Measurements and Prediction of Friction Drag of Rough Surfaces

Licentiate thesis, 2016

Owing to the increased sea transportation of goods, the environmental impacts of this activity are becoming more and more important, since a ship experiences resistance that directly affects its performance and fuel consumption.   The growth of marine organisms (fouling) on ship hulls increases the roughness of the hull surface, which in turn causes a rise in ship resistance with a consequent increase in fuel consumption and greenhouse emissions of up to 40%.   Antifouling coatings have been developed and used to counteract the effect of fouling on ships and boats, but a desirable characteristic of a good antifouling coating is of course a low contribution to drag. The immediate effect of an antifouling on a hull is to increase its roughness. Its effect on vessel resistance has been studied by some researchers, but there is no common agreement on the way the drag should be characterized, which implies finding the velocity decrement or roughness function, .   This thesis examines different approaches to characterizing the drag caused by antifouling paints. One of the approaches implies submicron resolution boundary layer measurements with Particle Image Velocimetry (PIV), which, to the best of our knowledge, has not been tried before. Characterization from torque measurements on rotating disks was also evaluated together with drag measurements on towed flat plates. These data have been used to validate resolved CFD simulations, and the outcome is a promising method for characterizing the drag of any arbitrary rough surface.