Repetitive bubble injection promoting frictional drag reduction in high-speed horizontal turbulent channel flows

Air lubrication adopting repetitive bubble injection was examined in a facility of high-speed turbulent channel flow. The bulk water velocity in the channel was controlled from 5.0 to 7.0 m/s, resulting in a channel Reynolds number Re-m > 10(5) and friction Reynolds number Re-tau > 10(3). Bubbles were injected with gas flow rates periodically fluctuating at frequencies of 1.0-6.0 Hz in the upstream section of the channel. In the downstream of the channel, a periodic pattern of bubble distributions and a periodic fluctuation of wall shear stress were observed at a frequency coinciding to the frequency of bubble injection. Measurements of the wall shear stress show that the ratio of the time-averaged drag reduction was improved by the repetitive bubble injection relative to the conventional method of continuous bubble injection. The largest ratio was obtained at a frequency of 2.0 Hz. Under this condition, the ratio of the drag reduction per unit volume of injected gas was almost twice that for continuous bubble injection, demonstrating the superiority of repetitive bubble injection in terms of the efficiency of drag reduction.