Civil Engineering Reference
In-Depth Information
The bursting response time of the near-wall flow induced
by a convected structure depends on several factors,
including the shape and type of the vortex. According to
Ersoy and Walker
[ERS 85], the timescale for regeneration of
a secondary structure by a mother vortex convected at a
distance
y
from the wall is linked to
=
a
Re
+
[5.13]
Δ∝
t
Γ
+
2
a
This relation is qualitative. However, it gives us vital
information. A “mother” structure is more capable of
regenerating a “baby” vortex when it is intense and is
situated near to the wall.
Figure 5.11.
Stages in the formation of structures: a) raising of the head of
the structure, stretching and intensification; b) induction of layers of
vorticity in the wake of the mother structure; c) roll-up; and d) complete
formation of the secondary structure. The wall shear stress schematically
shows the zones of activity associated with the phenomenon of turbulent
bursting. This figure is adapted from [MEN 98]
5.3.3.
Viscous response of the flow to the convection of a
vortex structure near to the wall
The group at Lehigh has also worked intensively on the
phenomenon of unsteady separation caused by a structure
convected near to the wall, as briefly discussed in the
previous section. Their investigation spans the period from
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