Department of Mechanical Engineering, University of Texas at Dallas, Richardson, Texax, USA

Reliable prediction of friction drag and roughness function ∆U⁺ induced by wall roughness nowadays is an open issue, despite the extensive effort of the scientific community. Recent studies illustrate how a single feature of the roughness geometry is not able to adequately predict ∆U⁺. In order to find a geometrical parameter able to match the stochastic nature of random roughness and its effect on the flow, several Large Eddy Simulations at Re_??= 400 over 2D rough walls were carried out. Designed throughout gaussian functions, the geometries are characterized by different roughness average heights, distributions and densities. The results confirm that the higher values of the roughness function are associate with higher roughness elements. With the aim to find a universal correlation between ∆U⁺ and rough surface, new geometrical quantities based on the area occupied by the roughness were investigated. The proposed parameter, called Effective Area, EA, is based on the correlation between the Effective Slope and the roughness area A. The achieved results show a good correlation between ∆U⁺ and EA.