Compressible Multiphase Flow: Cavitation and Collapse Phenomena


Cavitation is encountered in liquid flows which are subject to large pressure variations. It refers to the rapid growth of vapor nuclei in low pressure regions followed by an intense collapse of the resulting vapor pockets in zones of high pressure. This process is known for its detrimental impact onto engineering devices. The caused effects comprise, among other things, material erosion, noise and vibrations and thus limit the lifetime of the affected device.  In contrast, the power of caviation can also be harnessed, for instance, for biomedical applications, cleaning purposes or vegetable oil extraction. 


On the one hand, attempts to make use of the destructive power of cavitation needs precise control of the behavior of only a few gas bubbles. On the other hand, the design of engineering devices requires accurate models that are able to predict the collective behavior of cavitating and potentially turbulent flow. The latter typically involves thousands of vapor bubbles and includes a multitude of spatio-temporal scales. A detailed understanding of these complex processes is a particularly challenging task which cannot be captured by experimental measurements only, but also requires numerical investigations.


We perfom large-scale simulations of clouds of tens of thousands gas bubbles that are subject to a collapse process. Systems with such a large number of bubbles are computationally enabled by Cubism-MPCF, an open-source compressible multicomponent flow solver for high performance computing, which is developed and maintained by the members of the CSElab.


Figure 1 shows a close up of the outer surface of a cloud comprising 50 000 bubbles. The bubbles first collapse at the outer surface of the cloud. This process comes along with the formation of micro jets due to bubble-bubble interactions. The combined effect of individual collapsing bubbles results in the formation of an inward-propagating pressure wave and velocity front. While moving towards the core of the cloud, the pressure wave and the velocity front continuously increase in strength. Eventually, significant pressure amplifications are observed in the core region.


Fig.1: Large-scale simulation of collapse process of spherical cloud with 50’000 bubbles (movie).


Further simulations consider shock induced collapses of arrays of bubbles. Figure 2 depicts a rendering of the pressure field and the air/liquid interface for a linear array of bubbles that are subject to a shock coming from the left-hand side. The dense orange fields correspond to very high pressures while thin green fields corresponds to moderately high pressures.


Fig. 2: Volume rendering of high pressure regions of one collapsing bubble in linear array of bubbles (movie).




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