Fluid dynamic properties of irregular, multi-scale rough surfaces

Surfaces roughness affects energy efficiency and maintenance costs in many industrial
sectors. 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 random
rough surfaces with specified topographical parameters. Direct numerical simulations of turbulent channel
flow over the surfaces will be used to obtain their fluid dynamic properties with the aim to establish
relationships 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 better
turbulence models for typical industrial computational fluid dynamics simulations that can take
surface topography effects into account. This will provide the basis for a more accurate prediction of
the 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.

Under the 2008 Climate Change Act the United Kingdom has committed to reduce emissions by at least 80% in 2050 from 1990 levels. To achieve this goal, energy efficiency will need to be improved and sources of renewable energy, such as marine wave or tidal energy, developed. Surface roughness has a big impact on the energy efficiency of engineering systems. For example, surface roughness on tidal turbines can lead to a power loss of over 40% in cases of strong surface fouling. The cost associated with this loss of efficiency has been estimated to be in the order of several tens of millions of euros for large tidal farms.

One of the main effects of surface roughness is that it modifies the near-wall flow, which leads to an increase in the friction factor and the drag of the surface. The near wall flow is both influenced by the height and the topography of a rough surface. Since the relationship between surface topographical parameters and fluid dynamic quantities, such as the friction factor, is not clear, it is not possible to accurately predict the fluid dynamic effects of a given engineering rough surface. As a consequence of this, surface maintenance cycles may be scheduled at sub-optimal frequencies and systems that are known to be affected by surface roughness such as tidal turbines may be modelled inaccurately.

The main aim of this project is to develop empirical relationships between the topographical parameters of rough surfaces and their fluid dynamic properties to enable a more accurate and surface-specific prediction of the effects of surface roughness on near-wall turbulent flows. This is a pre-requisite for a better capturing of roughness effects in typical computational fluid dynamics codes used in industry, which rely on empirical approaches for simulating turbulent flows such as Reynolds-Averaged-Navier-Stokes turbulence models and wall-treatments. An improved modelling and prediction of roughness effects will help to improve the energy efficiency in many engineering systems, e.g. by more accurate scheduling of maintenance cycles for fluid machinery and ship hulls. Higher energy efficiency will help in mitigating climate change and improving air quality by reducing emissions of carbon dioxide and nitrogen oxides. This project will also make a contribution to addressing some of the challenges faced by the marine renewable energy sector, where surface roughness generated by salt-water corrosion and bio-fouling poses serious problems. By laying the foundations for more accurate, surface specific roughness modelling, this project contributes a building block to the development of resilient fluid machinery which can cope with the harsh conditions imposed by the marine environment.