Effect of Large-Scale Structure Generated in Low-Reynolds-Number Poiseuille Flow on Longitudinal Vortices

The direct numerical simulation of a spectral method is performed to study the flow structure of a turbulent channel flow at low Reynolds number, where the turbulent and quasi-laminar regions alternatively appear in the streamwise direction. The turbulent region is punctuated by the high-speed and low-speed fluids at the upstream and downstream sides, respectively. As a result, both the bulk and friction Reynolds numbers decrease from upstream to downstream regions, which makes the low Reynolds number effects pronounced at the downstream turbulent region; attenuation of turbulent diffusion into the wall vicinity and decrease in dissipation rate there. Differences between streamwise vortical structures in upstream and downstream turbulent regions are also investigated, which elucidates the effects of local mean flow on the vortices even in the wall vicinity as well as low Reynolds number effects.