Large- and Very Large-scale Structures - Transport and Coherent Structures in Wall Turbulence - page 416

Figure 6.27. Contours of a) the streamwise and b) wall-normal velocity at
15

y +

. The quantities are taken at their respective mean
square values. The size of the regions selected is shown in Figure 6.26

and

= Re

τ =

1,100

, which
are generic in wall physics, are classified as first-category
large-scale structures by Jimenez et al . [JIM 04], when

The structures whose size is

L +

and

L +

= 1, 000 = 100

. They essentially arise from the amalgamation of
the wakes induced by the QSV s with localized sites of
Reynolds shear stress. They possess spatial coherence only
in the buffer sublayer and are decoupled from the outer
layer. The underlying process, therefore, depends little on
the Reynolds number, if at all. The local spacing of the
streaks at a given position y + corresponds closely to an
energetic mode and to the peak of the premultiplied spectral
densities

+ +

LL >>

x z

, according to Tomkins and
Adrian [TOM 05]. The largest spanwise scales

k ++ for

L +

≈ 400

z uu

L +

=− 150 300

contain a significant proportion of the streamwise energy in