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Measurement technique and data acquisition
Therefore, the scale of the measurements had to be fixed to allow good quality records for high as
well as for small records with positive and negative sign. Two measures were taken to reduce the
value of the measured velocity.
1. The UVP-probes were inclined by 20° allowing a reduction of the highest velocities to 34%
(sin 20°) of the unprojected value (Fig. 4.21). The inclination was not increased to higher
values because a very small error in the frame geometry would lead to an important measurement
error. Furthermore, this inclination still gives a satisfying resolution for low velocities.
Despite this inclination, the high velocities were somewhat too big to be measured.
2. Therefore an interesting characteristic of the Doppler measurement device was explored. If
the measured velocity is higher than the maximum velocity, the UVP shifts the measured
value by minus 2 times the velocity range into the negative domain of the measurement
(ROLLAND, 1995). If the sign of the velocity is known, the recorded velocity profile can be
corrected, by shifting the negative values again into the positive domain (Fig. 4.20). This was
done for the high velocities in tangential direction whose sign was clearly given (see also
§ 4.6.2). The same procedure applies in analogy for negative velocities.
Real velocity
V max
Shift +2V max to
obtain real velocity
Measured velocity
Measurement domain
-V max
Figure 4.20: Shifting the measured velocity exceeding the measurement domain back to its correct position
Since the UVP-probes could not be placed vertically, the vertical control volume had a diameter of
up to 3 to 6 cm, depending on the considered distance to the measurement plate.
EPFL Ph.D thesis 2632 - Daniel S. Hersberger November 9, 2002 / page 87