pdf, 12 MiB - Infoscience - EPFL

Chapter 4 - Experimental setup and test procedure
were mounted on the measurement frame, allowing to record three groups of three 1D profiles
(constituting one 3D profiles) simultaneously to accelerate the data acquisition (see also HERS-
BERGER, 2002).
The used probes had an emitting frequency of 2 MHz. The frequency of the finally acquired profile
depends on the measurement depth; most samples were recorded at 77 Hz (measurement
depth of 30 cm). The technical specifications of the velocity profiler and the probes can be found
in Appendix 2.1.2. A multiplexer (Fig. 4.17, next to number. 5) allowed switching between the different
UVP-probes.
Velocity profiles were recorded for all measurement points on a 9 x 9 cm grid. But due to the big
amount of data, only the measurements in one cross-section is given for each frame position (center
of the measurement frame) in Appendix . These profiles are exactly perpendicular to the channel
axis (Fig. 4.19).
Frame Position
7
4
5
6
25°
10°
8
9 10
Probe
70° 85° +1m
support
55° 40°
9 probes
128 meas.
points
Instantaneous
measurement
(= 1 profil)
3
-1m
File
Header
1 bloc (1 probe)
(Profil 1)
Probe support
Position 1 (= 1 file)
Probe 1 = Bloc 1
… 9 blocs
…
(Profil 64)
128 points
Figure 4.19: Frame positions and file format of the velocity raw data
Several specific problems due to the complexity of the flow field in bends had to be solved:
1. The measured velocities in tangential direction were rather high (up to 1.5 m/s),
2. the radial velocities were an order of magnitude smaller than the velocities in tangential
direction and
3. the radial velocities were positive and negative due to the secondary flow.
page 86 / November 9, 2002
Wall roughness effects on flow and scouring