Large- and Very Large-scale Structures - Transport and Coherent Structures in Wall Turbulence - page 415

with large L + also contain concentrated zones of Reynolds
shear stress. The spanwise length scale of these zones is still
the spacing of the high- and low-velocity streaks: in

the inner sublayer. Packet formation in this zone is
independent of the Reynolds number. The artificial removal
of the vortical structures originating in the outer zone,
therefore, has no effect on this process, as was
clearly demonstrated by Jimenez et al . [JIM 04]. These
structures of large streamwise extent are, therefore,
“active”, in the terminology employed by Townsend
[TOW 76], and their regeneration is decoupled from the
outer scales.

L +

= 100

The outer structures do not affect the spatial scales
characteristic of the wall-normal fluctuations. The
fluctuations of v are governed by the QSV s: the streamwise
length scale of v is the length of the QSV s, which typically
ranges from , while the spanwise

characteristic scale is the diameter of the vortices, i.e.
30

L + = 300 to L + = 500

xv xv

. Figure 6.27 confirms these arguments, and shows
that there is a clear dissimilarity of scale between u and v
at

L +

=

zv

. The structure of the spanwise velocity
fluctuations is also fragmented in a similar way to v ,
although the spatial scale corresponding to w is larger
than that for wall-normal velocity (Figure 6.28). It is clear, in
any case, that the spanwise velocity does not exhibit a
streaky structure, unlike the streamwise fluctuations in u .
The local fluctuations in v adopt a satellite-like
arrangement around the long streaks of u and give rise
to a telegraphic succession of zones of Reynolds shear stress.
The streaks with large scales

y + = 15

, but
the production remains a local phenomenon governed by
the individual scales of v , which depend on the wall
parameters.

L transport uv

−