We focus on studying laser-induced cavitation under widely different physical conditions, from superheated jets to superfluid liquid helium. We use ultra high-speed video imaging to track cavitation bubble dynamics at frame-rates of up to 7 million frames-per-second. Cavitation is induced by focusing a 532 nm pulsed Nd-YAG laser at a spot with a minimum spot size of 150 μm and pulse duration of six ns, which forms high-pressure plasma, leading to a rapidly expanding bubble/void, which subsequently collapses. We mainly study two configurations: (1) laser-induced cavitation in liquid helium inside an optical cryostat and (2) laser-induced cavitation in HCP solid helium. Moreover, we report preliminary results of two promising studies: (3) laser-induced cavitation inside a highly turbulent flow within a square channel and (4) laser-induced break-up inside a cylindrical liquid jet, leading to its atomization. (1) Inside the liquid helium-4, we reach widely different thermodynamic conditions when adjusting the temperature between 1.4 to 5.1 degrees Kelvin. Below the lambda point at T = 2.17 K, the liquid is superfluid, with viscosity appr zero, while above this temperature, regular liquid helium approaches the critical point at ≃ 5.1 K. This greatly changes the cavitation dynamics with different amounts of vapor appearing during cavity growth and collapse, and revealed four regimes of cavitation bubble behavior. We also measure shock velocities and analyze their characteristics. (2) At pressures of roughly 25 atmospheres, superfluid helium (He-II) solidifies. With wavy time-evolving oscillations on its surface when disturbed, the interface between the solid and the superfluid exhibits fascinating behavior with wavy time-evolving oscillations on its surface when disturbed. The interface between liquid and solid can consequently behave similarly to a free surface. Here, we experimentally investigate laser-induced interfacial dynamics at temperatures between 1.2 K and 2 K and at pressures ranging from the melting pressure of approximately 25 atm to a maximum of 39 atm, which covers both the HCP and BCC structure of the solid, using ultra-high-speed imaging at frame rates up to 7 million frames per second. (3) The cavitation inside the turbulent flow; this study aims to investigate the mutual effect of the rapid straining outside the bubble on the coherent vortices within the liquid and the feedback from the modified turbulence on the shape of the vapor cavity, and to include time-resolved particle image velocimetry. (4) The cavitation inside a liquid jet helps break it up into fine spray, which is of interest for injectors in combustion engines.
|Date of Award||Aug 2023|
|Original language||English (US)|
- Physical Sciences and Engineering
|Supervisor||Sigurdur Thoroddsen (Supervisor)|
- solid Helium
- laser-induced cavitation.