“Cavitation” is a common hydrodynamic phenomenon. It occurs due to strong accelerations in a liquid, which reduce the local pressure. When this pressure is sufficiently low, vapor bubbles are formed. The formation and subsequent collapse of these bubbles can lead to loss of performance (for instance of ship propellers or pumps), noise, vibrations, and wear.
In general, cavitation is an unsteady phenomenon: vapor cavities grow due to coalescence of vapor bubbles, and these cavities will shed once they reach a certain size. These shed cavities travel downstream as vapor clouds, before ultimately collapsing.
Here we visualize the two main mechanisms that are considered to be responsible for the shedding: the ‘re-entrant jet’ and ‘condensation shock’ mechanisms. This is demonstrated in a benchmark flow: a converging-diverging nozzle (a.k.a. ‘venturi’). While both mechanisms lead to periodic shedding, they do so at different time scales. Interestingly, traces of both mechanisms can be seen for a wide range of flow conditions. Nevertheless, in general one of the two will be the dominant mechanism, setting the frequency of the shedding cycle. Why this is the case is not exactly clear. To investigate this, we combine various measurement techniques to provide quantitative insight into this flow. This include tomographic particle imaging velocimetry, high-speed imaging, time-resolved x-ray densitometry and pressure measurements. The combined data is used to demonstrate why pressure waves - emanating from the cloud collapse - turn into condensation shocks, thus becoming the dominant mechanism instead of the re-entrant jet