Oscillation of an ultrasonically driven gas bubble in an asymmetric confined domain

This study numerically investigates the behaviour of an ultrasonically driven gas bubble between two parallel rigid circular walls with a cylindrical micro-indentation in one wall using the OpenFOAM software. The primary objective is to determine the conditions that facilitate the removal of particulate contamination from the indentation using the bubble jet. To accomplish this objective, various values of the indentation diameter D^* were considered, ranging from 0 to 23. It was found that, when D^*<4.9, during the collapse phase, the bubble splits into two smaller sub-bubbles, within each of which a liquid jet emerges pointing towards the adjacent wall. Once the lower sub-bubble transforms into a toroidal shape, the high-speed liquid jet penetrates the indentation. This allows for the effective removal of particulate contamination present within the indentation specifically for cases where D^*<4. On the other hand, the lower sub-bubble completely vanishes when D^*>=4.9, rendering the removal of contamination impossible. It is important to note that for large values of D^*, there may exist situations in which numerical instability arises, resulting in the creation of artificial gas-filled regions within the liquid. We effectively resolve this issue through the proper treatment of the computational grid.