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that the viscous sublayer adjacent to the wall, which had
thitherto been incorrectly called the “laminar” sublayer, is
actually made up of high- and low-velocity streaks. These
streaks alternate in the spanwise direction
z
they are
sinuous and their spacing in the direction is remarkably
quasi-regular (see Figure 3.2). The earliest visual
observations revealed that the low-velocity streaks “become
detached” and oscillate before being ejected far from the wall
in the form of a sudden eruption, dispersed over “small
scales”. Eighty percent of turbulence production takes place
during these relatively short periods, covering around 10
inner units [KIM 71]. This phenomenon, overall, was called
“bursting” by the community of wall-turbulence researchers
at that time. It would be two decades before the realization
was made that bursting is merely a symptom of the passage
of vorticial structures, and that the phenomenon is not
linked to a mechanism caused by arbitrary instability. The
review published by Willmarth [WIL 75] gives a complete
overview of the opinions that were popular in the 1970s-
1980s.
z
Also, Theodersen [THE 52] was the first researcher to
advance the hypothesis that wall turbulence is governed by
coherent structures. He suggested that the spanwise
vorticity lines near to the wall come together and detach
from the wall under the influence of a local instability. The
velocity caused by the nascent eddy drives the center of the
structure toward the outer layer, which then takes the shape
of a horseshoe vortex (see Figure 3.3) This intuitive
suggestion put forward during a conference in 1952, which
received relatively little attention at that time, is, it turns
out, not too far from the reality of the situation. It is
certainly noteworthy that the arrangement proposed by
Theodersen [THE 52] already contained certain structural
elements relating to the transport of the momentum
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