Fluid/Material Coupled Numerical Analysis of Single Bubble Collapse Near a Pit on a Wall

Abstract: In order to elucidate the progression mechanism of cavitation erosion, the behaviors of a single cavitation bubble collapse near a pit on a wall and both the resulting pressure wave in fluid and stress wave in material are investigated in detail. To find out the mechanism of cavitation erosion, many experimental studies on the bubble collapse behavior near a flat rigid wall and the resulting material damage have been conducted so far. A lot of numerical studies using only fluid analysis have been also carried out. In recent years, a few studies on the bubble collapse near a more complex geometry were made and it is reported that more complex geometry has an effect on the bubble collapse behavior, jet formation and subsequent wave dynamics. It is, however, very challenging to introduce a material analysis and investigate detailed stress wave propagation in the material and its effect on the material damage i.e. cavitation erosion. This study tackles this problem using an in-house fluid/material two-way coupled numerical analysis method which considers reflection and transmission of plane waves with acoustic impedance at the fluid/material boundary. In the fluid domain, the locally homogeneous model of compressible gas-liquid two-phase medium is used for capturing the gas-liquid interface. The compressibility of two-phase flow is also considered in this model so that the propagation of pressure wave can be also be taken into account. The governing equations are the 3D compressible gas-liquid two-phase Navier-Stokes equations. In the material domain, the governing equations are composed of the motion equations and the time-differential constitutive equations assuming that the material is a homogeneous isotropic elastic medium, which can simulate the stress wave propagation in the material. Results show that the stress waves are concentrated at the bottom of the pit regardless of the initial bubble position. It is also found that the surface pressure in the fluid side does not necessarily correlate with the stresses in the material, suggesting the importance of material analysis. Moreover, under high pressure conditions, a rapid bubble collapse causes a gas phase generation at the bottom of the pit and its gas phase is contracted and collapsed by the pressure wave, which leads to pressure and stress peaks at the bottom of the pit. Furthermore, through the study of the effect of initial bubble position on its collapse behavior, it is confirmed that, when the initial bubble position is shifted horizontally, bubble collapses asymmetrically and the pressure waves tend to be directed away from a pit. This research numerically reveals that a single bubble collapse near a pit on a wall results in high strain energy concentration at the bottom of the pit, which gives rise to deeper erosion progression at the bottom of the pit.