Laser-induced cavitation near a rigid boundary in a sound field

• Applicant: Professor Dr. Claus-Dieter Ohl
• Subject Area: Fluid Mechanics
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 428644013

Cavitation denotes the formation of bubbles far from their equilibrium size, i.e., they are expanded to a much larger size than their rest radius. This can be achieved by either depositing energy in the liquid that explosively expands a void in the liquid (termed cavitation nucleation by energy deposition), or by supplying a low pressure or a tension such that a bubble expands from its equilibrium radius to a considerably larger size (termed cavitation by tension). For cavitation bubbles generated by either method, once reaching their maximum size they shrink in an accelerated fashion and focus energy from the liquid thereby compressing the bubble content to enormous pressures and temperatures, initiating chemical reactions, emitting shock waves, accelerating liquid jets, and eroding any material nearby. The advantage of cavitation through energy deposition is the excellent control of later bubble dynamics. Meanwhile, tension can be more easily achieved, for example, in an ultrasound field. The present proposal is a continuation of the successful DFG-funded previous project ending in September 2023. In that project, a solver was developed that accounts for the fluid-structure interaction between a collapsing or expanding cavitation bubble and a linearly elastic solid using direct numerical simulations. The solver is embedded in the OpenFOAM framework and uses the Volume-of-Fluid method. Next, we want to utilize this solver together with straightforward experiments to unravel the non-spherical cavitation bubble dynamics in an ultrasound field. Our focus lies in the interaction of the cavitation bubble with the sound field in the presence of a single nearby boundary as well as in a confined liquid filled gap. The cavitation bubble is generated with a laser pulse in the sound field of an acoustic horn (sonotrode). The project is split into three work packages. First we want to understand the importance of phase, pressure, and position of the bubble in a standing wave sound field on the non-spherical bubble dynamics. Then, the bubble will be generated close to a boundary where the attractive force of the boundary competes with the acoustic pressure gradient. This will lead to a complex boundary layer flow that we will resolve. The third part of the proposal studies thin gaps where the bubble dynamics are confined. Significant erosion is observed in thin gaps, and the aim of the third part is to understand the kind of bubble dynamics that cause it. All three work packages include experiments and simulations. The validated simulations will provide details about the forces and fluid flows that are difficult to resolve in the experiments.

• DFG Programme: Research Grants