Surface-specific Moody-diagrams: A new paradigm to predict drag penalty of realistic rough surfaces with applications to maritime transport

The sea-water that flows over a ship hull forms a turbulent boundary layer that is responsible for the skin-friction drag incurred by the ship. This boundary layer is influenced by the "roughness'' of the hull surface, which increases the drag penalty by up to 80% compared to a smooth surface in some applications. This highlights the urgent need to understand the "roughness'' effects of surface coatings and their degradation on the efficiency, economy and emissions of ship transportation. In this project, we propose a transformative approach where we tackle this pressing problem using three complementary methods. First, we will carry out Direct Numerical Simulations (DNS) of turbulent flow over surfaces that have been obtained from surface scans of various ship hulls. These results will be complemented by laboratory experiments and measurements in a towing tank of flows over replica of the same scanned surfaces. Finally, a DNS-Embedded Large Eddy Simulation (DELES) methodology will be developed and used to predict the influence of realistic topographies on drag.This holistic approach will provide the necessary data to gain fundamental understanding of the flows over such rough surfaces and enable development of a new data-rich paradigm for predicting the effects of roughness. We will specifically focus on maritime transport by developing a new surface-specific Moody-diagram approach that can be used by coatings manufacturers and ship operators to generate a realistic estimate of the drag penalty of coatings and fouling. This information can then be used to make operational decisions such as duration between dry-docking, quality of surface finish when in dry-dock, choice of specific coatings for specific surface finish and the variations in performance during service. This new approach can easily be extended to different sectors and new surface-specific Moody-diagrams can be developed for a whole range of applications including oil, gas and water transport pipelines, aircraft fuselage, trains, propellers etc. This project has financial support from a leading antifouling coatings manufacturer as well as collaborators at the University of Melbourne and the US Naval Academy who share our mutual interest in this research area.

In this project, a new paradigm for the prediction of roughness effects will be developed, focussing on roughness types prevalent in the marine environment (knowledge). The new surface-specific Moody-diagrams that will emerge from this project will enable both civil and military ship operators to make well-informed decisions on scheduling of ship dry-docking maintenance cycles and accurate predictions for the roughness induced variations of ship performance (economy). Optimal scheduling of maintenance cycles will lead to significant reductions in fuel consumption by ships and commensurate lowered emissions of greenhouse gases and particulate matter. This will contribute to achieving the UK's goals under the Climate Change Act 2008 (society). Since particle pollution exposure has been linked to health conditions such as decreased lung function, aggravated asthma and heart attacks, a reduction in particulate matter emissions by ships will help in improving the health of people living in coastal areas (society). The PDRAs involved in this project will gain new skills in experiments and simulation, international collaboration and public outreach (people). In addition, the project will be communicated to people who operate small vessels such as motorboats and yachts for leisure activities with the aim to raise their awareness of the impact of marine roughness on fuel consumption and emissions, improving their skills in assessing roughness impact on vessel performance and encouraging them to adopt better maintenance practices (people).