3D flow simulations and pressure measurements for the evaluation of cavitation dynamics and flow aggressiveness in ultrasonic erosion devices with varying gap widths.

Cavitation induced wall loads at an ultrasonic horn facility are analyzed by 3D flow simulations and temporally high-resolved pressure measurements for varying gap widths between horn and stationary erosion specimen. Piezoelectric polyvinylidene fluoride (PVDF) probes are placed at different radial wall positions at the stationary specimen opposite of the oscillation horn and yield a declining flow aggressiveness with increasing radial position and gap width. The measurement results are reproduced by virtual probes in CFD simulations. Pressure measurement results yield a measure of flow aggressiveness in terms of wall load collectives that correlate well with incubation times obtained by erosion tests. A maximum aggressiveness at the specimen at 0.5mm gap width is obtained. Subharmonic frequencies associated with horn-attached void cavities increase with gap width which is well captured by the simulation. Due to the revealing of 3D flow patterns by the validated CFD results, detailed flow mechanisms associated with flow aggressiveness are discussed. The subharmonic frequency characteristics vs. gap width is associated with the shielding of the inner attached cavity region for small gaps and prevents the cavity from subharmonic collapse for several horn cycles. This shielding is less pronounced for larger gaps and leads to a shorter life time of the attached cavity and therefore to higher subharmonic frequencies.