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In wall-bounded shear flow the primary coherent structure is the streamwiseroll and streak (R-S). Absent of an associated instability the R-S has beenascribed to non-normality mediated interaction between the mean flow andperturbations. This interaction may occur either directly due to excitation ofa transiently growing perturbation or indirectly due to destabilization of theR-S by turbulent Reynolds stresses. A fundamental distinction between thedirect and the indirect mechanisms, which is central to understanding thephysics of turbulence, is that in the direct mechanism the R-S is itself thegrowing structure while in the indirect mechanism the R-S emerges as aself-organized structure. In the emergent R-S theory the fundamental mechanismis organization by the streak of Reynolds stresses configured to support itsassociated roll by the lift-up process. This requires that a streak organizesturbulent perturbations such as to produce Reynolds stresses configured toreinforce the streak. In this paper we provide detailed analysis explainingphysically why this positive feedback occurs and is a universal property ofturbulence in shear flow. DNS data from the same turbulent flow as that used inthe theoretical study (Poisseullle flow at R=1650) is also analyzed verifyingthat this mechanism operates in DNS.

Mechanism of Roll-Streak Structure Formation and Maintenance in Turbulent Shear Flow