Full scale friction resistance of rough marine surfaces

Abstract

This thesis aims to delineate a methodology for the estimate of added viscous drag by roughness of coated surfaces at full scale ship condition. Most of the current approaches rely on formulations that correlate the geometry of the roughness with its frictional properties. However, no universal agreement is found and indirect methods of friction extrapolation from velocity measurements expose to relevant errors. A novel methodology is proposed, based on experimental testing of coated surfaces and computational fluid dynamics. In fact, major contribution to the viscous drag at full scale is dependant on the ratio between two geometric scales: the characteristic dimension of the roughness and the size of the wall scales of motion. This ratio can be effectively reproduced and tested in a flume type facility. Then, experimental data can be used to inform a modified formulation of wall function that permits the simulation of the full scale ship. Examples of application, from the design of experiments to the computational results are produced and discussed.