Large- and Very Large-scale Structures - Transport and Coherent Structures in Wall Turbulence - page 386

Figure 6.10. Schematic view of structures in a packet

Figure 6.10 gives a schematic representation of the events
associated with a packet of three coherent structures:
primary (PS), secondary ()

SS and tertiary (TS). The PS is
the most intense. We have assumed that a given structure
regenerates a vortex with the opposite vorticity, as discussed
in the previous chapter. Each vortex induces QII - and QIV -
type events, with tilted shear layers detectable by VITA (or
variable-interval space averaging (VISA)) at the point where
they intersect. The scale of an individual structure rarely
surpasses , and is governed by the inner variables.

Their coherent alignment in the mean flow direction and the
wake associated therewith extends up to at least

0.3 Λ

0

. The
long zones of deficient momentum are linked to the outer
scale.

3 Λ

0

The notion of attached/detached structures goes back to
Townsend [TOW 76]. The clusters of structures which
penetrate the buffer sublayer, whose minimum distance from
the wall is , are called “attached structures”, as

opposed to packets of “detached” structures, whose minimum
distance from the wall is greater than

y + < 20

min

, according to
Del Alamo and Jimenez [ALA 06b]. Detached structures

y + > 20

min