Study of Cavitation Bubble Collapse near a Wall by the Modified Lattice Boltzmann Method

Experimental study of temporal dynamics of cavitation bubbles selectively attached to the solid surfaces of different hydrophobicity under the action of ultrasound

In this work, we experimentally investigated the dynamics of vapor-gas bubbles arising in distilled water under the action of ultrasound (US), near and on the surface of solid plates with various surface properties. In the experiments, we used the plates made of Teflon, acrylic glass, and amorphous quartz, with various hydrophobic properties (contact angle). The experiments showed a significant effect of surface properties on the dynamics of bubbles oscillating near and on a solid surface under the influence of ultrasound. In the case of a hydrophobic surface (Teflon), steady attachment of bubbles is observed, the surface area covered by the bubbles grows according to a law close to linear, and then it reaches a plateau. For less hydrophobic surfaces, the drift and rising of bubbles along the plates are observed, as a result of which, the area covered by the bubbles grows less rapidly over time. When the ultrasound is switched off some bubbles located near and on the surface of the acrylic plate float and drag other bubbles with them, differ from the surface of Teflon. This behavior of the bubbles limits both their maximum possible diameter and the maximum solid surface area covered by the bubble. In addition, experiments showed a significant effect of the concentration of gas dissolved in a liquid on the process of bubble formation: a decrease in gas concentration led to a qualitative change in the time dependence of the surface area covered by the bubbles; in the case of long-term degassing of water using ultrasound, the formation of extended bubble clusters on all solid surfaces becomes impossible.

Simulation of collapsing cavitation bubbles in various liquids by lattice Boltzmann model coupled with the Redlich-Kwong-Soave equation of state

A computational technique based on the pseudo-potential multiphase lattice Boltzmann method (LBM) is employed to investigate the collapse dynamics of cavitation bubbles of various liquids in the vicinity of the solid surface with different wettability conditions. The Redlich-Kwong-Soave equation of state (EoS) that includes an acentric factor is incorporated to consider the physical properties of water (H_{2}O), liquid nitrogen (LN_{2}), and liquid hydrogen (LH_{2}) in the present simulations. Accuracy and performance of the present multiphase LBM are examined by simulation of the homogenous and heterogeneous cavitation phenomena. The good agreement of the results obtained based on the present solution algorithm in comparison with the available data confirms the validity and capability of the multiphase LBM employed. Then, the cavitation bubble collapse near the solid wall is studied by considering the H_{2}O, LN_{2}, and LH_{2} fluids, and the wettability effect of the surface on the collapse dynamics is investigated. The obtained results demonstrate that the collapse phenomenon for the H_{2}O is more aggressive than that of the LH_{2} and LN_{2}. The cavitation bubble of the water has a shorter collapse time with an intense liquid jet, while the collapse process in the LN_{2} takes a longer time due to the larger radius of its bubble at the rebound. Also, this study demonstrates that the increment of the hydrophobicity of the wall causes less energy absorption by the solid surface from the liquid phase around the bubble that leads to form a liquid jet with higher kinetic energy. Therefore, the bubble collapse process occurs more quickly for hydrophobic surfaces, regardless of the fluids considered. The present study shows that the pseudopotential LBM with incorporating an appropriate EoS and a robust forcing scheme is an efficient numerical technique for simulation of the dynamics of the cavitation bubble collapse in different fluids.

Study on the Collapse Process of Cavitation Bubbles Near the Concave Wall by Lattice Boltzmann Method Pseudo-Potential Model

In this paper, the lattice Boltzmann pseudo-potential model coupled the Carnahan–Starling (C-S) equation of state and Li’s force scheme are used to study the collapse process of cavitation bubbles near the concave wall. It mainly includes the collapse process of the single and double cavitation bubbles in the near-wall region. Studies have shown that the collapse velocity of a single cavitation bubble becomes slower as the additional pressure reduces, and the velocity of the micro-jet also decreases accordingly. Moreover, the second collapse of the cavitation bubble cannot be found if the additional pressure reduces further. When the cavitation bubble is located in different angles with vertical direction, its collapse direction is always perpendicular to the wall. If the double cavitation bubbles are arranged vertically, the collapse process of the upper bubble will be quicker, as the relative distance increases. When the relative distance between the bubbles is large enough, no second collapse can be found for the upper bubble. On the other hand, when two cavitation bubbles are in the horizontal arrangement, the suppression effect between cavitation bubbles decreases as the relative distance between the bubbles increases and the collapse position of cavitation bubbles moves from the lower part to the upper part.

Study on Bubble Cavitation in Liquids for Bubbles Arranged in a Columnar Bubble Group

In liquids, bubbles usually exist in the form of bubble groups. Due to their interaction with other bubbles, the resonance frequency of bubbles decreases. In this paper, the resonance frequency of bubbles in a columnar bubble group is obtained by linear simplification of the bubbles’ dynamic equation. The correction coefficient between the resonance frequency of the bubbles in the columnar bubble group and the Minnaert frequency of a single bubble is given. The results show that the resonance frequency of bubbles in the bubble group is affected by many parameters such as the initial radius of bubbles, the number of bubbles in the bubble group, and the distance between bubbles. The initial radius of the bubbles and the distance between bubbles are found to have more significant influence on the resonance frequency of the bubbles. When the distance between bubbles increases to 20 times the bubbles’ initial radius, the coupling effect between bubbles can be ignored, and after that the bubbles’ resonance frequency in the bubble group tends to the resonance frequency of a single bubble’s resonance frequency. Fluent software is used to simulate the bubble growth, shrinkage, and collapse of five and seven bubbles under an ultrasonic field. The simulation results show that when the bubble breaks, the two bubbles at the outer field first begin to break and form a micro-jet along the axis line of the bubbles. Our methods and conclusions will provide a reference for further simulations and indicate the significance of the prevention or utilization of cavitation.