Turbulent boundary layers over rod- and cube-roughened walls

Abstract

Direct numerical simulations (DNSs) of spatially developing turbulent boundary layers (TBLs) over two-dimensional (2D) rod-roughened walls and three-dimensional (3D) cube-roughened walls were performed to investigate the effects of the streamwise spacing of roughness elements on the properties of the TBLs. The inspection of the Reynolds stresses showed that except for the 2D rough walls with p_x /k = 2 and 3, the effects of the roughness on the 2D and 3D rough walls extend to the outer layer and that the magnitude of the Reynolds stresses in the outer layer increases in proportional to p_x /k. However, such results do not account for the variations with p_x /k in the form drag and roughness function, which have maximum values at p_x /k = 8 and 4 for the 2D and 3D rough walls, respectively. Finally, we examined turbulent structure through instantaneous analysis, linear stochastic estimation, and dynamic mode decomposition to the time-evolving flow fields to scrutinize the spatial organization of the outer-layer structures over several different rough walls.

Keywords:

• turbulent boundary layers
• boundary layer structure
• turbulence simulation

Acknowledgement

This work was supported by the Creative Research Initiatives (No. 2012-0000246) program of the National Research Foundation of Korea, and partially supported by the World Class University (R31-2008-000-10045-0) program of the National Research Foundation of Korea.