  Although the roll-streak (R-S) is fundamentally involved in the dynamics of
wall-turbulence, the physical mechanism responsible for its formation and
maintenance remains controversial. In this work we investigate the dynamics
maintaining the R-S in turbulent Poiseuille flow at R=1650. Spanwise
collocation is used to remove spanwise displacement of the streaks and
associated flow components, which isolates the streamwise-mean flow R-S
component and the second-order statistics of the streamwise-varying
fluctuations that are collocated with the R-S. This streamwise-mean/fluctuation
partition of the dynamics facilitates exploiting insights gained from the
analytic characterization of turbulence in the second-order statistical state
dynamics (SSD), referred to as S3T, and its closely associated restricted
nonlinear dynamics (RNL) approximation. Symmetry of the statistics about the
streak centerline permits separation of the fluctuations into sinuous and
varicose components. The Reynolds stress forcing induced by the sinuous and
varicose fluctuations acting on the R-S is shown to reinforce low- and
high-speed streaks respectively. This targeted reinforcement of streaks by the
Reynolds stresses occurs continuously as the fluctuation field is strained by
the streamwise-mean streak and not intermittently as would be associated with
streak-breakdown events. The Reynolds stresses maintaining the streamwise-mean
roll arise primarily from the dominant POD modes of the fluctuations, which can
be identified with the time average structure of optimal perturbations growing
on the streak. These results are consistent with a universal process of R-S
growth and maintenance in turbulent shear flow arising from roll forcing
generated by straining turbulent fluctuations, which was identified using the
S3T SSD.
