Investigation of the bubble dynamics beneath a free surface within confined spaces

The present paper investigates the bubble dynamics beneath a free surface within confined spaces. The bubble dynamics are analyzed utilizing the high-speed photography system and the Kelvin impulse theory. Specifically, the high-speed photography is employed to observe the bubble collapse behaviors, exploring the influence factor of the bubble deformation and the collapse jet. Based on the Kelvin impulse theory, the characteristics of the flow field and the bubble Kelvin impulse are investigated, revealing the interaction mechanisms between the bubble and the free surface. The present research provides an essential reference for the microfluidic control, and the main conclusions are summarized as follows: (1) Two distinct bubble collapse jet morphologies are identified: angular shape and arc shape. The jet morphologies transition from arc shape to angular shape as the bubble-free surface distance decreases and the confined space height increases. (2) During the oscillation process, the bubble first moves to the free surface for a short distance, then accelerates away from it, which can be effectively predicted by the Kelvin impulse and the Bjerknes force acting on the bubble. (3) Among the factors affecting the liquid velocity, the contribution of the bubble accounts for the largest proportion (exceeding 60%), followed by the contributions of the free surfaces and the bubble volume oscillation.