Frictional Drag Reduction on a Ship by Air Lubrication

To meet the International Maritime Organization's zero-emission targets, air lubrication systems are essential for reducing frictional drag on ships. This study focuses on bubble drag reduction and introduces a method to enhance its performance using artificial void waves. While natural void waves—spatiotemporal fluctuations in void fraction—are known to promote drag reduction, they occur uncontrollably in conventional constant air injection. We propose repetitive bubble injection, generating periodic high-density bubble clusters (artificial void waves) by pulsing the airflow. Experimental results from horizontal channel flows and a 36-m flat-bottomed model ship demonstrate that the artificial wave significantly improves drag reduction compared to continuous injection at equivalent average air flow rates. This improvement is attributed to the locally increased void fraction within the generated waves, which effectively modifies turbulent structures near the wall. Furthermore, when this method was applied to an actual ship, the net energy savings increased from 4% to 5%. To further optimize control parameters of this method, we also introduce novel laboratory-scale experimental setups—a belt-driven system and an expanding channel—that successfully reproduce void waves by simulating the developing boundary layer of a ship's bottom. These systems provide a platform for elucidating the detailed mechanism of void wave propagation and maximizing the efficacy of air lubrication.