency (BPF) is observed at approximately 105 Hz caused by the rotational motion of the propeller for all kinds of flow patterns. In addition, when cavitation occurs, the amplitude of pressure pulses increases significantly across a broad frequency spectrum larger than 2 BPF. For cases at an advance coefficient less than 0.75, the cavitation pattern varies from NC to TVSC with decreasing cavitation number. A regular tip vortex cavity first appears and connects on the propeller blade tip, then the well-developed tip vortex grows continually and a large-.scale transparent sheet cavity attaches on the back surface. Finally, the tip vortex almost disappears with only small-scale cavities shed downstream at the trailing edge of the sheet cavity at low cavitation number. In contrast, for cases at an advance coefficient larger than 0.75, the cavitation pattern varies from NC to VC as the cavitation number decreases. Concurrently, under the influence of vortices, an indistinct foamy cavity forms across the entire leading edge of the propeller front surface, accompanied by the shedding of small-scale cavities at the blade tips. Meanwhile, the small-scale shedding cavities are primarily composed of microbubbles with radii predominantly less than 100 μm. Additionally, the statistical probability density functions of the bubble radii adhere to a gamma distribution. Moreover, the mechanism leading to the formation and evolution of induced pressure pulses is analyzed and quantitatively verified based on experimental results. As cavitation develops with the decrease of cavitation number, the amplitude increases in the mid-frequency band spanning from 2 to 13 BPF for all kinds of cavitation patterns. Meanwhile, a distinct dominant frequency is observed in the mid-frequency region, which is associated with the fragmentation and shedding of small-scale cavities. Finally, the mechanism causing the pressure pulses in the high-frequency band higher than 13 BPF is investigated. The high-frequency pressure pulses are relatively small and stable when there is no cavitation observation or a stable vortex tube appearance at the propeller blade tip. In contrast, the pressure pulses fluctuate at a decrease rate of 6 dB per octave, caused by the collapse of microbubbles in the high-frequency region.