Three-dimensional PIV measurements of bubble drag and lift coefficients in restricted media

Abstract

A hybrid scheme combining Particle Image Velocimetry and Shadow Image Velocimetry has been used for a full-volume, three-dimensional, transient study of the shape, trajectory and forces acting on air bubbles rising in stagnant tap water in restricted media. The bubble Reynolds number ranged from 400 to 650. The three-dimensional reconstruction of the bubble was accomplished by combining images from two orthogonal views. This reconstruction process allowed for measurement of dimensions, orientation, trajectory, rotation, velocity and acceleration of an individual rising bubble. These parameters were then used to compute drag and lift forces acting on the bubble. Instantaneous values of drag and lift coefficients were then determined. These experimental results were compared to known experimental data and values obtained from correlations found in scientific literature. It was found that correlations intended for determining drag coefficient values should be adequately modified when necessary to account for wall impact, since the drag coefficient magnitude is considerably higher than that predicted by such correlations at Re below 550. Regarding the bubble lift coefficient, instantaneous data scatter noticeably as a function of Re, but average values agree within the range of known data. The major contributors to the uncertainty in this experiment were the capability of accurately reconstructing the 2D shape of the bubbles from distorted and/or incomplete PIV images and determining the bubble centroid. An overall error of 7% was computed for the drag coefficient, but it rises up to 44% for the lift coefficient.