Reversal of lift on a bubble in a linear shear flow under low Morton number condition*

In a linear shear flow, bubble changes its lateral moving direction depending on Morton number and Reynolds number. Such a phenomenon has been known as ‘reversal of lift.’ Several correlations on the lift have been made by experimental or numerical methods. However, current data in the literature for low Morton number condition, such as atmospheric air-water, are lacking or show large discrepancies. To fill the gap, we present experimental measurement results on the lift on a bubble in a linear shear flow made by special vanes in the water channel. Observing aspect ratios and rising velocities of tested bubbles, it is confirmed that these are fully contaminated. Our data on the time-averaged lift coefficient show relatively large standard deviation than that of the more viscous case, though, the sample means of them present the consistent tendency to these of clean bubbles of various Morton number conditions. The physical modeling of the phenomenon is discussed with the detailed analysis of the oscillatory motion and shape of bubbles.