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In-Depth Information
Fig. 44 Proposed flow
structure in the scour hole in
front of the cylindrical pier
A
B
cusp
1
2
3
The interaction between the downflow and the horseshoe vortex inside the scour
hole leads to the formation of a cusp, separating the region in the scour hole mainly
shaped by the downward-flow and the region shaped by the horseshoe vortex and
the separation vortexes. Indeed, the bed topography between zones 1 and 2, i.e., the
observed cusp, would thus be formed by the upward movement of the jet flow that
incorporates the horseshoe vortex and the flow upward from the bottom of the
vortex. In zone 3, the separate flow is the result of adverse gradient of pressure
which results from the sudden deepening of the bed. Among the areas 2 and 3, there
are structures in counter-rotation.
This model is compatible with the one proposed by Unger and Hager (
2007
), but
not with those by Qadar (
1981
) or Muzzammil and Gangadhariah (
2003
). It follows
that the scour mechanism depends fundamentally on the downward flow and not on
the intensity of the horseshoe vortex as defended by Shen et al. (
1969
).
5 Conclusion
This text was prepared for the 30th International School of Hydraulics, in Wiejce,
Poland, September 2010. It presents a sample of work performed at Instituto
Superior T´cnico, Technical University of Lisbon, Portugal using LDA and PIV
techniques in the scope of fluvial hydraulics.
The work was divided in three parts, the first of which devoted to the presentation
of basic concepts of LDA and PIV. The second part addressed the characterization of
organized turbulence in open-channel flows over flat rough mobile beds and over flat
rough fixed (armored) beds. In the third part, the scour mechanism associated to a
vertical cylinder mounted on a mobile bed was characterized and discussed.
The first application, the characterization of turbulent open-channel flows over
flat rough mobile beds and over flat rough fixed beds, required detailed turbulent
