Characterisation of Marginally Turbulent Square Duct Flow

We have performed direct numerical simulation of fully developed turbulent flow in a straight duct with square cross-section. The main objective of the present study is to determine the minimal requirements for maintaining turbulence in duct flows [1]. A detailed analysis of this limit regime allows to elucidate the dominant mechanisms governing the marginally turbulent state, where the self-sustaining coherent structures, i.e. streamwise vortices and streaks, have a cross-streamwise length scale comparable with the duct width. It is therefore expected that the coherent structures are of direct relevance to the appearance of secondary flow of Prandtl’s second kind.