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Chapter 4 - Experimental setup and test procedure
During the measurements, the (water and bed) levels were recorded in an Excel file. For each
measurement point, 32 values were recorded at 5 Hz and stored in a spreadsheet. The average of
these 32 values was computed in a further step (Visual Basic Macro) and recorded in a file summarizing
all data points over the whole channel. The coordinates were transformed from the local
measurement frame coordinates to absolute frame coordinates relative to the bend (Fig. 4.1 and
Fig. 4.19). The so obtained Excel spreadsheet was exported to a text file for further treatment with
Matlab. A scheme of the performed Matlab treatment 1 is described in the flowchart on Fig. 4.24.
The Matlab treatment combined the automatically acquired levels, the manual readings on the
outer side wall and the manually recorded levels in the straight inlet and outlet reaches. The computation
was performed in two steps. First, the different input files were combines for each individual
test (water respectively bed level). Then first graphics of the obtained raw data were plotted
and analyzed to make sure that the data contained no errors (procedure dh_level1 on Fig. 4.24). In
a second step the pretreated data sets were combined and compared to each other. At this stage
the following plots were computed and printed: water and bed level changes compared to the initial
bed level (Appendix 4 and 5), longitudinal plots (Appendix 8) and cross-section plots
(Appendix 7) containing measured water and bed levels as well as the initial bed level (procedure
(dh_level2 on Fig. 4.24).
4.6.2 Velocity treatment
In order to extract the 3D velocity field in six cross-sections over the whole channel (10°, 25°, 40°,
55°, 70° and 85°), the acquired binary velocity file (see § 4.4.4) needed some treatment.
First, the nine 1D records were extracted from the raw data file (Fig. 4.26) and written to a text
file. The high velocities in tangential direction exceeding the measurement domain (negative sign)
were then corrected to be located in the positive range (see § 4.4.4). Then the time-averaged measured
components (average over 64 velocity profiles) at a given location had to be projected in a
cylindrical coordinate system relative to the bend (tangential, radial and vertical velocities). Assuming
that the measured velocity components are a, b and c (Fig. 4.21), the velocity components in
a point are given with (Fig. 4.25):
v θ
=
a
---------------------------
+ b–
2 ⋅ c
, v
– a + b
, (4.1)
2 ⋅ sinα
r = ----------------- v
a+
b
2 ⋅ sinα
z = -------------------
2 ⋅ cosα
1. The developed Matlab treatment needed about 4400 lines of program code.
page 94 / November 9, 2002
Wall roughness effects on flow and scouring