EXPERIMENTAL STUDY ON THE MOTION, DEFORMATION, AND INDUCED FLOW OF RISING BUBBLES

The rising bubbles with controllable releasing frequency were generated by continuously injecting air into the quiescent water. The motion, deformation, and induced flow of these bubbles were measured in a three-dimensional (3D) manner using shadowgraphy and stereoscopic particle image velocimetry (SPIV). This work investigates the characteristics of the bubble motions and the interface oscillations based on the shadowgraphy data. The bubble images from SPIV are employed to reconstruct their three-dimensional geometries via a newly proposed method. The bubble motions, the spatial distributions of bubbles, and the bubble-induced flows are compared for different airflow rates. The conditional average method is employed to statistically extract the fluctuating flow field induced by a single bubble. It is found that the typical shape of the bubble is approximately ellipsoidal, and its aspect ratio exhibits periodic oscillation, with the dominant frequency in good agreement with the theoretical prediction. When the airflow rate is high, the trajectory of the bubbles exhibits stronger randomness, and the distribution within the horizontal cross-section tends to be isotropic. As the height increases, both the rising speed of the bubble and the induced flow velocity present a temporary rapid increase before stabilizing. Within the range of the airflow rate considered in this experiment, the inducing effect of each bubble is less sensitive to the airflow rate. Accordingly, the bubble-induced turbulence could be viewed as the superposition of the induced flows of individual bubbles.