Numerical observations of turbulence structure modification in channel flow over 2D and 3D rough walls

The effects of wall roughness on turbulence structure modifications were explored by numerical experiments, carried out using Large Eddy Simulation techniques. The wall geometry was made using an archetypal artificial method, thus to achieve irregular two- and three-dimensional shapes. The proposed roughness shapes are highly irregular and are characterised by high and small peaks, thus it can be considered a practical realistic roughness. Their effects are analysed comparing the turbulence quantities over smooth, 2D and 3D rough walls of fully developed channel flow at relatively low friction Reynolds number Reτ=395. Both transitional and fully rough regimes have been investigated. The two rough surfaces were built in such a way that the same mean roughness height and averaged mean deviation is obtained. Despite of this, very different quantitative and qualitative results are generated. The analysis of the mean quantitative statistics and turbulence fluctuations shows that deviations are mainly concentrated in the inner layer. These results support the Townsend's similarity hypothesis. Among the geometrical parameters, which characterise the wall geometries, roughness slope correlates well with the roughness function δU+. Specifically, a logarithmic law is proposed to predict the downward shift of the velocity profile for the transitional regime. Instantaneous view of turbulent organised structures display differences in small-scale structures. The flow field over rough surfaces is populated with coherent structures shorter than those observed over flat planes. The comparative analysis of both streaks and wall-normal vortical structures shows that 2D and 3D irregularities have quite different effects. The results highlight that 3D rough wall are representative of a more realistic surface compared to idealised 2D roughness.