Large-Scale Flow Structure in Turbulent Poiseuille Flows at Low-Reynolds Numbers

Abstract

Direct numerical simulations of a spectral method are performed to study the turbulent channel flow at the low-Reynolds numbers, where the quasi-laminar and turbulent regions simultaneously appear and form the stripe pattern, which is floated downstream with a constant bulk mean velocity. In the turbulent region, many quasi-streamwise vortical structures are followed by low-speed streaks upstream. In the quasi-laminar region, however, quasi-streamwise vortices are rarely observed, though vortex roll with weak streamwise vorticity is regularly found around the channel centre. The turbulent regions are fixed by subtracting their advection velocity from the mean flow. After being temporarily averaged, turbulence statistics are further averaged over the string direction. As a result, turbulent regions are found to be an interface between upwind high-speed fluids and downwind low-speed fluids, where the flow is redirected into the spanwise direction. We investigate the detailed flow structure of the striped turbulent region using the linear vorticity equations.