Near-Wall Coherent Structures: History, Identification and Detection - Transport and Coherent Structures in Wall Turbulence - page 144

The conditional averages corresponding to the ejections
detected by yield results similar to those obtained by
the quadrant method (Figure 3.10). However, there are
certain differences, particularly with regard to the duration
of the detected events. There is also a slight skewness of
around . These differences, though, are not significant,
and detection by the technique is relatively satisfactory,
although one-to-one correspondence between the and
events is by no means perfect. The degree of coherence
of the detected events is also satisfactory (Figure 3.11).

u − l

u c

t +

= 0

u − l

u − l

QII

3.3.3. Detection by VITA and VISA

The variable integration time averaging (VITA) method
[BLA 76, BLA 83] detects strong shear layers, combined with
temporal accelerations or decelerations ,
linked to the phases of ejections and sweeps, respectively,
upstream and downstream of the detection point. The
technique is based on the local variance defined by

du dt > 0 du dt < 0

2

T

⎛

T

⎞

v v

t + t +

2 2

1 ⎜ 1 ⎟

() () ()

∫

2

∫

[3.6]

σ t = u tdt − utdt

⎜ ⎟

V

T T

⎜ ⎟

T T

V V v v

t −

⎝

t −

⎠

2 2

y + Figure 3.11. Coherence of detec tion by the u-l m eth od at = 15 . A:

(

; B:

uv uu vv

u c v c uu vv

c