Sliding motion of bubbles in an inclined turbulent channel flow

The sliding motion of a bubble near an inclined channel flow was experimentally investigated to understand the variation of the motion by the inclination in turbulent boundary layers. The inclination of the channel was controlled from 0° to 80°, and Reynolds number of channel flow was 22,000. The qualitative visualization confirmed that there is no significant variation in bubble shape from 0° to 40°, however, bubbles tend to be elongated perpendicular to the wall from 50° resulting from a balance between buoyancy and drag. An optoacoustic measurement technique was adopted, and the optical measurement offers the velocity and diameter of individual bubbles while the ultrasound measurement provides the maximum distance between the wall and bottom of the bubbles with liquid velocity profiles. It was confirmed that the bubble diameter is reduced with the increase of inclination while the bubble height decreases. In addition, the bubble velocity accelerated by the buoyancy, but it slightly increased from 40°. Based on the variables from the optoacoustic measurement, the drag coefficient of the bubble was obtained using a force balance between drag and buoyancy. Finally, we proposed a correlation of drag coefficient using Bond number, Weber number, and ellipticity of bubble.