Numerical simulation of fluid dynamics and flow aggressiveness for hydroacoustic cavitation at ultrasonic horns

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In spite of the broad application of ultrasonic horn devices e.g., in chemistry, process engineering, biology or material testing, details of the aggressive cavitating flow field, named as Hydroacoustic Cavitation in what follows, are not understood yet. As an example, the focus is on the indirect method for material testing, where a stationary material specimen is placed opposite to the oscillating horn. This exceedingly aggressive flow leaves cavitation erosion profiles on the specimens, which effectively serve as material sensors and provide topography maps of flow aggressiveness. By a variation of the gap width between oscillating horn and stationary specimen, available measurement data show a characteristic redistribution of flow aggressiveness, which could not be explained by recent CFD approaches. Thus, the project aims at an improvement and validation of 3D flow simulation methods whose application on hydroacoustic cavitation at ultrasonic horn devices should contribute to a better understanding of the flow physics, which is associated to the erosion patterns. Beyond turbulent and thermal effects, the focus is on an extension of the simulation towards the effect of non-condensable air and its damping impact on erosion. Based on a homogeneous mixture ansatz, a hybrid Volume-of-Fluid (VOF) / Euler-Euler-2-Fluid (EE2F) -scheme will be developed which resolves the phase interface if a sufficient spatial resolution is available. For the disperse distribution on the EE2F part of the method, class-based population balance and bubble interaction models are employed. The phase transition is described by a compressible scheme with thermo-dynamical cavitation model which means the assumption of thermal and mechanical equilibrium and the use of an equation of state. The implementation is based on in-house extensions of OpenFOAM. With a phase interface resolving 1D single bubble model, the detailed mass and heat transfer is evaluated for a multiplicity if single bubbles, and for a scale-bridging description of air de- and absorption rates, the resulting rate distribution is passed via source term to the 3D EE2F solver. Beyond hydroacoustic cavitation at ultrasonic horn devices, the method is validated on cavitation-induced air release at an orifice flow, which corresponds to classical hydrodynamic cavitation.