Geoscience Reference
In-Depth Information
bed (details
in
Nogueira et al.
2008
). Figure
32d
shows the arrangement of the
rough elements.
The Particle Image Velocimetry (PIV) was used to measure the two-dimensional
instantaneous velocities of the flow. The double-pulsed Nd:YAG laser illuminates a
given section of the flow which is recorded by a digital camera. The particles in the
fluid scatter the light incident, which is detected by the camera. The frames are then
split in interrogation areas, being possible to calculate the displacement vector for
each one of these areas, using spatial correlation techniques.
Calibration was performed by recording a triangular scale (Fig.
33a
). The
cylinder placed in the flow field causes oscillation of the flow surface upstream
the obstacle so, to inhibit optical problems due to the impact of the laser light with
surface waves, the water surface was covered with an acetate sheet (Fig.
33b
).
The velocity measurements were performed under clear water conditions, being
the flow discharge established based on the concept of critical flow velocity. Three
different criteria (cf. Raudkivi
1998
) were used to estimate the value of critical
velocity, being adopted the value
U
cr
ΒΌ
0.286 m/s correspondent to a flow dis-
charge of 20 l/s. Table
3
summarizes the flow variables and channel parameters of
the experiments.
The variables in Table
3
are
Q
, the flow discharge;
h
, the water depth;
U
, the
depth average critical velocity;
B
, the channel width; and
D
is the cylinder diameter.
A scour hole was allowed to develop for 5 min around the cylinder placed in the
mobile bed. The scour hole can be seen in Fig.
34a
. Subsequently, the sediment bed
was fixed with spray varnish to prevent the movement of the bed material during the
measurements although maintaining the roughness characteristics of the bed.
The tests were performed in a superficial region of the scour hole, in the
symmetry axis and 30
from it, and inside the scour hole, 45
from the symmetry
axis. Measurements in a horizontal plane, 5 mm from the bed, were also performed.
Further details on the experimental arrangements can be found in Fig.
34
. Figure
35
Fig. 33 Additional material used in experiments. (a) graduated object used for lens-focusing and
calibration purposes, and (b) acetate sheet placed on the flow surface, downstream view
Table 3 Flow variables and
channel parameters
D
50
(mm)
Q
(m
3
/s)
h
(m)
U
(m/s)
B
/
D
(
)
B
/
h
(
)
0.837
0.20
0.10
0.286
14.58
7.00
