Large- and Very Large-scale Structures - Transport and Coherent Structures in Wall Turbulence - page 385

structures. The streamwise extent of these zones can reach
up to two or three times the outer scale . We also see that

the number of structures per packet is five or six - i.e. double
that which is observed at

Λ

0

. According to Adrian et
al. [ADR 00] and Adrian [ADR 07], the packets of vortices
probably contain more structures when the Reynolds number
increases. 3 It is also interesting to note that the packets
extend from the wall to the logarithmic sublayer. Tardu
[TAR 02] found all of these characteristics in the analysis of
the conditional averages found by a specific method of
grouping of the ejections in packets.

Re

τ =

500

Figure 6.9. Low momentum zones induced by packets in a boundary layer
at , according to [ADR 00]. The figure is reproduced from [ADR

07]. Readers may also consult [CHR 01] for the additional statistical proofs
supporting the existence of packets of structures. For a color version of this
figure, see www.iste.co.uk/tardu/wallturbulence.zip

Re

τ =

2, 250

3 Note, however, that in the absence of a detailed statistical analysis
similar to that performed by Tardu [TAR 02], it is difficult to draw a
conclusion about the number of structures per packet and its dependence
on the Reynolds number. Indeed, we can see in Figure 6.9 that there is a
first packet of three structures, followed by a second packet, whose
individual elements are separated by larger intervals: this may actually be
two distinct packets rather than a single packet.