Near-Wall Coherent Structures: History, Identification and Detection - Transport and Coherent Structures in Wall Turbulence - page 145

The detection function for shear layers with acceleration is

du

⎧

1, if σ ( ) t > H u , and > 0

⎪ V V

[3.7]

Dt () =

dt

⎨
⎪ ⎩

0 else.

The number of events detected decreases exponentially
with the threshold . The threshold deduced from [BOG H V

86] is = 0.35.

H V

Figure 3.12 shows the conditional averages of the events
detected by VITA at . The conditional averages
obtained with different parameters generally collapse well
when they are stated in relation to the local global threshold.
T hus, , rendered dimensionless by the global threshold
in Figure 3.12, is qualitatively and quantitatively
similar to that presented by Alfredsson and Johansson
[ALF 84] at the same point of detection , but obtained
with different parameters and . We can also see
a significant acceleration of around the point of detection,
and the conditional averages are asymmetrical (or skewed)
near to the wall in particular. These characteristics are
amply documented in the literature [ALF 84, SHA 86].

y + = 15

u c

H V uu

y +

T V + = 10

H V = 1

u

y +

Figure 3.12. Conditional averages found by VITA at . The

= 1 5

T +

detection parameters a re and H = 0. 35 . A: uH u ; B: = 13

c

V

V

( c

vH v ; and C: vv

uv H uu

c

V V