Influence of the free surface and rigid wall on the bubble dynamics within confined spaces

The present paper investigates the influence of the free surface and rigid wall on the bubble dynamics within confined spaces, based on the image method and high-speed photography experiments. A theoretical model of the velocity field and Kelvin impulse, which accounts for the bubble motion, is established. The influences of the rigid wall and the free surface on the Kelvin impulse distribution, the collapse jet development, and the bubble splitting are quantitatively analyzed. Based on the contribution ratio of the Kelvin impulse by the rigid wall and the free surface, the dominant regions occupied by the two boundaries are delineated. The conclusions are given as follows: (1) the jet direction is much more affected by the bubble-free surface distance than by the bubble-wall distance. The region between the rigid wall and free surface can be divided into three domains: the wall domain, the combined domain, and the free surface domain. The free surface dominates in most liquid areas, reaching 60% when its proportion exceeds 2/3. (2) The closer the bubble is to the rigid wall or free surface, the greater the Kelvin impulse it experiences. Furthermore, the distribution of the Kelvin impulse is symmetrical about the angle bisector, and its direction points from the free surface to the rigid wall. (3) The sensitivity of the Kelvin impulse direction is high near the rigid wall, while it is relatively low near the junction of the rigid wall and free surface, indicating that the rigid wall greatly influences the direction of force acting on the bubble.