Measurement and modeling of surface erosion in acoustic cavitation

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The topic of the project is acoustic cavitation: Intense ultrasonic fields in liquids generate and activate bubbles that undergo strong volume oscillations. The most relevant effect for applications is the strong collapse (implosion) of bubbles where shock waves, chemistry, and luminescence can occur. A key phenomenon is the erosion of any solid material by cavitation. It is linked with the intense conditions during the strong bubble collapse when it appears close to objects, i.e. high pressure and temperature peaks directly at the material surface.The project is an extension of the previous work on “Measurement and modeling of acoustic cavitation bubble populations” where cavitation in the bulk liquid in simple geometries was investigated. Now the experimental and numerical techniques will be expanded to realistic application conditions of acoustic cavitation where solid objects are submerged and more complex liquids are employed. Particular target applications are solid surface cleaning and metal erosion in acids for recycling.The experimental part of the project is conducted at Georg-August-University Göttingen, and the numerical work is done at TU Clausthal. The experiments comprise studies on cavitation in the main frequency range from 20 to 50 kHz and in liquids of variable surface tension and of high viscosity, employing high-speed imaging, sound field, luminescence, and erosion measurements. The core of the investigations is concerned with the behavior of cavitation in the presence of solid objects that scale approximately from the acoustic wavelength to 1/15th thereof.For this purpose, artificial bubble sources and controlled bubble interaction with solids are experimentally realized under variation of geometry and liquid parameters. The case of multiple objects is studied in fixed bed and in fluidized bed conditions, and sound wave penetration and scattering, cavitation extension and structures, and bubble activity at the solids are assessed and optimized. The numerical work extends previously developed codes for sound field propagation and dissipation in cavitating liquids. Bubble-bubble and bubble-solid interactions are included to reach more realistic descriptions of acoustic cavitation in the presence of objects. Surface erosion prediction models are further developed and adjusted on basis of single-bubble numerical studies. Simulations are extended to multiple solid objects submerged in the liquid, both in fixed and floating conditions. Numerical and experimental results are continuously compared to ensure that the essentials are captured by the model, aiming at a coherent description of the relevant three-phase systems of liquid, solid and cavitation bubbles.