Near-Wall Coherent Structures: History, Identification and Detection - Transport and Coherent Structures in Wall Turbulence - page 141

y +

Figure 3.7. Conditional averages a t issued from quadrant II

= 15

technique. Wall-normal velocity (A), stream w is e v elocity

v c vv u c uu

(

(B) and Reynolds shearstress (C) uv uu vv

c

In Figure 3.7, we can see that the maximum of
does n ot r eal ly coincide with the minima of and of
. This is understandable because the events
detected by the quadrant technique are conditioned
primarily by the intermittent values of , which do not
necessarily occur when and/or reach their similar local
maxima. The phenomenon is similar to the effect described
by Blackwelder [BLA 77] and Yule [YUL 79]. However, these
differences are very slight with regard to the ejections
detected by the quadrant technique. This demonstrates that
the technique is truly objective, in that it actually identifies
“puffs” of low-axial-velocity fluid, stretched significantly from
the wall toward the outer zone.

v c vv

u c uu

(

uv uu vv

c

uv ( t )

u v

The difference between and indicates the
degree of coherence of the events identified, as shown by
Antonia et al. [ANT 90a, ANT 90b]. The larger that
difference is, the larger the incoherent part, and vice versa .
Figure 3.8 shows the perfect coherence of the ejections
detected by the quadrant technique in the interval .
In addition, the incoherent parts are large both upstream
(

()
c

u c v c

uv

t +

− 5 < < 5

t + ) and downstream ( t + ). > 0 < 0