Fluid dynamic properties of irregular, multi-scale rough surfaces

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Surfaces roughness affects energy efficiency and maintenance costs in many industrialsectors. Rough surfaces impair the performance of turbomachinery and marine energy harvesting sys-tems. Surface roughness caused by fouling increases the drag of ships and aircraft. An accurate prediction of the impacts of roughness is a prerequisite for the design of resilient systems and the economic scheduling of maintenance cycles for machinery affected by surface roughness built-up e.g. due to surface fouling or erosion. Surface roughness increases fluid dynamic drag and cause a downwards shift in the near-wall velocity profile called the roughness function. The fluid dynamic roughness effect is influenced both by the roughness height and the roughness topography. Engineering rough surfaces with the same roughness height but different topographies can give rise to roughness function values that differ by a factor of four. While the relationship between roughness height and drag is well understood, the relationship between roughness topography and fluid dynamic properties remains unclear, making an accurate prediction of the fluid dynamic properies of a rough surface impossible.In this project, surface simulations methods from tribology will be used to generate realistic randomrough surfaces with specified topographical parameters. Direct numerical simulations of turbulent channelflow over the surfaces will be used to obtain their fluid dynamic properties with the aim to establishrelationships between topographical parameters and quantities such as the fluid dynamic drag, the roughness function and near-wall turbulence intensity levels. The new relationships will enable the development of betterturbulence models for typical industrial computational fluid dynamics simulations that can takesurface topography effects into account. This will provide the basis for a more accurate prediction ofthe impact of roughness in a wide range of engineering systems including the marine energy sector,where bio-fouling and corrosion lead to strong surface roughness effects.