Complex dynamics of cavitation bubbles near objects

• Applicant: Dr. Robert Mettin
• Subject Area: Fluid Mechanics
• Term: since 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 361418507

The project aims to elucidate the mechanisms how imploding bubbles can erode hardest materials and clean surfaces. It is well-known that the strong collapse of bubbles is the relevant process for energy concentration by cavitation, which is the appearance and action of gaseous voids in fast liquid flows or intense ultrasonic fields: Cavitation bubble collapse can lead to strong heating, chemical reactions and plasma inside the bubble, and to severe pressure waves and shocks in the liquid. However, bubble implosions near objects are remarkably complicated events that are not yet completely understood. Collapses can be accompanied by strong bubble deformations, splitting, rapid jet flows through the bubble, vortex generation and shear stresses. All these phenomena can sensitively depend on the geometry of the solid surface and bubble characteristics.In previous work, we have shown numerically that bubbles expanding and collapsing directly at a solid can develop extremely fast jet flows towards the boundary. Such fast jets origin from self-impact of the inrushing bubble wall, and they have been essentially overlooked in the existing literature. Their speed (~1000m/s) exceeds conventional collapse jet flows (~100m/s) by an order of magnitude, implying a high relevance for erosion and cleaning. The first working hypothesis is that these peculiar liquid jets can occur under a variety of conditions. The respective research objective is to explore and characterize further geometries of bubble collapse at objects, particularly with respect to fast jets. The second hypothesis is that for acoustically excited bubbles at a solid surface, equally involved and partly unknown bubble dynamics can be expected. Accordingly, bubbles situated near an object and driven by a sound field are investigated for a variety of acoustic and geometric parameters.To clarify these scientific questions on bubble dynamics, advanced experimental and numerical studies are conducted. Experimental techniques comprise nucleation of individual bubbles by focused laser pulses and high-speed imaging of bubble shape and shock waves. Bubbles are placed near objects of various shapes, and additionally sound fields can be applied. Numerical studies employ the Finite Volume Method that is able to capture shock waves and changes of bubble topology. Experimental and numerical work is complementary in form of validation of the models and computation of details not resolved by the experimental methods. Thus, experiments and numerics are tightly connected in the project.Results on the complex behavior of collapsing and acoustically driven bubbles at objects will lead to a better understanding of the action of cavitation, directly linked to better control and optimization of technical and medical applications.

• DFG Programme: Research Grants
• International Connection: Austria
• Partner Organisation: Fonds zur Förderung der wissenschaftlichen Forschung (FWF)
• Cooperation Partner: Dr. Christiane Lechner