Ph.D. DEFENSE COMMITTEE

Advisor:  Professor William Roberts

Chair:  Professor Cafer Yavuz

External Committee Member:  Professor Claus-Dieter Ohl

Internal Committee Member:  Professor Tadd Truscott

Abstract

Cavitation induced by ultrasound and acoustic waves is a physical phenomenon widely used nowadays in many sectors, from sonochemistry and process intensification to biomedical applications like histotripsy. Given their wide adoption, acoustic transducers and ultrasonic reactors operate at various frequencies and on different fluid mixtures. However, considering the large number of variables, the optimal combination of parameters for a specific application is often unknown. The present dissertation aims to clarify some aspects of acoustic cavitation phenomena, which are crucial for optimizing the design of transducers and ultrasonic reactors. One key parameter required to calibrate and operate an acoustic system is the cavitation threshold. The thesis presents a study on cavitation inception induced by the propagation of 24 kHz acoustic waves in water. A rigorous and repeatable methodology, based on high-speed imaging and hydrophone measurements, is introduced to establish the cavitation onset in acoustic systems. The tensile strength of the liquid is estimated by applying the Rayleigh analytical solution to the data extracted from the high-speed sequence. From the results, the frequency arises as the most relevant parameter determining the inception of cavitation bubbles in a continuous medium. This introduces the need for a novel cavitation number. The framework was then extended, through an acoustic analogy, to a case of impulsive cavitation. Cavitation in different fluids was also investigated to confirm the findings. Despite their different physical properties, all the liquid media exhibit a similar behavior in terms of the cavitation threshold and expansion velocity of the nucleated bubble. This sheds light on the nucleation mechanism that has originated cavitation in the lab-scale acoustic system. The extent of cavitation leveraging sonochemistry was also explored. The dependency between the dynamics of the vapor field structures and the hydroxyl radicals was analyzed through coumarin dosimetry and high-speed imaging. Finally, experiments were conducted with a calibrated Schlieren technique to study fluid dynamic effects. Thanks to calibration and tomographic reconstruction procedures, the pressure fields induced by acoustic and shock waves were quantitatively estimated. All the methodologies introduced can be applied to study phenomena at other frequencies. 

Biography

Gianmaria Viciconte obtained bachelor’s and master’s degrees from the University of Florence, Italy. After graduating in 2019, he joined as a researcher the REVIP (Reverse Engineering and Virtual Prototyping) Laboratory at the Department of Industrial Engineering of the University of Florence. In 2020, he moved to Saudi Arabia to start his doctoral studies. He is currently a Ph.D. candidate in Mechanical Engineering in the PSE division at King Abdullah University of Science and Technology (KAUST), conducting research activity in the group of Professor William L. Roberts. His research interests mainly focus on the fundamental aspect of cavitation induced by acoustic waves, sonochemistry, and ultrasonic reactor design optimization. 

Zoom link:  https://kaust.zoom.us/j/94913102689