Large- and Very Large-scale Structures - Transport and Coherent Structures in Wall Turbulence - page 380

Tardu [TAR 95b, TAR 02] combined the time lag between
two energetic events with a pattern-recognition method 1 to
identify PS and SS . Packets of events in VITA, where

t t

⎛ ⎞

⎜ ⎝ ⎠ does not represent a break which can be exploited
to determine the clustering parameter Ge

P

e

>

t t

e e

tt , have thus been
able to be identified objectively. Figure 6.5 summarizes the
frequency distribution of packets of ejections and solitary
ejections identified by the quadrant II and u -level
techniques. We can see that the frequency of the SS
increases in the viscous sublayer and in the lower buffer
sublayer, reaching . However, the

regeneration rate of the packets increases in a broader area,
and reaches a plateau later on, at around

f +

at

y + = 0.01 > 30

SS

. From
this, we can conclude that the clusters are continually
regenerating until the logarithmic equilibrium zone is
reached, although the existence of this sublayer, in the
universal sense, is debatable at small Reynolds numbers. At
the time of writing this topic, it was not possible to present
other results obtained with large Re
τ

y +

= 100

values.

Figure 6.5. Distribution of the frequency of packets and solitary structures
at Re = 500 , as found by Tardu [TAR 02]

r

1 The conditional averages related to the first ejections of packets have
greatly different signatures from those of SS [TAR 02].