Large- and Very Large-scale Structures - Transport and Coherent Structures in Wall Turbulence - page 378

% . We use the notation

to represent the relative

Φ aAQ = % %

Q Q Q

amplitude. The shear modulation uy is confined in a

Stokes sublayer whose thickness decreases with the imposed
frequency (Figure 6.2) in the high imposed frequency regime.
The details concerning the near-wall turbulence subject to
forced velocity oscillations go beyond the context of this topic.
Interested readers can consult [TAR 94] and the various
references cited therein for details.

∂∂

Figure 6.3. The ratio of the relative amplitude of the cadency of the PS
and the SS to the relative amplitude of wall friction in an unsteady
channel flow, as found by Tardu and Binder [TAR 97]. The amplitude in
the center of the channel is 20%. The Reynolds number is

Re = 500

τ

Figure 6.3 shows the relative amplitude of the rate of
regeneration of PS and SS , in relation to the amplitude of
the wall friction as a function of the imposed frequency f + .
We can clearly see that the regeneration of the PS strictly
follows the inner scale, whereas the modulation of the
frequency of the SS is greatly dependent on f + . These
results show that the packets of quasi-streamwise vortices
are “attached” to the buffer sublayer in which the shear
oscillations Uy are confined. The characteristic time of

the packets is the interval separating two successive

∂ ∂