Wall Shear Rates Induced by a Single Cavitation Bubble Collapse

Abstract

When a cavitation bubble collapses in vicinity to a solid surface, high flow velocities are induced. They involve remarkably high but unsteady wall shear rates. Even though they are crucial in ultrasonic surface cleaning or cavitation erosion, and their knowledge is needed for validation of numerical methods, they have not been measured so far due to experimental difficulties. Here, a wall shear rate raster microscope was developed. It bases on an electrochemical principle and involves a model to solve the appropriate convection-diffusion equation. As wall shear rate sensor, a microelectrode was flush-mount into a solid surface. With this method, wall shear rates on micrometer and microsecond scales can be resolved. The wall shear rates produced during the collapse of a single, laser generated bubble (maximum radius about 400 µm) were measured in planes. Via the synchronously performed high speed imaging, the wall shear rates can be clearly related to details and the different stages of the bubble dynamics. This way the respective impacts of the flow phenomena involved on the generation of wall shear stress were evaluated. For example, the jet that accompanies the bubble collapse was resolved in terms of wall shear rates during its impact on the wall and its subsequent spreading in radial direction. The above experimental data are compared with results of numerical simulations of the collapse of a single bubble obtained with a compressible two-phase flow solver that uses barotropic equations of state.

Numerical simulation of a confined cavitating gas bubble driven by ultrasound

“…Recalling that the simulations are axisymmetric, in three dimensions the maximum shear can be visualised to occur in concentric circular zones (annular rings) whose centre is directly beneath the bubble surface. Recent experimental measurements of wall shear stress and corresponding 3D simulations by Reuter et al 23 for a bubble collapsing near a single rigid wall suggest that this is indeed the case.…”

“…Improved agreement with experimental measurements for validation purposes is of course desirable, but the rapidly fluctuating strong gradients generated in the thin liquid-film between a bubble and nearby boundaries pose significant experimental challenges. Nevertheless, Reuter et al 23 have achieved comparable agreement between experimental measurements and computational simulations in OpenFOAM for an RGC bubble collapsing near a single rigid wall. However, the maximum shear stress reported therein was only ∼ 5 kPa, which still is orders of magnitude lower than that observed in the present configuration.…”

“…An order of magnitude discrepancy is often found between experimental measurements of the shear stress and numerical computations. 27 One drawback of experimental measurements is that, to date, these have been limited to a single point location, 21,23,98 which is restrictive as opposed to the complete spatial and temporal distribution available in simulations. The proposed VOF model is therefore a useful tool that complements experimental investigation and provides insight into the mechanisms underlying bubble-cleaning.…”

“…It is also less than the ∼ 25 kPa computed maximal shear stress reported by Koukouvinis et al 96 who simulated the growth and collapse of a laser-generated bubble. This discrepancy may be due to the fact that Reuter et al 23 only compute the collapse stage of the bubble, assuming that at maximum expansion the bubble is perfectly spherical, which is rarely the case for bubbles expanding in vicinity of a wall. 48,49 Besides, the differing bubble sizes (∼ 400 µm in Reuter et al 23 compared to ∼ 700 µm in Koukouvinis et al 96 ), may also result in a different mechanical action on the surface.…”

“…For instance, the strong shear flow along the boundary is one of the main mechanisms responsible for cleaning, but only a couple of experimental techniques have been successful in measuring its strength. 21,23 On the other hand, using Computational Fluid Dynamics (CFD) simulation it is possible to obtain a more comprehensive understanding of the mechanisms responsible for cleaning since the full details of the flow-field evolution are computed and there are also no limits on the location and number of measurements that can be made.…”