Rebounds of deformed cavitation bubbles

Presented here are experiments clarifying how the deformation of cavitation bubbles affects their rebound. Rebound bubbles carry the remaining energy of a bubble following its initial collapse, which dissipates energy mainly through shock waves, jets, and heat. The rebound bubble undergoes its own collapse, generating such violent events anew, which can be even more damaging or effective than at first bubble collapse. However, modeling rebound bubbles is an ongoing challenge because of the lack of knowledge on the exact factors affecting their formation. Here we use single-laser-induced cavitation bubbles and deform them by variable gravity or by a neighboring free surface to quantify the effect of bubble deformation on the rebound bubbles. Within a wide range of deformations, the energy of the rebound bubble follows a logarithmic increase with the bubble's initial dipole deformation, regardless of the origin of this deformation.

Shock waves from nonspherical cavitation bubbles

Nicolas Dorsaz, Mohamed Farhat, Philipp Kobel, Danail Obreschkow, Outi Supponen, Marc Tinguely

We present detailed observations of the shock waves emitted at the collapse of single cavitation bubbles using simultaneous time-resolved shadowgraphy and hydrophone pressure measurements. The geometry of the bubbles is systematically varied from spherical to very nonspherical by decreasing their distance to a free or rigid surface or by modulating the gravity-induced pressure gradient aboard parabolic flights. The nonspherical collapse produces multiple shocks that are clearly associated with different processes, such as the jet impact and the individual collapses of the distinct bubble segments. For bubbles collapsing near a free surface, the energy and timing of each shock are measured separately as a function of the anisotropy parameter zeta, which represents the dimensionless equivalent of the Kelvin impulse. For a given source of bubble deformation (free surface, rigid surface, or gravity), the normalized shock energy depends only on zeta, irrespective of the bubble radius R-0 and driving pressure Delta p. Based on this finding, we develop a predictive framework for the peak pressure and energy of shock waves from nonspherical bubble collapses. Combining statistical analysis of the experimental data with theoretical derivations, we find that the shock peak pressures can be estimated as jet impact-induced hammer pressures, expressed as p(h) = 0.45(rho c(2) Delta p)(1/2) zeta(-1) at zeta > 10(-3). The same approach is found to explain the shock energy decreasing as a function of zeta(-2/3).

Amer Physical Soc2017

Collapse phenomena of deformed cavitation bubbles

Outi Supponen

Cavitation bubbles are a topic of long-standing interest owing to the powerful phenomena associated with their collapse. Their unique ability to focus energy typically causes damage in hydraulic machinery (turbines, pumps, propellers, ...) but, if managed correctly, can also be beneficial in numerous applications such as cleaning practices and biomedical sciences. Here the complex problem of cavitation, often multi-scale both in time and space, is reduced to a simplified case study of the collapse of a single, initially spherical bubble. We study the bubble's energy distribution into its distinct collapse phenomena, namely the micro-jets, shock waves and luminescence, and aim to quantify and predict how such a distribution is affected by the bubble's deformation. Combining experiments with statistical analysis, numerical simulations and theoretical models, we seek to quantify and predict the key properties characterising each of the collapse phenomena. The deformation of bubbles is characterised by the liquid micro-jets formed during their non-spherical collapse. A unified framework is proposed to describe the dynamics of such jets, driven by different external sources, through an anisotropy parameter

\zeta

, which represents a dimensionless quantity of the liquid momentum at the bubble collapse (Kelvin impulse). The bubbles are carefully deformed in variable gravity aboard European Space Agency parabolic flights or by introducing surfaces nearby. Through high-speed visualisation, we measure key quantities associated with the micro-jet dynamics (e.g. jet speed, impact timing), which, upon normalisation, reduce to straightforward functions of

\zeta

. This is verified by numerical simulations based on potential flow theory. Below a certain threshold, all of these functions can be approximated by useful power laws of

\zeta

that are independent of the micro-jet driver. For bubbles collapsing near a free surface, we identify and measure the shock waves generated through distinct mechanisms, such as the jet impact onto the opposite bubble wall and the individual collapses of the remaining bubble segments. The energy carried by each of these shocks is found to vary with

\zeta

. We find that for bubbles that produce jets, the shock wave peak pressure may be approximated by the jet-induced water hammer pressure as a function of

\zeta

. Following such an approximation, we also develop a semi-empirical model to explain the shock energy variation with

\zeta

. Finally, an innovative luminescence detection system is built to overcome the challenge of measuring the spectra (300-900nm) of the weak, small, rapid and migrating flash light from individual bubble collapses. We find an approximately exponential quenching of the luminescence energy as a function of

\zeta

. Surprisingly, the blackbody temperature of luminescence does not vary with

\zeta

. Multiple peaks are measured within a time frame of approximately 200ns, implying non-uniform gas compression during the collapse. Overall, these results help in predicting bubble collapse characteristics in known pressure fields and can be useful for numerical benchmarking.

EPFL2017