Wall-sliding bubbles in inclined turbulent channel flow

The characteristics of sliding bubbles inside turbulent boundary layers on the walls inclined with a range of 5°–80° were experimentally investigated to clarify how the motion of bubbles varies with inclination. Optical visualization provided information on the diameters and velocities of individual bubbles, and the ultrasonic pulsed Doppler method was used to obtain their bottom position and velocity profiles in the liquid-phase flow. The optoacoustic measurements revealed that the interaction between drag and buoyancy affected the variation in bubble shape and motion with changes in inclination. An empirical formula is proposed to predict the ellipticity of bubbles. The drag coefficient is obtained based on a force balance equation, revealing that the drag coefficient depends on the forces acting on bubbles hydrostatically and hydrodynamically. A model equation for the drag coefficient, applicable over broad tilt angles, is suggested using the Weber and Bond numbers of bubbles and the inclination angle.