The dynamics of cavity clusters in ultrasonic (vibratory) cavitation erosion

Abstract

The erosion of solids caused by cavitating liquids is a result of the concerted collapse of clusters of cavities. In vibratory cavitation equipment the clusters grow and collapse adjacent to a solid surface and are typically of hemispherical or cylindrical form. In the present paper the collapse process of these clusters is described and the collapse equations are developed and solved. The theoretical results are compared with results from high-speed photography of the clusters and with the initial stages of cavitation erosion on metal specimens. Experimental and theoretical results show that the collapse of a cavity cluster is driven by the ambient pressure and the collapse proceeds from the outer boundary of the cluster towards its center. During the collapse the pressure at the inward-moving cluster boundary increases continuously, and at the cluster center it rises significantly above the ambient pressure. Therefore the collapse velocity of the individual cavities increases towards the cluster center, which explains that the erosion, being caused by the individual cavities, occurs predominantly in this region. Likewise, the pressure increase at the cluster boundary explains why materials of even high strength can be eroded by collapse of cavity clusters at atmospheric ambient pressure.

The study of surface processes under electrochemical control in the presence of inertial cavitation

“…This implies that prior to any event that induces a re-passivation transient, the electrode is essentially shielded by the collapsing bubble. Nevertheless, it has also been proposed that multiple cavitation bubble collapses can occur which could potentially cause erosion of a surface [16]. However, the experimental data contained in this paper does not allow the actual mechanism of erosion to be ascertained at this time.…”

Effect of nuclei concentration on cavitation cluster dynamics

“…These observations suggest to model the shrinkage of the cluster boundary as a shock front. The speed,Ẋ, at which the cluster collapses is identified with the propagation speed of the shock front 23,24 which is given by:…”

Mass transfer enhancement produced by laser induced cavitation

“…This range of sizes, and the unpredictability of the times and locations of cavitation nucleation, hinders examination of individual hydrodynamic or ultrasonic cavitation events: measurement of individual events may be required to validate models, or to correlate the results of one sensor with those of another, as is done in this paper with mass transfer sensing and high-speed photography. Furthermore hydrodynamic or ultrasonic cavitation tends to produce clusters [4][5][6] and clouds of events within a particular reactor. While this may be useful for bulk processing of materials, it makes the study of the cavitation processes that are occurring more difficult as the locality of each event is hard to predict and interactions between bubbles (cluster effects) may also occur.…”

“…Finally, it should be noted that due to the high gain (used to ensure a good signal to noise ratio for this system) employed (10 7 V A -1 ), the response time of the current follower is likely to be limiting 5 . Hence the absolute current magnitude at the event peaks will be restricted by op-amp response.…”

“…It is interesting to note that the mass transfer enhancements (where strong rebound was observed -see figure 4) are most prominent after the primary collapse and rebound: whereas the acoustic signature as a result of bubble action is strongest 5 The response time of the current follower employed was measured by applying a potential step across a dummy resistor (1 x 10 7 Ω) at a gain of 1 x 10 7 V A -1 . The maximum response time was found to be However, it should be noted that the video sequence shown in figure 1 was not recorded at the same time as the electrochemical data shown in figures 2 through 4.…”