Cavitation is the formation of vapor bubbles within a liquid and is caused by a low-pressure region generated by the passing of a solid object such as a projectile or propeller or where pressure waves pass through regions of density discontinuity. There are a number of industrial applications in which cavitation is an unwanted product of a flow process.
ESI has conducted a number of studies detailing the effects of cavitation for a number of industrial/research activities in order to provide an understanding of the process and the possible detrimental effects cavitation may produce, especially where a cavitation bubble collapses.
Figure 1: Hydrodynamic Ram Event Showing the Growth of a Cavitation Bubble and Its Collapse
For example, in the case of a Hydrodynamic Ram (HRAM) event, the growth and collapse of a large cavitation bubble generated by the transfer of kinetic energy, from a projectile as it moves through a liquid container, is transferred to the surrounding fluid resulting in the formation and growth of an expanding bubble interface and a corresponding drop in local static pressure below the vapor pressure of the liquid. Fig 1 shows a sequence of 6 images of how a large cavitation bubble grows when a projectile penetrates a water filled container. Images 1 – 3 shows its expansion to a maximum size in image 4 (approximately 10” (25.4 mm) in diameter). The following two images 5 & 6 shows how the bubble collapses in on itself forming a re-entrant jet, as detailed on the right side of the cavitation bubble in image 5. This implosion may produce extremely large pressures, as high as 1000 psi, and can result in catastrophic failure of the liquid container.
A further area of interest is that due to the momentum exchange and the resulting forces that are produced when the re-entrant jet collapses within the cavitation bubble (Figure 2). This typically occurs when a pressurized volume of gas is suddenly opened into a liquid producing a sudden uncorking of the pressurized gas that expands outwards into the surrounding liquid. This uncorking event forms a large bubble that grows and expands to a maximum size before collapsing.
Figure 2: Diagrammatic Sketch of the Evolution of a Re-Entrant Jet
Furthermore, if the bubble begins to collapse near a solid boundary, the inward-directed jet, as shown in the four images in Figure 3, may produce severe damage to a solid surface. Image 2 shows the start of the jet, with it expanding downwards in image 3, and finally beginning to collapse in image 4.
Figure 3: The Evolution of a Re-Entrant Jet