Experimental observations of the behaviour of a bubble inside a circular rigid tube

This paper is concerned with the dynamics of a spark-generated bubble inside a long, rigid, circular tube with two open ends submerged horizontally in a tank filled with water. The behaviour of the bubble was found to be sensitive to two geometrical parameters: the dimensionless tube radius α=R _T/R _Bmax and the dimensionless eccentricity ε=E/R _Bmax, where R _T is the inner radius of the tube, E is the distance from the bubble center at inception to the axis of symmetry of the tube, and R _Bmax is the maximum equivalent bubble radius (∼10 mm). The expansion, collapse and rebound of the bubble were captured using a high-speed camera both for the case where α > 1 and α < 1, with and without eccentricity, respectively. Some new features of the bubble dynamics were observed. In particular, a bubble initiated with eccentricity in a tube for which α < 1 was seen to migrate to the distal part of the tube at the end of collapse with formation of a jet also in that direction. This is distinct from the case of a bubble collapsing near a flat surface. A similar phenomenon has been observed previously in the case of a microbubble collapsing in a blood vessel under ultrasound excitation, but was attributed to the elasticity of the vessel wall. The present study suggests that it may in fact be due simply to the geometry of the system. A cloud of microbubbles was observed shortly after the start of rebound of the bubble. Our analysis shows that the microbubbles should be generated from nuclei in tap water with radii in the range of 10 ⁻⁸–10 ⁻⁶ m.