Experimental and numerical investigations on cavitation bubble dynamics near a solid boundary

• Main Author: Yang, Yuanxiang
• Other Authors: Tan Soon Keat
• Format: Theses and Dissertations
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Civil engineering::Water resources; DRNTU::Engineering::Environmental engineering::Water treatment; DRNTU::Engineering::Environmental engineering::Water supply
• Online Access: https://hdl.handle.net/10356/59113
• Institution: Nanyang Technological University

Cavitation bubble dynamics near a solid boundary have been studied for quite a long time, primarily in the field of hydrodynamic science, to explain the cavitation corrosion and damage observed on ship propellers and hydraulic machines. Recently, however, there has been increasing interest in applying acoustic cavitation in certain other fields, like ultrasonic cleaning, sonochemistry, shockwave lithotripsy, and sonoporation. Although the occurrence areas of these cavitation bubbles are quite different (depending on the specific field), the physical fundamentals of these processes are more or less the same. This study attempts to elucidate the corrosion/cleaning mechanism of cavitation bubbles through investigations on a single cavitation bubble dynamics near a solid boundary. The experiments on laser-induced cavitation bubble dynamics near a solid boundary were first presented. A single cavitation bubble was generated using a focused Q-switched Nd: YAG laser pulse. The bubble’s maximum expanded volume was dominated by the energy of the incident laser pulse. The inception position of the single cavitation bubble had an important influence on the bubble dynamics, which was finely controlled using a micro-translation stage. Direct observations of the bubble evolutions using a high-speed video camera revealed detailed deformations of the cavitation bubble near the solid boundary: the non-spherical bubble generation; the nearly spherical shape during the first expansion phase; the formation of a liquid jet resulting from the asymmetry of the flow field; the counterjet only appearing in a certain range of stand-off distances; the various profiles after the jet impact, etc. Indirect detections of the acoustic signals released by the oscillating bubbles, using a hydrophone system, provided a much more accurate estimation of the bubble oscillation periods than those obtained from the bubble videos captured using the high-speed camera. The statistics of the non-dimensional bubble oscillation periods could be used to roughly indicate the bubble inception positions when the maximum expanded bubble radii were known. Temporal evolutions of the bubble size were measured in this study using a novel method involving the application of AutoCAD. The energy deposited in the bubble was estimated using the maximum expanded volume after the jet impact. Following this, numerical simulation works were conducted to show some detailed bubble dynamics which could not be observed using the experimental methods. A mathematical model based on the potential flow theory was used to describe the proposed problem. An approximate perturbation method was also developed using the method of matched asymptotic expansions to include the influence of the compressibility of the liquid. It was concluded that the velocity potential near the bubble surface satisfied Laplace’s equation. A dimensionality reduction of the initial 3D potential problem to a final 1D solution was made in a cylindrical polar coordinate. Special treatments on the solid boundary and toroidal bubble were taken carefully. A numerical model based on the mixed Eulerian-Lagrangian (MEL) method and the boundary integral method (BIM) for bubble dynamics in a weakly compressible liquid near a solid boundary was created. Finally, the numerical results calculated using the MEL-BIM model were verified using available analytical results and previous numerical results. Reliable comparisons were made between the numerical results and the experimental results obtained in this study, including those for the non-spherical bubble shape, the evolution of the equivalent bubble radius, and the movement of the bubble centroid. An important advantage of the numerical method was the ability to calculate the flow field dynamics near the bubble surface. The velocity vectors and pressure contours around the bubble surface could be simulated under a fixed grid across the liquid field. The jet impact dynamics and the induced hammer pressure that impinged on the solid boundary were calculated and proved using previous experimental measurements. The formation of the counterjet was also elucidated using the numerical simulations.