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ECN publication
Calibration and measuring procedure of a spheric velocity probe [ECN-R--95-031]
Published by: Publication date:
ECN 1996
ECN report number: Document type:
ECN-R--95-031 Other
Number of pages: Full text:
56 Download PDF  

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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