Regeneration and Self-Maintaining of Coherent Structures - Transport and Coherent Structures in Wall Turbulence - page 309

conditional velocity fields v and w , respectively, at x + = 0

and . We can see the appearance of two counter-

rotating structures, with a vortex where

x + = 30

near to the
logarithmic sublayer, on the right-hand side, and a “baby”
structure with

ω < 0

x

nearer to the wall on the left-hand

ω > 0

x

side. 14

Figure 5.16. Mean square values in inner variables for
the production terms in the transport equation applying
to streamwise vorticity, according to [BRO 93]

The persistence of certain structural elements shown in
Figure 5.15 is noted in many publications. The high- and
low-velocity streaks induced by the QSVs, along with the
thin walls of vorticity in the normal direction, result from the
kinematics engendered by the structures, as discussed in
detail in section 5.2. The regeneration of a layer of
streamwise vorticity whose sign is opposite to that of the
“mother” structure, visible at the start of the process shown
in Figure 5.15, was also observed in quasi-2D models and in
other DNS (e.g. in [ORL 94]).
14 Remember that more sophisticated methods such as invariant Q and
λ -2 techniques are needed to gain a more appropriate detection of the

vortex structures.