Step response of turbulent structure inside a horizontal bubbly channel flow due to sudden change in void fraction

Abstract

Performance of bubbly drag reduction (BDR) can be improved by repetitive bubble injection (RBI) which provides spatiotemporal fluctuation of local void fraction, termed void waves, to enhance momentum exchange between turbulence and bubbles. The interaction has a variety of time scale as multiphase turbulent flow, and the optimal frequency for RBI has not been clarified to date. We measured temporal change of Reynolds shear stress profile in a horizontal turbulent channel flow when bubble injection was suddenly stopped, i.e., the step response of BDR. The results obtained by particle tracking velocimetry revealed a significant overshoot of the stress before returning to the state of single-phase flow.