Observations of shock waves in cloud cavitation

TLDR

Cloud cavitation, formed by periodic breakup and collapse of a sheet or vortex cavity, often causes severe noise and damage, but the precise mechanism enhancing these effects remains unclear. The study aims to investigate the large impulsive surface pressures produced by cloud cavitation and correlate them with high‑speed motion‑picture images. High‑speed imaging is used to correlate the impulsive surface pressures with cavitation dynamics. The study reveals that shock waves form during cloud collapse, with global shock waves from coherent collapse producing the largest pressures and noise, while crescent‑shaped and leading‑edge structures from less‑coherent collapse generate smaller radiated noise but comparable interior pressure pulses, indicating that shock‑wave dynamics, rather than single‑bubble collapse, drive noise and damage in cavitating flows.

Abstract

This paper describes an investigation of the dynamics and acoustics of cloud cavitation, the structures which are often formed by the periodic breakup and collapse of a sheet or vortex cavity. This form of cavitation frequently causes severe noise and damage, though the precise mechanism responsible for the enhancement of these adverse effects is not fully understood. In this paper, we investigate the large impulsive surface pressures generated by this type of cavitation and correlate these with the images from high-speed motion pictures. This reveals that several types of propagating structures (shock waves) are formed in a collapsing cloud and dictate the dynamics and acoustics of collapse. One type of shock wave structure is associated with the coherent collapse of a well-defined and separate cloud when it is convected into a region of higher pressure. This type of global structure causes the largest impulsive pressures and radiated noise. But two other types of structure, termed ‘crescent-shaped regions’ and ‘leading-edge structures’ occur during the less-coherent collapse of clouds. These local events are smaller and therefore produce less radiated noise but the interior pressure pulse magnitudes are almost as large as those produced by the global events. The ubiquity and severity of these propagating shock wave structures provides a new perspective on the mechanisms reponsible for noise and damage in cavitating flows involving clouds of bubbles. It would appear that shock wave dynamics rather than the collapse dynamics of single bubbles determine the damage and noise in many cavitating flows.