Downstream persistence of frictional drag reduction with repetitive bubble injection

Frictional drag reduction using repetitive bubble injection is investigated in a turbulent boundary layer beneath a 36-m-long flat-bottom model ship towed at 8.0 m/s. Bubbles are injected with air flow rates periodically fluc-tuating at a repetition frequency of 0.5-2.0 Hz. The drag-reduction ratio reaches 24%, which is a 5% improvement relative to the conventional method of continuous bubble injection. Measurement of the local wall shear stress acting on the bottom plate and optical visualization of bubbly flow reveal that the transition of the status of bubble dispersion causes the streamwise decay of local drag reduction. The downstream persistence of the drag-reduction effect can be improved by a factor of 15 through the use of repetitive bubble injection. The estimation of the full-scale performance indicates that repetitive bubble injection provides approximately 7 times the frictional drag reduction provided by continuous bubble injection for long ship hulls exceeding 130 m.

Automated summary

Frictional drag reduction using repetitive bubble injection in a turbulent boundary layer beneath a towing model ship was studied. The drag reduction ratio reached 24%, which was a 5% improvement compared to continuous bubble injection. The transition of bubble dispersion status was found to cause the streamwise decay of local drag reduction. Repetitive bubble injection improved the downstream persistence of the drag-reduction effect by a factor of 15. The estimation of full-scale performance indicated that repetitive bubble injection provided approximately 7 times the drag reduction of continuous bubble injection for long ship hulls exceeding 130m.