Geoscience Reference
In-Depth Information
4 PIV Study of the Flow in the Scour Hole of a Cylinder
4.1 Overview
Enforcing the structural integrity of bridges requires the capacity to predict pier
local scour morphology. The complex nature of the interaction between turbulent
flow hydrodynamics, local morphology, and sediment transport in the near field of
the pier justifies the important volume of research performed on these topics.
The contribution of the present work is mostly in the characterization of the
turbulent flow in the scour hole in front of a bridge pier, idealized as a wall-mounted
cylinder of vertical axis, intercepting the full depth of an open-channel flow with a
developing boundary layer. Such a cylinder changes the pressure field around it.
The adverse pressure gradient upstream the cylinder modifies the pattern of the
approaching flow, which becomes downward in the upstream face of the obstacle,
while the remaining flow goes around the cylinder in the downstream direction
(Morton
1987
; Raudkivi
1998
). If the pressure gradient becomes sufficiently strong,
the interaction between the downward and the approaching flow results in flow
separation near the base of the cylinder, where it develops, in a turbulent flow, a
complex and unsteady vortex system known as the horseshoe vortex. This pheno-
menon has already been studied, but the contribution of the horseshoe vortex in the
scour development is not totally clarified.
In this study, the local scour mechanism is again studied and characterized. The
main goal of the present work is the visualization and characterization of the
downward-flow and of the turbulent flow field in the separated region in front of
the cylinder. In particular, the downward-flow and the vortex system inside a scour
hole in its early stages of development are investigated in order to identify the most
relevant scour mechanisms.
The work was essentially laboratorial and involved the use of Particle Image
Velocimetry (PIV) for the visualization and quantification of the velocity field.
Velocity measurements were performed in the vertical plane of symmetry in front
of the cylinder, in vertical planes at 30
and 45
from the symmetry plane.
Measurements in horizontal planes were also performed on selected experiments.
4.2 Theoretical Considerations
In front of the cylinder, the separation area is characterized by a complex structure,
eventually dominated by horseshoe vortex systems. Morton (
1987
) demonstrated
that the vorticity mechanism is governed by the pressure field. The component of
the vorticity normal to a vertical plane in the flow direction depends on the pressure
gradient in the flow direction:
e
y
r
ðwÞ
@
x
p
u
ðwÞ
@
z
j
z
0
¼
(9)
