DNS of turbulent channel flows with two- and three-dimensional roughness

Abstract

DNS results for turbulent channel flow with two- and three-dimensional roughness elements are discussed. The focus is on surfaces where the separation between consecutive roughness elements is small. In these cases, the roughness function is strongly influenced by the shape of the roughness elements. Usually, the roughness function is plotted against geometrical parameters and a satisfactory correlation does not emerge. Alternatively, a rather good collapse of the data is achieved when the roughness function is plotted against the rms of the normal velocity on the plane of the crests. This good relationship was observed previously for two-dimensional geometrical disturbances (Orlandi, P., Leonardi, S. and Antonia, R. A., 2006, Turbulent channel flow with either transverse or longitudinal roughness elements on one wall. Journal of Fluid Mechanics, to appear), here it is extended to three-dimensional geometries that are more often found in practical applications. Three-dimensional roughness has a strong influence on the overlying flow than two-dimensional roughness. This is shown through visualizations of the instantaneous velocity and vorticity, and through the comparison among the turbulent intensities and Reynolds shear stress distributions.

Acknowledgments

The support of a MIUR 60% grant is acknowledged. We would like to thank Prof. R.A. Antonia for the fruitful discussions and R.J. Smalley for the careful reading of the draft. The suggestions of anonymous reviewers are also appreciated.