Characteristics and Mechanisms of the Zigzag and Spiral Movement of Rising Bubbles in Still Water

Abstract

When a bubble rises freely in still water, it often moves along a zigzag or spiral trajectory. In order to explore the mechanism of this movement, an experiment was conducted to record the changes in the movement trajectory and bubble shape. The results show that this movement can be explained by the swing of trailing vortices and the change in vorticity. There is asymmetric shedding of the trailing vortices. The change in bubble velocity caused by the shedding of the bubble trailing vortices will lead to an asymmetric change in the vorticity of the trailing vortices. Two factors lead to an asymmetric change in the drag force of the trailing vortices on both sides of the bubble, resulting in the zigzag trajectory. Only when the aspect ratio λ reaches 2.0 will the bubble move along the zigzag. The trailing vortices moving in two orthogonal directions will lead to a spiral trajectory. The movement of the trailing vortices not only changes the trajectory of the bubble but also changes its shape. The effect of the trailing vortices on the bubble can be equivalent to a low-pressure area around a bubble. When a bubble moves along a zigzag trajectory, the low-pressure area at the trailing of a bubble swings back and forth in a plane, and the bubble is flatter. When moving along a spiral trajectory, the low-pressure area rotates around the trailing of the bubble and becomes more spherical. Compared with a zigzag trajectory, a bubble has a higher velocity and lower frequency when moving along a spiral trajectory.

Keywords

• zigzag and spiral trajectory
• the swing of trailing vortices
• the change of vorticity
• oscillation frequency
• low-pressure area