Visualization of the flow around a bubble moving in a low viscosity liquid

Abstract

A new technique to visualize the flow around a bubble rising in a low viscosity fluid is presented. With this technique it is possible to observe streak lines of the flow as well as the shape and position of the bubble. The visualization of the streak lines is obtained by open-diaphragm photography of laser-sheet illuminated micro-tracers. The shape and position of the bubble is obtained, in the same photo plate, by simultaneously illuminating the flow with a stroboscopic light. The experiments were performed in a closed acrilic tank of $50\times50\times50 \mbox{ cm}^3$, in which bubbles were injected using a capillary tube. Filtered water was used as the working fluid. Pure Nitrogen was used to form the bubbles. Experimental results were obtained for a range of bubble sizes. The size of the injected bubbles was controlled by a fixed volume switch valve. We have identified a change of the bubble trajectory, from rectilinear to zig-zaging, as its volume increases, in accordance with previously reported studies. A characteristic change of the velocity field around the bubble is observed when the trajectory instability appears. We conclude that the point of inflection in the velocity-volume plot is directly related to the appearance of the trajectory instability.