Nonlinear interaction and coalescence features of oscillating bubble pairs: Experimental and numerical study

Nonlinear interaction and coalescence features of oscillating bubble pairs are investigated experimentally and numerically. The spark technique is used to generate in-phase bubble pairs with similar size and the simulation is performed with the compressible volume of fluid (VOF) solver in OpenFOAM. The initial conditions for the simulation are determined from the reference case, where the interbubble distance is sufficiently large and the spherical shape is maintained at the moment of maximum volume. Although the microscopic details of the coalescing behaviors are not focused, the compressible VOF solver reproduces the important features of the experiment and shows good grid convergence. We systematically investigate the effects of the dimensionless interbubble distance gamma (scaled by the maximum bubble radius) and define three different coalescing patterns, namely, coalescence due to the expansion in the first cycle for gamma < 1.1 (Pattern I), bubble breaking up and collapsing together with coalescence at the initial rebounding stage for 1.1 < gamma < 2.0 (Pattern II), and coalescence of the rebounding toroidal bubbles for 2.0 < gamma < 3.65 (Pattern III). For Pattern I, prominent gas flow and velocity fluctuation can be observed in the coalescing region, which may induce the annular protrusion in the middle of the coalesced bubble. For Patterns II and III, migration of the bubbles toward each other during the collapsing and rebounding stages greatly facilitates the bubble coalescence.