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may be over 20 times greater than the integral scale. VLSMs
tend to be established at large Reynolds numbers, where the
inner and outer scales are clearly separate, and they arise in
an equilibrium sublayer where no viscous effects are
experienced. The VLSMs transport not only a significant
portion of the streamwise turbulent intensity, but also of the
Reynolds stress. The dynamic role (active or passive) of
VLSMs is addressed in the second part of this chapter.
6.2. Clusters of coherent structures
6.2.1.
History
In publications dating from the 1960s relating to
visualizations of flows by using dyes or hydrogen bubbles,
the terms “ejection” and “bursting”, respectively, describe the
processes of individual lifts of the low momentum fluid from
the wall and the consecutive disintegration of the fluid
filaments. These two terms were first distinguished by the
authors of [RUN 63], [KIM 71] and [OFF 74, OFF 75], who
noted the passage of “small parcels of fluid” comprising
multiple active events arising from the same streak, which is
why the term “bursting” was adopted. The ejections
belonging to the same burst are near to one another in time
(or space) and the flow is relatively inert between them. The
same terminology was later adopted by many research teams
([BOG 86], [BOG 87] and [LUC 87] to name a few).
Figure 6.1 shows the cumulative probability distribution
t
t
⎛
⎞
P
e
>
⎠
of the time
t
separating two ejections detected by
⎜
⎟
t
t
⎝
e
e
y
+
the quadrant-
II
technique at
=
12
, in a homogeneous
turbulent channel flow at
. We see a break in
Re
τ
=
450
t
t
⎛
⎞
P
e
>
⎠
, which defines a time
Ge
tt
separating the
⎜
⎟
t
t
⎝
e
e
energetic events that constitute clusters of structures and
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