Dynamics of cavitation bubbles near granular boundaries

Abstract

We study the dynamics of a cavitation bubble near beds of sand of different grain sizes. We use high-speed imaging to observe the motion of the bubble and the sand for different values of the stand-off parameter $\gamma$ (dimensionless bubble-boundary distance) between 0.3 and 5.3. Compared with a rigid boundary, we find that a granular boundary leads to bubbles with shorter lifetimes and reduced centroid displacements. Above $\gamma \approx 1.3$ , the behaviour of the bubble is almost independent of the granularity of the sand. When the stand-off parameter lies between $0.6$ and $1.3$ , a mound of sand develops beneath the bubble, which can force the latter to assume a conical shape as it collapses. For $\gamma \lesssim 0.6$ , the bubble develops a bell-shaped form, leading to the formation of thin and surprisingly fast micro-jets ( $v_{jet} > 1000\,{\rm m}\,{\rm s}^{-1}$ ). Moreover, between $\gamma \approx 1.3$ and $\gamma \approx 0.3$ , granular jets erupt from the sand surface following the bubble collapse. We additionally develop a simple numerical model, based on the boundary integral method, to predict the dynamics of a cavitation bubble near a bed of sand, which we replace by an equivalent liquid. The simulations are remarkably consistent with experimental observations for values of $\gamma$ down to $1.3$ . We also show how the anisotropy parameter $\zeta$ , a dimensionless version of the Kelvin impulse, can be adapted for the case of a nearby bed of sand and predict the displacement of the bubble centroid for $\zeta \lesssim 0.08$ .

Bubble collapse near porous plates

“…This effect is visible in data sets we have used previously (Andrews & Peters 2022) and can be found in some other publications such as figure 5 in Požar, Agrež & Petkovšek (2021) and figure 4( a ) in Sieber et al. (2022).

Figure 9.Frames from a high-speed recording of a bubble collapsing above a non-porous boundary.

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“…However, a recent study has reported use of an aluminium off-axis parabolic mirror (Sieber et al. 2022) so the degradation concern may not be as significant as assumed originally.…”

“…Notably, the gold surface absorbs most of the laser energy, but is less likely to degrade when immersed in water when compared with mirror materials designed to operate with a 532 nm laser (Obreschkow et al 2013). However, a recent study has reported use of an aluminium off-axis parabolic mirror (Sieber et al 2022) so the degradation concern may not be as significant as assumed originally.…”

“…Broadly, they could be categorised as connected or unconnected. One example of an unconnected porous material is a bed of sand, such as those investigated by Sieber, Preso & Farhat (2022). The sand creates pores which are permeated by water, but the grains are able to separate which allows for significant deformation of the bed.…”

Ring Vortex Dynamics Following Jet Formation of a Bubble Expanding and Collapsing Close to a Flat Solid Boundary Visualized via Dye Advection in the Framework of OpenFOAM

“…Similar vortex dynamics as the ones in Reuter et al [30] were observed by Sieber et al [34] with a bubble close to a sand bed. The sand gravels acted as tracer particles after the bubble collapse and were partially carried away from the bed by vortex shedding after the bubble dynamics have ceased.…”