Experimental study of single bubble rising near vertical wall in hele-shaw cell

Abstract

• Experimental investigations on the vertical wall effect on bubble rising in a confined 2D Hele-Shaw cell. • 2D force balance involved added-mass effect to calculate forces acting on the rising bubble. • Clear visualisation of the wake structure in the rear of the rising bubble. A confined single air bubble rising near a vertical wall in a 2D Hele-Shaw cell was investigated experimentally with high-speed imaging. Wall effect was studied by varying the initial distance between the wall and the capillary needle to release the bubble, comparing to a confined-rising bubble without wall. The bubble of diameter of 3.65 mm rises first in a vertical path, then changes to zigzag and migrates away from the wall. Characteristics of the rising bubble were analyzed. Force balance equations considering added-mass effect were proposed to calculate forces on the bubble. With a rheoscopic fluid, clear wake structure is observed. A single threaded wake is formed once released, then a symmetric vortex pair is attached to the bubble. Vortex shedding triggers zigzag and a Kármán vortex street is eventually generated. The presence of the wall provokes earlier zigzagging and complicated wake structure in the near-wall area.