The Turbulent Boundary Layer and Frictional Drag Characteristics of New Generation Marine Fouling Control Coatings

Abstract

The turbulent boundary layer over rough surfaces has been a widely studied research topic since most of the engineering wall-bounded turbulent flows develop under the influence of surface roughness. Accordingly, the research on rough wall turbulent boundary layer has gone a long way. However, unresolved major problems can still be found in this area such as the unsatisfying correlation of roughness and frictional drag for irregular engineering surfaces and the discrepancies about the validity of wall similarity. This study aims to contribute to further understanding of the rough-wall turbulent boundary layer flows developed over marine fouling control coatings along with the investigation of their friction drag properties. Two-dimensional Laser Doppler Velocimetry (LDV) experiments were conducted consisting of zero-pressuregradient turbulent boundary layer measurements over surfaces coated with marine fouling control coatings together with smooth and rough references in the Emerson Cavitation Tunnel of Newcastle University by using flat plate test models. Six different surfaces were included in the experimental campaign, which consist of one hydraulically smooth reference, a sand grit surface and four surfaces coated with fouling control coatings including Self-Polishing Copolymer (SPC) and Foul(ing) Release (FR) types, applied either by spraying or rollering. The mean velocity, local skin friction drag and roughness functions were calculated and discussed for the tested surfaces. In complementing the boundary layer tests, roughness measurements of the test surfaces were carried out by using a laser profilometry. Two new relations were proposed for the correlation of the roughness properties and roughness functions within the covered Reynolds number range. However; further work is needed in order to ensure the validity of the proposed relations at higher Reynolds number ranges.