Frictional drag reduction caused by bubble injection in a turbulent boundary layer beneath a 36-m-long flat-bottom model ship

The reduction of frictional drag caused by bubble injection was experimentally investigated in a turbulent boundary layer beneath a 36-m-long flat-bottom model ship. For this model ship towed at 8 m/s, the drag reduction increased exponentially (with a power index of 1.2) with the air flow rate, and the drag-reduction ratio reached 28% at the maximum air flow rate. The local wall shear stress measured at 23 locations on the flat-bottom surface revealed a reduction in the local friction coefficient of 50% immediately downstream of the bubble injector and 20% at the stern. This downstream decay proceeded nonlinearly, inferring that the spatial transition of local drag-reduction characteristics depends on the status of bubble dispersion. We established a formula characterizing the streamwise decay from measurements of the transition of drag-reduction characteristics along the 36 m length of the model. This formula can be used to evaluate the downstream persistence of bubbly drag reduction and estimate the full-scale performance of air lubrication technology.