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Figure 6.16.
Peak of conditional averages of the wall-normal velocity
corresponding to the packets of structures in the inner sublayer. This figure
is reproduced from [TAR 95b]
6.6. Clustering of packets. Very large-scale motions
The packets of structures in the inner layer develop and
gradually reach the logarithmic sublayer. The characteristic
length scale in the streamwise direction is the outer scale, as
indicated in Figure 6.14. The coherent alignment of the
packets in the logarithmic sublayer gives rise to VLSMs,
whose streamwise length may be up to
(see
Figure 6.17). The role of VLSMs in the logarithmic sublayer
5
becomes significant at large Reynolds numbers, typically
20
Λ
0
, for which a zone of constant stress
uv
+
of
notable breadth in direction
y
+
is clearly established. The
VLSMs are, therefore, capable of transporting a considerable
amount of the Reynolds shear stress and the turbulent
energy, in zones where
uv
+
−
Re
τ
>
1700
is high and is near to 1, but
where the mean shear stress, and therefore the mean
production
−
+
, is low. The “active” or
“passive” nature of this transport mechanism is an
5 The coherence of the
VLSM
s gradually fades beyond the logarithmic
sublayer toward the wake layer, according to [GAN 03].
+
+
+
Puv dUdy
=−
(
)
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