Roughness effect in an initially laminar channel flow

The possibility of generating and maintaining turbulence in an initially laminar channel flow is investigated for two Reynolds numbers and 2100 (based on bulk velocity, and half-height of the channel, ). The study is carried out through a direct numerical simulation based on the lattice Boltzmann method (LBM). The channel consists of two parallel walls separated by a distance where the roughness elements are mounted on both walls. It was observed that when the transverse square bars span half the width of the channel and are mounted in a 'staggered' formation, the flow becomes fully turbulent with strong similarities to fully rough wall turbulent channel flows at much higher Reynolds number, as reported in the literature. For example, the rough wall mean velocity profile exhibits a significant downward shift when compared to the smooth wall one. Also, the turbulent kinetic energy budget is similar to its counterpart in rough wall turbulent channel flows at much higher Reynolds numbers than the present ones. It is further shown that the present velocity spectra compared very well with that obtained in a rough wall turbulent boundary layer. Finally, some elements of the possible physical mechanism allowing the generation, growth and sustainability of turbulence are proposed.