Large- and Very Large-scale Structures - Transport and Coherent Structures in Wall Turbulence - page 379

ejections, and is - a value which is one order of

magnitude smaller than the characteristic scale

Δ= t + 20

PS

t +

.

Δ= 200

SS

The effective distortion time induced by Uy is,

therefore, notably short for the group of energetic events,
which accounts for the robust response of PS to wall shear
stress. Readers can consult similar arguments based on the
rapid distortion theory in [TAR 97].

∂ ∂

6.3. Characteristics of packets of structures at
moderate Reynolds numbers

The clustering of structures begins at (only) moderate
Reynolds numbers, and this organization is not a
manifestation of large-scale structures arising from the outer
layer. Figure 6.4 shows the perspective view of lambda-2
structures in the half of a turbulent channel flow at
590

. The group of quasi-streamwise structures and the
solitary vortices are clearly visible in this figure.

Re

τ =

Figure 6.4. Structures detected by the lambda-2 technique in a fully
developed channel flow at Re 0 , according to Bauer [BAU 14]. The

clustering of the structures (indicated by the circle) and the solitary quasi-
streamwise vortices are clearly visible in this figure

τ =