Regeneration and Self-Maintaining of Coherent Structures - Transport and Coherent Structures in Wall Turbulence - page 359

layer in order to take place. These same authors conducted
another DNS to determine the minimum distance from the
wall beyond which the turbulence can be sustained without
there being any effect from the outer layer. Beginning with a
wider turbulent channel, where

L +

, they filtered the
fields in space and time until the flow became laminar once
again at

= 300

y + > 95 . The turbulence continued to be sustained

y + < at 65 in spite of these particularly severe conditions.

5.10. Formation of arch vortices. Generation of new
streamwise structures

The detailed dynamics of the structure of near-wall
turbulence is complex, and there are a number of aspects
related with the generation of QSVs. The regeneration of
QSVs is accompanied by the creation of 3D shear layers, at
the peak of which, small arch-shaped spanwise vortices may
form [HEI 00]. These structures are different from the
hairpin head vortices, which detach far from the wall.
However, they regenerate in the lower buffer sublayer. The
size of these vortices increases, and the spanwise vorticity

inherent in these structures, by way of the twisting term

ω

z

- which, according to equation [5.48], is reduced to

ω∂ ∂ uz

z

( )( )

38 - gives rise to the QSVs. The DNS
performed by Heaist et al. [HEI 00] with a small Reynolds
number

∂∂ ∂∂ vx uz

indicate that 30% of QSVs in the lower
buffer sublayer are created by this mechanism. In
Figure 5.40, we have illustrated the scenario set up by the
above authors [HEI 00], showing the formation of an arch
vortex at

Re τ = 150

y +

and its rotation in the streamwise
direction, regenerating a new QSV.

= 6.5

38 We can see that, still, the introduction of a dependency in direction x
is necessary here.