  Many exact coherent states (ECS) arising in wall-bounded shear flows have an
asymptotic structure at extreme Reynolds number Re in which the effective
Reynolds number governing the streak and roll dynamics is O(1). Consequently,
these viscous ECS are not suitable candidates for quasi-coherent structures
away from the wall that necessarily are inviscid in the mean. Specifically,
viscous ECS cannot account for the singular nature of the inertial domain,
where the flow self-organizes into uniform momentum zones (UMZs) separated by
internal shear layers and the instantaneous streamwise velocity develops a
staircase-like profile. In this investigation, a large-Re asymptotic analysis
is performed to explore the potential for a three-dimensional, short
streamwise- and spanwise-wavelength instability of the embedded shear layers to
sustain a spatially-distributed array of much larger-scale, effectively
inviscid streamwise roll motions. In contrast to other self-sustaining process
theories, the rolls are sufficiently strong to differentially homogenize the
background shear flow, thereby providing a mechanistic explanation for the
formation and maintenance of UMZs and interlaced shear layers that respects the
leading-order balance structure of the mean dynamics.
