Skin friction reduction on smooth and rough surfaces using air lubrication

Air lubrication systems have been shown to reduce skin friction drag in both laboratory and full-scale settings, but there has been little research into the interactions between air lubrication systems and rough surfaces. These surfaces are ubiquitous to all in-service ships. A flow channel and floating element balance were designed and assembled to address 1) whether the presence of additional surface roughness significantly affects the amount of air needed for significant skin friction drag reduction and 2) whether the percent drag reduction differs between roughened and smooth surfaces after the optimal air injection rate is obtained. Three different surfaces were tested: a smooth acrylic and two roughened acrylic panels. The results show that air lubrication has the potential to decrease the frictional drag coefficients by as much as 30 % at laboratory scale bulk Reynolds numbers (Re H ∼2.8 × 10 4 to 3.3 × 10 4 ). However, roughened surfaces required higher air injection to reach similar drag reductions. The practical implication is that the increased air injection rates will result in a decrease of ALS efficiency as the ship's hull becomes more fouled and/or coating roughness increases. • No significant differences in drag reductions after reaching critical air flux. • Roughened surfaces show comparable air drag reductions to smooth surfaces. • Roughened surfaces show a larger critical air flux. • Air lubrication can reduce drag on roughened surfaces.