Abstract
Cavitation is a process where a liquid evaporates due to a pressure drop and re-condenses violently. Noise, material erosion and altered system dynamics characterize for such a process for which shock waves, rarefaction waves and vapor generation are typical phenomena. The current paper presents novel results for collapsing vapour-bubble clusters in a liquid environment close to a wall obtained by computational fluid mechanics (CFD) simulations. The driving pressure initially is 10 MPa in the liquid. Computations are carried out by using a fully compressible single-fluid flow model in combination with a conservative finite volume method (FVM). The investigated bubble clusters (referred to as “clouds”) differ by their initial vapor volume fractions, initial stand-off distances to the wall and by initial bubble radii. The effects of collapse focusing due to bubble-bubble interaction are analysed by investigating the intensities and positions of individual bubble collapses, as well as by the resulting shock-induced pressure field at the wall. Stronger interaction of the bubbles leads to an intensification of the collapse strength for individual bubbles, collapse focusing towards the center of the cloud and enhanced re-evaporation. The obtained results reveal collapse features which are common for all cases, as well as case-specific differences during collapse-rebound cycles. Simultaneous measurements of maximum pressures at the wall and within the flow field and of the vapor volume evolution show that not only the primary collapse but also subsequent collapses are potentially relevant for erosion.
Highlights
Collapsing single bubbles have been extensively investigated in the past and results obtained by experiments, theory and simulations are broadly represented in the literature [1], [2]
Since this area is relatively large compared to the size of individual bubbles, pressure signals caused by single bubble collapses are blurred over the sensor area
This study shows the effects of the cloud interaction parameter and of the stand-off distance on the collapse of a cloud of vapour bubbles in a liquid ambient
Summary
Collapsing single bubbles have been extensively investigated in the past and results obtained by experiments, theory and simulations are broadly represented in the literature [1], [2]. There is a simple theory provided by Brennen et al [6], clusters of bubbles are difficult to investigate experimentally. One reason is that a repeatable generation of a cluster of well-defined bubbles (radii, position) is hardly achievable. To estimate the interaction among bubbles a cloud interaction parameter β was introduced [4]. This parameter is defined as α0·(1- α0)·A20/R20, where α0 is initial cloud void fraction and A0 and R0 are the cloud and the bubble radius, respectively
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