Analytical approximations for the collapse of an empty spherical bubble

The violent collapse of vapor bubbles in cryogenic liquids

Vapor bubbles in cryogenic fluids may collapse violently under subcooled and pressurized conditions. Despite important implications for engineering applications such as cavitation erosion in liquid propellant rocket engines, these intense phenomena are still largely unexplored. In this paper, we systematically investigate the ambient conditions leading to the occurrence of violent collapses in liquid nitrogen and analyze their thermodynamic characteristics. Using Brenner's time ratio χ, the regime of violent collapse is identified in the ambient pressure-temperature parameter space. Complete numerical simulations further refine the prediction and illustrate two classes of collapses. At 1 < χ < 10, the collapse is impacted by significant thermal effects and attains only moderate wall velocity. Only when χ > 10 does the collapse show more inertial features. A mechanism analysis pinpoints a critical time when the surrounding liquid enters supercritical state. The ultimate collapse intensity is shown to be closely associated with the dynamics at this moment. Our study provides a fresh perspective to the treatment of cavitation in cryogenic fluids. The findings can be instrumental in engineering design to mitigate adverse effects arising from intense cavitational activities.

An empirical experimental investigation on the effect of an external electric field on the behaviour of laser-induced cavitation bubbles

This study is an attempt to empirically investigate the behaviour of laser-induced cavitation bubbles under the influence of an external electric field. As such two targets (copper and iridium) were subjected to a high-power Nd:YAG laser beam while being submerged in a liquid. Three different liquids were chosen for this purpose viz. acetone, ethanol, and distilled de-ionized water. The choice of the liquids was made with the underlying assumption that the conductivity of the liquids would play a significant role in responding to the applied external electric field and thus dictate the behaviour of the cavitation bubbles. A probe-beam method known as a beam deflection setup was employed for this experiment and the results were analyzed using the Rayleigh-Plesset model. The results revealed that the maximum radii of the cavitation bubbles increased in response to an increasing electric field. This effect was more pronounced in the presence of acetone medium and decreased successively while using ethanol and water media owing to their varying magnitudes of electrical conductivity. The bubble collapse speeds and their energies were also measured and similar trends were observed in both cases. The results from cavitation bubble dynamics were then applied to a Gilmore model and the sizes of the NPs synthesized using laser ablation with and without an external electric field were calculated using classical nucleation theory.

Laser-induced cavitation bubbles and shock waves in water near a concave surface

The interplay among the cavitation structures and the shock waves following a nanosecond laser breakdown in water in the vicinity of a concave surface was visualized with high-speed shadowgraphy and schlieren cinematography. Unlike the generation of the main cavitation bubble near a flat or a convex surface, the concave surface refocuses the emitted shock waves and causes secondary cavitation near the acoustic focus which is most pronounced when triggered by the shock wave released during the first main bubble collapse. The shock wave propagation, reflection from the concave surface and its scattering on the dominant cavity is clearly resolvable on the shadowgraphs. The schlieren approach revealed the pressure build up in the last stage of the collapse and the first stage of the rebound. A persistent low-density watermark is left behind the first collapse. The observed effects are important wherever cavities collapse near indented surfaces, such as in cavitation peening, cavitation erosion and ophthalmology.

A smoothed particle hydrodynamics study of the collapse for a cylindrical cavity

In this study, we propose a mesh-free (particle-based) Smoothed Particle Hydrodynamics model for simulating a Rayleigh collapse. Both empty and gas cavities are investigates and the role of heat diffusion is also accounted for. The system behaves very differently according to the ratio between the characteristic time of collapse and the characteristic time of thermal diffusion. This study identifies five different possible behaviours that range from isothermal to adiabatic.

Experimental investigations of the particle motions induced by a laser-generated cavitation bubble

The interactions between a laser-generated cavitation bubble and a spherical particle are investigated experimentally with the aid of high-speed camera. Both the cavitation bubble dynamics and its induced particle moving dynamics are clearly recorded and analyzed qualitatively and quantitatively. Influences of two essential parameters (including the bubble-particle distance and the particle/bubble radius ratio) on the phenomenon are given and discussed. Furthermore, the underlying physical mechanisms are discussed based on the calculations of the radiation pressure and the generations of the micro-jet. Our results reveal that the distance between the cavitation bubble and the particle shows significant influences on the phenomenon. For different radius ratios, the maximum particle displacement varies especially for the small stand-off distance.

Motion of a Free-Settling Spherical Particle Driven by a Laser-Induced Bubble

We document experimentally four different interactions of a laser-induced bubble and a free-settling particle, with different combinations of the geometric and physical parameters of the system. Our force balance model shows that four nondimensional factors involving the particle radius a, the maximum bubble radius R_{max}, the initial separation distance l_{0} between the particle center and the bubble center, the fluid viscosity μ_{f}, and the particle and fluid densities ρ_{p} and ρ_{f}, respectively, in detail l_{0}/R_{max}, a/R_{max}, ρ_{p}/ρ_{f}, and μ^{*}=μ_{f}T_{c}/ρ_{f}R_{max}^{2}, where T_{c}=0.915R_{max}sqrt[ρ_{f}/(p_{∞}-p_{v})], influence the particle-bubble dynamics, and reasonably predict the maximum particle velocity and the limiting condition when the particle starts to "bounce off" the bubble during bubble growth. In particular, we also discover the high-speed ejection of the particle, and a cavity behind the particle, in cases when initially the particle is in very close proximity to the bubble. These observations offer new insights into the causal mechanism for the enhanced cavitation erosion in silt-laden water.

Particle motion induced by bubble cavitation

Cavitation bubbles induce impulsive forces on surrounding substrates, particles, or surfaces. Even though cavitation is a traditional topic in fluid mechanics, current understanding and studies do not capture the effect of cavitation on suspended objects in fluids. In the present work, the dynamics of a spherical particle due to a cavitation bubble is experimentally characterized and compared with an analytical model. Three phases are observed: the growth of the bubble where the particle is pushed away, its collapse where the particle approaches the bubble, and a longer time scale postcollapse where the particle continues to move toward the collapsed bubble. The particle motion in the longer time scale presumably results from the asymmetric cavitation evolution at an earlier time. Our theory considering the asymmetric bubble dynamics shows that the particle velocity strongly depends on the distance from the bubble as an inverse-fourth-power law, which is in good agreement with our experimentation. This study sheds light on how small free particles respond to cavitation bubbles in fluids.