Civil Engineering Reference
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the wall. One of the most noteworthy characteristics of
unsteady separation is the appearance of a swirling focal
point. Figure 5.13 shows a typical case, analyzed by Atik
et al.
[ATI 04], corresponding to a negative circulation
,
0
Γ<
and
defined in equation [5.18]. This case corresponds
to a hairpin structure of relatively high intensity, convected
with a relative velocity
β =
0.2
. The value of the parameter
α =
1/ 6
is low, and indicates that the disturbance is
essentially 2D in nature. Figure 5.13 shows the
instantaneous streamlines obtained by Atik
et al.
[ATI 04] at
the time of separation in the plane of symmetry
y
(ordinate)
and
p
=
0.1
3
D
. The
separation is essentially due to the adverse pressure
gradient induced by the structure. We can see a violent
ejection of the rotational flow away from the wall toward the
potential zone, which is illustrated by the bursting behavior
of the streamlines in Figure 5.13. It is this behavior of
viscous/inviscid interaction that led the group from Lehigh to
propose their mechanism as a possibility to account for the
regeneration of coherent structures in the inner layer.
However, there are a variety of questions which need to be
asked in this regard. First, the basic flow in the Lehigh
model is purely laminar, and the “background turbulence” in
the inner sublayer may delay or even prevent unsteady
separation. Second, the violent bursting which is
characteristic of unsteady separation takes place relatively
far from the wall
6
, whereas the origin of the regeneration of
the QSVs is in the lower buffer sublayer, which is adjacent to
the wall. The existing DNS do not have the spatiotemporal
resolution needed for detecting bursting events similar to
those shown in Figure 5.13, and at present, these questions
remain to be answered.
x
(abscissa), respectively, at
and
z
=
1
z
=
0
6 Depending on the value of the parameter
which determines the 3D
p
3
D
nature of the initial disturbance.
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