Development of a field instrument to quantify frictional drag of a heterogeneous biofilm

The presence of a relatively low form biofilm has been shown to significantly increase power requirements and resulting greenhouse gas (GHG) emissions from shipping. The International Maritime Organization has set ambitious goals in reducing GHG emissions from international shipping by at least half in 2050 compared with 2008. One of the main ways to inhibit biofilm growth and resulting GHG emissions is through proper coating selection. There are numerous test sites and testing campaigns whose purpose is to evaluate various coating types to identify top performing coatings and underlying formulations. The performance of these coatings is typically determined through visual inspection. Determining the equivalent sand grain roughness (k_s) of the test panels would allow for a direct comparison of the frictional drag of the surfaces and aid in the development of numerical methods quantifying frictional drag of biofilms on full-scale ships. The present work investigated the feasibility of a field deployable instrument that could determine the frictional drag and resulting k_s values of coating test panels immersed in a marine environment with a heterogeneous biofilm. Several sources of uncertainty where examined: 1) flow asymmetry due to the presence of a smooth wall (instrument) and rough wall (biofilm/coating), 2) flow development length, and 3) streamwise heterogenous roughness of the biofilms. The effects of these elements on the overall principal dimensions of the instrument are discussed. Results from this instrument, namely equivalent sand grain roughness of 220 and 500-grit sandpaper and a surface with a light biofilm, are compared with previous work reported in the literature.