Energy Focusing of Cavitation Bubbles in Flows near Boundaries

• Applicant: Professor Dr. Claus-Dieter Ohl
• Subject Area: Fluid Mechanics ; Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 537676318

Cavitation bubbles are mostly empty bubbles that frequently occur in high-speed liquid flows. After reaching a maximum volume, they shrink in an accelerated fashion. During this so-called collapse, cavitation bubbles focus the kinetic energy of the liquid, giving rise to localized high temperatures and pressures. If this occurs near a rigid structure, i.e. a hydrofoil or the blades of a pump, they may be eroded. Although the ability of cavitation bubbles to focus energy has received a lot of scientific attention, the effect of an external flow on this phenomenon has hardly been investigated. Yet our recent findings point out that slight asymmetries during the collapse, e.g. due to a background flow, can enhance the energy focusing drastically. Although most cavitation occurrences in real-world applications are in the presence of a flow, their effect on the energy focusing and in particular on erosion has not been studied on a single bubble level. In the present proposal, we want to elucidate the importance of flows on bubbles collapsing near rigid boundaries. For this, we expose cavitation bubbles to two kinds of flows: a stagnation point flow induced by a submerged wall jet, and pressure driven shear flows. The shear flows studies are split into a simple planar flow and a radially expanding shear flow. We will utilize high-speed imaging, high bandwidth acoustic measurements, axisymmetric Volume-of-Fluid simulations, and analysis of the eroded volume with confocal laser scanning microscopy. The aim is to understand how a flow affects the energy focusing, thus if certain flows prevent or enhance cavitation erosion. The implications of such findings would be significant. They could guide cavitation erosion models in computational fluid dynamics for more realistic effects of single bubble collapse. Also, the design of cavitation-prone flows such as through narrow constrictions of nozzles, propellers, flow bends, or even for artificial heart valves would benefit from the understanding of how to avoid erosion. More importantly, adding a background flow could help to enhance energy focusing to achieve beneficial effects from cavitation. This could be nanoparticle generation, cavitation peening of materials, or increased efficiency in sonochemical reactors. Thus, we see this proposal as a starting point in a research field that has limited activities so far, but yet has high impact potential.

• DFG Programme: Research Grants