Numerical 3D flow simulation of ultrasonic horns with attached cavitation structures and assessment of flow aggressiveness and cavitation erosion sensitive wall zones

Abstract

As a contribution to a better understanding of cavitation erosion mechanisms, a compressible inviscid finite volume flow solver with barotropic homogeneous liquid-vapor mixture cavitation model is applied to ultrasonic horn set-ups with and without stationary specimen, that exhibit attached cavitation at the horn tip. Void collapses and shock waves, which are closely related to cavitation erosion, are resolved. The computational results are compared to hydrophone, shadowgraphy and erosion test data. At the horn tip, vapor volume and topology, subharmonic oscillation frequency as well as the amplitude of propagating pressure waves are in good agreement with experimental data. For the evaluation of flow aggressiveness and the assessment of erosion sensitive wall zones, statistical analyses of wall loads and of the multiplicity of distinct collapses in wall-adjacent flow regions are applied to the horn tip and the stationary specimen. An a posteriori projection of load collectives, i.e. cumulative collapse rate vs. collapse pressure, onto a reference grid eliminates the grid dependency effectively for attached cavitation at the horn tip, whereas a significant grid dependency remains at the stationary specimen. The load collectives show an exponential decrease towards higher collapse pressures. Erosion sensitive wall zones are well predicted for both, horn tip and stationary specimen, and load profiles are in good qualitative agreement with measured topography profiles of eroded duplex stainless steel samples after long-term runs. For the considered amplitude and gap width according to ASTM G32-10 standard, the analysis of load collectives reveals that the distinctive erosive ring shape at the horn tip can be attributed to frequent breakdown and re-development of a small portion of the tip-attached cavity. This partial breakdown of the attached cavity repeats at each driving cycle and is associated with relatively moderate collapse peak pressures, whereas the stationary specimen is rather unfrequently stressed at the end of each subharmonic oscillation cycle by the violent collapse of the complete cavity.

Assessment of Flow Aggressiveness at an Ultrasonic Horn Cavitation Erosion Test Device by PVDF Pressure Measurements and 3D Flow Simulations

“…5 (a): this is in good agreement with practical experience as well as topographic measurements, as reprinted in Fig. 5 (b) from [13], where the erosion depth for the stationary specimen and horn tip is shown.…”

“…Loading of the stationary specimen is dominated by subharmonic collapse, i.e. the collapse of large vapor structures which form over a multiple of the horn driving period [13]. The weak grid dependence of numerical sensor signals presented here is attributed to the large sensor size: while numerical pressure in a single cell strongly depends on grid size, cell-face area weighted integration across a large number of cells to obtain a sensor force diminishes grid dependency.…”

“…A homogenous mixture of liquid and vapor within each cell is assumed. The method has previously been implemented into our in-house code 'Hydrub' and applied to study erosion sensitive wall zones and erosion mechanisms at ultrasonic horns [13]. By counting events exceeding threshold values for pressure and void collapse rate, erosion indicators and a map of dimensionless erosion probability are obtained.…”

“…By counting events exceeding threshold values for pressure and void collapse rate, erosion indicators and a map of dimensionless erosion probability are obtained. Details on the numerical scheme and the assessment of erosion sensitive wall zones by erosion probability can be found in Mottyll and Skoda [13]. Results within the present publication are generated with our in-house OpenFOAM implementation 'HydrubFOAM' [14].…”

“…The grid within the gap and its immediate surrounding is optimized for minimal distortion and volume change. This is a necessary prerequisite for the erosion indicator methodology (as described in [13]). Cell volumes within the entire gap differ by ±12 % from average.…”