Ship drag reduction by air lubrication

The research aims at finding the mechanisms that are responsible for the ship’s frictional drag reduction by air lubrication of the boundary layer flow. The proposal focuses thereby on the efficiency, persistence, and scaling laws of these mechanisms.

One major aim of this proposal is to study drag reduction in a stationary and stable flow to better understand the fundamental mechanisms. To this end, a Taylor-Couette set-up will be used to study the behavior of two-phase flow in a boundary layer and the influence of wall characteristics such as roughness and hydrophobicity. This setup with two co-rotating cylinders has the advantage that it allows for statistically stationary flow and accurate resistance measurements by means of the applied torque on the rotating drum. Moreover, the bubble distribution in this stationary case will be measured, and its effect on the overall torque will be theoretically analyzed.

A second aim is to study the mechanisms for drag reduction in a developing non-stationary flow past a flat plate. This is considered a good model for the flow around a ship. To this end, a Perspex water channel with a developing boundary layer past a flat plate will be used. Air film stability, its break up into bubbles and the direct injection of bubbles will be visualized by high-speed imaging and velocity fields will be obtained by particle image velocimetry (PIV). These techniques will provide insight into the complex bubble fluid interaction in a developing boundary layer flow. A combination of conventional and experimental local shear stress measurements provides the downstream development of drag reduction, which will be related to air film thickness, bubble size and distributions, flow velocity etc.

A third aim finally, is to investigate the influence of setup size and scaling of flow parameters by additional measurements in a larger water channel (higher Reynolds number).