Uniform momentum zone scaling arguments from direct numerical simulation of inertia-dominated channel turbulence*

Inertia-dominated wall-sheared turbulence is composed of an inner and outer layer, the former occupied by the well-known autonomous inner cycle while the latter composed of coherent structures with spatial extent comparable to the flow depth. Outer-layer structures instantaneously manifest as regions of quasi-uniform momentum – relative excesses and deficits about the Reynolds average – termed uniform momentum zones (UMZs). Successive UMZs exhibit abrupt wall-normal gradients in streamwise momentum, which cannot be explained by the notion of attached eddies, for which the vertical gradient goes as (x₃)^-1  (x₃ is wall-normal position). Using DNS data of channel turbulence across inertial regimes, we recover vertical profiles of Kolmogorov length a posteriori and show that η ~ (x₃)^1/4, thereby requiring that ambient wall-normal gradients in streamwise velocity scale as (x₃)^-1/2 . The data reveal that UMZ interfaces are responsible for these relatively larger wall-normal gradients. The DNS data afford a unique opportunity to interpret inner- and outer-layer structures simultaneously: UMZs – and associated outer-layer dynamics – can be explained as the product of inner-layer bluff-body-like interactions, wherein wakes of quasi-uniform momentum emanate from the inner layer.