78th Annual Meeting of the APS Division of Fluid Dynamics (Nov 23 — 25, 2025)

V028: Friction Reduction Through Air Lubrication

Air-lubrication is an innovative approach to reduce fuel consumption in ships by injecting air bubbles under the hull. While this technique promises 10-20% fuel savings, the physical mechanisms behind drag reduction are not fully understood.Using a Navier-Stokes solver for multiphase flow, we performed a series of numerical simulations to study bubble dynamics beneath a ship's hull. By systematically varying the Froude number (inertial forces over gravitational forces) and Weber number (inertial forces over surface tension), different behaviors of the air layer were analyzed.The simulations reveal fascinating bubble dynamics that depend on flow conditions. At low Froude numbers, bubbles adhere to the surface forming a protective air cushion. As the Froude number increases, the flow entrains bubbles and pulls them away from the wall. Meanwhile, higher Weber numbers deform initially spherical bubbles into elongated shapes that align with the water flow, creating distinctive streak patterns visible in the visualization.A surprising finding emerges: perfectly spherical bubbles often increase drag rather than reduce it. The key insight is that bubbles must deform to be effective lubricants. Optimal conditions occur when deformable bubbles merge together, creating a continuous air layer that acts like a slippery coating on the surface.The best performance achieves 40% drag reduction when low Froude and high Weber numbers allow highly deformable bubbles to coalesce into a stable air film. This fundamental understanding helps engineers design effective air-lubrication systems for real ships, potentially revolutionizing marine fuel efficiency and reducing maritime emissions.