Observations of shock waves in cloud cavitation

Abstract

This paper describes an investigation o l 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.

Cloud Cavitation: Observations, Calculations and Shock Waves

“…This lecture will review recent experimental and computational findings which confirm that, under certain conditions, the collapse of clouds of cavitating bubbles involves the formation of bubbly shock waves and that the focussing of these shock waves is responsible for enhanced noise and potential damage in cloud cavitation. The recent experiments of Reisman et al (1998) complements the work begun by Mørch and Kedrinskii and their co-workers and demonstrates that the very large impulsive pressures generated in bubbly cloud cavitation are caused by shock waves generated by the collapse mechanics of the bubbly cavitatting mixture. Two particular types of shocks were observed: large ubiquitous global pressure pulses caused by the separation and collapse of indiviual clouds from the downstream end of the cavitation and much more localized local pressure pulses which occur much more randomly within the bubbly cloud.…”

“…A valuable first effort to do this was put forward by Kubota et al (1992) but by limiting the collapse of individual bubbles they prevented the formation of the large pressure pulses associated with bubble collapse. Wang et al (1994 and Reisman et al (1998) present accurate calculations of a simple spherical cloud subject to a low pressure episode and show that, for a large enough initial void fraction, the collapse occurs as a result of the formation of a shock wave on the surface of the cloud and the strengthening of this shock by geometric focussing as the shock propagates inward.…”

Cavitation erosion degradation of Belzona®coatings

“…The dissolved gas is the source of cavitation nuclei [14]. The dynamic load at the moment when cavitation bubble collapses lasts for a few microseconds Brought to you by | MIT Libraries Authenticated Download Date | 5/10/18 9:43 PM or nanoseconds [15]. Also the borehole environment is very favourable for formation of different types of corrosion outbreaks [7].…”

The speed of sound in a gas–vapour bubbly liquid

“…Pressure waves in the cavitating region are responsible for much of the unsteady features of cavitation. For example, the experiments of Reisman et al [1] showed that condensation shocks cause large pressure spikes which influence the behaviour of sheet cavities (e.g. [2,3]), and recent work has demonstrated that condensation shocks are responsible for the transition from stable to cloud-shedding cavities [4].…”