Comparison of turbulent boundary layer streamwise development over two- and three-dimensional rough surfaces

Abstract: Turbulent boundary layers (TBLs) above two-dimensional (2D) square-bar and three-dimensional (3D) sandpaper roughness are studied experimentally to investigate streamwise development and Reynolds number effects for two types of roughness. Previous studies have studied 2D square-bar roughness and 3D sandpaper roughness separately. There is a need to perform rough wall TBL measurements and systematical comparisons for 2D and 3D roughness. In this study, we performed TBL measurements over 2D and 3D rough walls at various streamwise stations spanning from x = 0.62m to 1.90 m, and we operated at various free-stream velocities to obtain Reynolds numbers ranging from Re? ? 1500 to 5500. The roughness heights from peak to valley of 2D and 3D roughness are similar, with k = 1.5mm and 1.219 mm, respectively. The mean velocity and turbulence intensity profiles of the two rough-wall TBLs show a consistent streamwise development with the literature results. The turbulent intensity profiles show the outer region collapsing at a wall-normal position farther away from the wall for the 2D roughness, compared with the 3D roughness result. In addition, the 2D roughness TBL shows a higher skewness than the 3D result in the near-wall region. The streamwise development of the turbulence intensity and turbulence energy distribution for 2D and 3D roughness are revealed in the analysis Keywords: turbulent boundary layer, rough wall, boundary layer structure, hot-wire experiment