Title:

Calibration and measuring procedure of a spheric velocity probe [ECNR95031]


Author(s):



Published by:

Publication date:

ECN

1996


ECN report number:

Document type:

ECNR95031

Other


Number of pages:

Full text:

56

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Abstract:
In order to measure the magnitude and the direction of the flow towards awind turbine rotor, a spheric velocity probe has been developed. This probe
has a spheric head with a centre tap and two pairs of symmetrically opposite
taps. The position of these taps with respect to the probe axis is given by a
base angle of 15 degrees. The actual inflow quantities (the azimuthal inflow
angle, the longitudinal inflow angle and the inflow velocity) are obtained by
measuring pressure differences in the five taps, and applying estimated
relations. These estimated relations are based on inviscid flow theory, and
consequently only valid for supercritical Reynolds numbers. For subcritical
Reynolds numbers the boundary layer over the sphere causes an effective
sphere radius which is larger than the geometric one. As a consequence, an
effective base angle smaller than the geometric base angle must be used.
Since the original analytical estimates for the inflow quantities perform
poorly, in particular if the wind speed is low, iterative estimates for these
quantities have been derived. These iterative estimates account for the
difference between a general reference pressure (which is employed in a field
experiment) and the pressure at infinity (which is not known in a field
experiment). The calibration of the spheric velocity probe in a wind tunnel
revealed that for wind speeds beyond 10 m/s the probe has an operational
envelope which is given by an effective inflow angle of 15 deg if an
effective base angle of 14 deg is used. In this case the errors in the
azimuthal inflow angle, the longitudinal inflow angle and the inflow velocity
are 1.5 deg , 2.5 deg and 0.4 m/s, respectively. The operational envelope
increases with the wind speed. At a wind speed of 35 m/s the probe can be
employed up to an effective inflow angle of 25 deg with an error in the
azimuthal inflow angle of 1.5 deg, an error in the longitudinal inflow angle
of 2.5 deg, and an error in the inflow velocity of 1.0 m/s. 6 figs., 5 tabs.,
6 refs., 3 appendices
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