Experimental study on the flow field inside soap bubble at different incident velocities

Bubbles are widely present in nature. However, previous scientific studies have primarily focused on the development of the outer contour of the bubble while neglecting the changing behavior of the internal flow field due to the difficulty in implementing experiments. This study designs a simple experimental device that can conveniently observe changes in the flow field inside the bubble while avoiding the tedious operation and high costs associated with the particle image velocimetry (PIV) system. Accordingly, this experiment investigates the development process of the flow field inside the bubble and the velocity conditions required for bubble formation for different incident velocities and Reynolds numbers. The study first examines the minimum flow velocity necessary for bubble formation. Then, under low-speed conditions, the flow inside the straw is laminar, and the flow field inside the bubble exhibits a single vortex structure. Under high-speed conditions, the flow inside the straw transitions to turbulent flow, and the flow field inside the bubble exhibits a four-vortex structure. The formation process of this four-vortex structure shows variations as the flow velocity increases. In addition, this study proposes corresponding physical models for bubble formation under low and high flow velocities and verifies the models.