Experimental investigation on a large deformable bubble motion rising in the vicinity of the vertical wall

Abstract

In the present study, the freely rising bubble behaviour near the vertical wall has been measured and analyzed while varying the wall configurations such as the initial distance between the bubble center and the wall and the wall material. The bubble Reynolds and Weber numbers are about 1100 and 4.4, respectively, based on the equivalent bubble diameter and these are relatively high values compared to those in the previous studies. The current bubble which has wobbling shape originally rises with a zig-zag trajectory indicating the two-dimensional movements. The initial distance between the bubble and the wall, and the surface boundary condition of the wall (no-slip, hydrophobic and porous, etc.) are considered to investigate the wall effect on the bubble motion. Though the bubble rises in the vicinity of the wall, the maintenance of two-dimensionality of the bubble rising path is confirmed while it is varied widely as bouncing, sliding and departing. In rising bubble motion, especially, the surface boundary condition significantly affects the bubble behaviour. So, first we analyze the rebounding motion of the bubble fitting with the sinusoidal function and apply the relation between the energy components with the bubble motion. The energy dissipation shows different trends according to the initial distance and wall boundary conditions. The hydrophobic condition causes larger amount of energy dissipation than no-slip condition when the bubble slides on the wall and as s/req increases (non-colliding motion appears), the dissipation has rather smaller value compared to no-slip condition. However, for the porous wall, the energy dissipation value is smallest among all conditions. Also, to deeply investigate the energy dissipation, the coefficient of restitution is calculated showing a good correlation with the variation of the energy components.