DEWI - Direction Sensitivity of (Vector) Anemometers(vector

DEWI - Direction Sensitivity of (Vector) Anemometers(vector风速计受风向影响的报告)

Harald Mellinghoff, Olaf Haack, Renhard Kluin

DEWI GmbH, Germany

Summary

This paper looks at the impacts that different types of sensors experience when they are subjected to

tilted inflow and effects following a rotation. We see that not only sonic anemometers with the transducer

pairs in wake position show significant directional sensitivities. Moreover, classic cup anemometers

which feature an asymmetric shape around the vertical axis have been tested. Depending on the

closeness of the asymmetry in structure to the rotor, high impacts on the measurement behavior have

been observed. As a consequence, we suggest to assess these features for anemometers in use in the

wind energy and to account for the effects in the mounting uncertainty contribution of the measurements.

similar to the one introduced for cup anemometers 1. Introduction

will have to take that into account.

A further common feature of such sensors is, that 1.1 MEASNET guideline development

they are not symmetric around the vertical axis. With the revision No. 2 from October 2009 of the

Thus it can be expected that the relative position to MEASNET guideline for anemometer calibrations,

the inflow has some impact on the reading. Usually the procedure has been opened to other sensor

these sensors have internal correction functions and types as well. The basic intention was to open up for

the following experiments give an indication to which the use of 2D and 3D sonic anemometers.

extend they can compensate these effects. The guideline notes that the operational

characteristics shall be assessed for directional

2. Measurement program sensitivity and other effects that could influence the

measurement behavior like temperature.

In the framework of a house internal research The MEASNET procedure is currently written for the

program two sets of measurements have been assessment of a single directional orientation

conducted in the wind tunnel. The open wind tunnel measuring the horizontal wind speed component

of University of Oldenburg with an area of 8000 cm2 only. Yet, if we start studying the directional

and a turbulence intensity of about 0.2 % has been sensitivity, not only a rotation around the vertical

used. axis is of interest, the tilt angle can be studied as

well. Both measurements have been performed at

2.1 Azimuth rotation tests DEWI in the past. A series of tests with different cup

These tests rotated the anemometer around the and sonic anemometers has been performed in the

vertical axis. The stepping width is 5 degrees. The past.

starting point is either defined with respect to the

North marker of the sensor or with reference to the 1.2 Increased scope of sensors

sensor cable if it does not inside the supporting tube. The classic cup anemometer is a complex

As a standard setting, DEWI places the sensor instrument when it comes to describing the details of

cables downwind the tube. The tests have been the flow in the rotor volume. So far the empirical

measurmements with try and error optimization of conducted at three wind speeds in the typical wind

the design are the standard apporach to know the speed range of wind energy use at 5, 10 and 15 m/s.

measurement behaviour of a cup anemometer. The The ratio of the wind speed at the assessed azimuth

sensors are pretty simple when it comes to the angle and the reference position has been

principle of operation and the signal forming calculated and plotted over direction.

process. The calibation function gives a linear

relation between the sensor output signal, usually a 2.2 Tilt angle tests

frequency and the wind speed, averaged at least Anemometers are subjected to the flow in the wind

over a full revolution of the cups. tunnel at three different wind speeds. The

Other types of sensors are under discussion for the anemometer is mounted tilted in such a way that the

revision of the standard [2] for the measurement of center of the measurement volume of the

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