Near-Wall Coherent Structures: History, Identification and Detection - Transport and Coherent Structures in Wall Turbulence - page 194

2

Q ≥ ε

th

1
27

6

Δ≥ ε

th

[3.70]

2

λε
λε

≤−
≥

2 th

cI th

This intuitive proposition can only yield acceptable results
if the structures are sufficiently intense. Chakraborty et al.
[CHA 05] apply the criteria [3.70] to the identification of the
core of the Burgers vortex analyzed in section 3.11. Figure
3.30 shows the results for two structures - one with a low
Reynolds number of Re

Γ =

10 and the other more intense,

with Re Γ = 30 . In each case, the threshold ε th is varied, and

the dimensionless radius of the vortex is determined in

line with the criteria [3.70]. We can see that the differences
observed in the case of the vortex with lesser intensity
become considerably less for the more intense structure of
30

*

r γ

. The case of the Burgers vortex is generic, and
cannot be used to draw general conclusions about the
detection of wall structures, which are far richer from the
topological point of view. All the detection criteria which we
have analyzed are based on the topology defined by the
velocity gradient tensor, with the exception of

Re

Γ =

detection.
The techniques differ when we wish to precisely determine
the core of the vortex - particularly for low-intensity
structures, but still coincide overall, at least from a
qualitative standpoint.

λ

2

3.13. Lagrangian detection

We will only briefly discuss the Lagrangian detection of
structures, which is a relatively new domain. It certainly
constitutes an objective approach, but at the same time, it
requires the tracking of fluid particles, and therefore
relatively cumbersome calculations.