General Points - Transport and Coherent Structures in Wall Turbulence - page 53

remarkably insensitive to Re for reasons that have not yet

been elucidated. One of the plausible arguments is that this
insensitivity arises from the universality of the spatial
distribution of the QSVs in the buffer sublayer. The wall-
normal vorticity is dominated by the term

τ

at

ω∂∂

y uz

∝

y + , particularly near to the vortex structures, where the

uz

≤ 15

shear layers are engendered by purely kinematic
effects. They form thin walls of vorticity separating the
QSVs. We can, therefore, consider the fact that

∂∂

, where

+ is the velocity scale linked

ω∂∂

+

∝ uz u

ω

+ +

∝

+

λ

+

u

y z ω

y y

+ , is the spanwise spacing between the QSVs
(directly linked to the spacing of the high- and low-velocity
streaks, as we will see later on). It occurs that

to

ω + and

λ

z

y

λ + depends

z

only slightly on Re , which could, in part, explain why

+ is

σ

τ

ω

y

independent of the Reynolds number, if only for Re τ ≤ 1, 1 0 0 .