Void-wave drag reduction in turbulent flows investigated through different scale experiments

Frictional drag of ships can be reduced by injecting bubbles into the turbulent boundary layer beneath the hull. Author's group has been developing a novel method, repetitive bubble injection (RBI), for improving the efficiency of drag reduction from the conventional method, continuous bubble injection (CBI). The air flow rate of the bubble injection periodically fluctuates in RBI to generate spatiotemporal fluctuation of local void fraction, termed void wave. In this work, the drag reduction caused by the void wave was investigated at two facilities, i.e., a horizontal channel and a towing tank. In the first experiment using the turbulent horizontal channel flow at 5.0-7.0 m/s, RBI at the repetition frequency of 2.0 Hz produces doubled efficiency of CBI at the same bulk void fraction. The second experiment used a 36-m-long flat-bottom model ship towed at 8.0 m/s. RBI at the repetition frequency of 0.5 Hz produces a 13% reduction of the resistance of the model ship and a 3% improvement relative to CBI. The series of experiments demonstrated the superiority of RBI to CBI in terms of time-averaged drag-reduction ratio. We will discuss the factor determining the optimal frequency of void waves for drag reduction based on the experimental results in different-scaled facilities.