Motion and drag reduction effect of bubbles in a turbulent boundary layer developed beneath a moving wall

Abstract

We examined the air lubrication by bubble injection with a new experimental system simulating a zero-pressure-gradient turbulent boundary layer beneath a moving belt at the water surface in a lab-scale facility. By varying the air injection rate and wall speed, we estimated the impacts of the injected bubble on turbulence statistics and wall frictional drag. Velocity field measurements showed an increase of mean velocity indicating the reduction of the wall frictional drag, which was qualitatively validated by measurements of the torque on the motor driving the belt. This means that we can estimate the local drag reduction from the mean velocity increment. The Reynolds shear stress was suppressed for the case of the highest wall speed for which a detailed investigation on instantaneous velocity fluctuation is ongoing.