Droplet generation by the oscillation of two spark-generated bubbles near a confined opening

Cavitation bubbles can be both beneficial and detrimental. One of their beneficial applications is to produce droplets as they oscillate near a confined free surface. In this paper, the oscillation of two spark-generated bubbles beneath a confined free surface and the resultant droplet generation are studied numerically using boundary element method. Three different configurations are considered: (i) a vertical cylinder (theta = 0 degrees), (ii) a vertical nozzle (theta = 45 degrees) and (iii) a perforated horizontal flat plate (theta = 90 degrees). The influences of the effective parameters, including the initial pressure and locations of the bubbles, and the configuration geometry on the dynamics of the bubbles, the free surface and the resultant droplet are studied. It was found that by decreasing the inter-bubble distance, the droplet size increases and its pinch-off occurs earlier. In general, the effect of the distance of the upper bubble from the free surface on the droplet behavior is much more significant than that of the inter-bubble distance. Furthermore, through the augmentation of the upper bubble's strength parameter, the droplet size is reduced and the moment of droplet pinch-off is postponed. Finally, the droplet size and its pinchoff time increase from the vertical cylinder to the flat plate case.