Downstream persistence of frictional drag reduction with repetitive bubble injection

• Published in: Ocean engineering Vol. 272; p. 113807
• Main Authors: Tanaka, Taiji, Oishi, Yoshihiko, Park, Hyun Jin, Tasaka, Yuji, Murai, Yuichi, Kawakita, Chiharu
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 15.03.2023
• Subjects: Bubbly drag reduction; Full-scale estimation; High Reynolds number; Model ship; Turbulent boundary layer; Model ship; BDR; Full-scale estimation; Turbulent boundary layer; RBI; Bubbly drag reduction; CBI; High Reynolds number
• ISSN: 0029-8018; 1873-5258
• DOI: 10.1016/j.oceaneng.2023.113807

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 fluctuating 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. •Air lubrication with repetitive bubble injection is examined in a turbulent boundary layer beneath a 36-m-long model ship.•The ratio of frictional drag reduction of the model ship is improved by 5% relative to continuous bubble injection.•The local wall shear stress was directly measured by shear-stress sensors at 23 locations on the bottom plate.•The downstream persistence of the drag-reduction effect is improved by a factor of 15.•The estimation of the full-scale performance indicates that repetitive bubble injection provides 7 times the drag reduction.