  In wall-bounded shear flow the primary coherent structure is the streamwise
roll and streak (R-S). Absent of an associated instability the R-S has been
ascribed to non-normality mediated interaction between the mean flow and
perturbations. This interaction may occur either directly due to excitation of
a transiently growing perturbation or indirectly due to destabilization of the
R-S by turbulent Reynolds stresses. A fundamental distinction between the
direct and the indirect mechanisms, which is central to understanding the
physics of turbulence, is that in the direct mechanism the R-S is itself the
growing structure while in the indirect mechanism the R-S emerges as a
self-organized structure. In the emergent R-S theory the fundamental mechanism
is organization by the streak of Reynolds stresses configured to support its
associated roll by the lift-up process. This requires that a streak organizes
turbulent perturbations such as to produce Reynolds stresses configured to
reinforce the streak. In this paper we provide detailed analysis explaining
physically why this positive feedback occurs and is a universal property of
turbulence in shear flow. DNS data from the same turbulent flow as that used in
the theoretical study (Poisseullle flow at R=1650) is also analyzed verifying
that this mechanism operates in DNS.
