Drag reduction promoted by repetitive bubble injection in turbulent channel flows

Abstract

Amount of drag reduction by bubbles depends on the void fraction provided in boundary layers. It means that certain power must be consumed to generate bubbles in water and it worsens the total power-saving performance. To obtain sufficient drag reduction with lower amount of the bubbles, RBI (Repetitive Bubble Injection) is suggested. In order to estimate effect of RBI, the wall shear stress and velocity vector fields around bubble cluster generated by RBI in a horizontal channel flow are measured. This multiple measurement leads to deeper understanding of bubble-originated drag reduction phenomena, in particular for unsteady process of boundary layer alternation. In the present experiments, the results have demonstrated that the bubble cluster promotes the drag reduction more than conventional type injection, named CBI (Continuous Bubble Injection), for the same void fraction supplied.