UNSTEADY FORCES ON SPHERICAL BUBBLES

An experimental investigation of freely rising spherical bubbles through a quiescent liquid is presented. The objective of the experiments is to examine the validity of a recently proposed history force expression for clean, spherical bubbles at finite Reynolds number (Mei et al. 1994). Excellent agreement between the measured and predicted bubble trajectory is obtained when using the proposed history force expression; the data presented herein thus provide an indirect validation of the history force expression. The range of Reynolds number and Weber Number based on the terminal velocity are 13 to 212 and 0.03 to 0.69, respectively. The history force expression is only applicable when the quiescent liquid is free of contaminants. Otherwise, both the steady and unsteady forces are difficult to predict because the liquid/vapor interface is partially immobilized.