Investigation of friction drag change by bubbles using double optic fiber probe

Abstract

The shape of double optical fiber probe and analysis method for measuring bubbles near wall in turbulent boundary layer are proposed. The spanwise position of the probe is made to coincide, and it is tilted upward in order to enable the measurement of small bubbles and the measurement near wall. The measured data are filtered using a histogram, velocity ratio at the upstream and downstream sides of the bubble, and velocity gradient to eliminate the effects of bubble mismatching and deformation. The effectiveness of this method is confirmed in comparison with the result by Shadowgraphy. The bubble exists only near the wall and the bubble interface velocity near the wall is high when the friction drag is increased. The bubble interface velocity near the center of the channel is accelerated with increasing void fraction when the friction drag is reduced. The acceleration rate is on the same order as the rate of change of the drag reduction effect.