Open Field Cup Anemometry

DEWI Magazin Nr. 19, August 2001

53

SummaryWithin the EC co-funded project "Identification of Variables for Site Calibration and PowerCurve Assessment in Complex Terrain" (SiteParIden, contract: JOR3-CT98-0257) largedeviations between cup anemometers of different type under natural conditions have beenidentified. Deviations in the order of 2 % have been found to be common even in flat ter-rain conditions although all compared cup anemometers have been calibrated accordingto the latest standards in the wind tunnel. These large deviations are of major significan-ce for the whole wind energy industry because the result of wind turbine power perfor-mance tests as well as site assessments are strongly dependent on the type of cupanemometer in use. The deviations between the cup anemometers have been found to beinfluenced by site characteristics. The most important flow and turbulence parametersinfluencing the cup anemometer deviations have been identified. From these results fol-lows a first attempt to correct cup anemometer measurements and to classify cup anemo-meters as well as an advice for further research on the field of cup anemometry.

1. Introduction

In the SiteParIden project detailed com-parisons of cup anemometers of diffe-rent type under natural conditions havebeen performed. Surprisingly large devi-ations between measurements with dif-ferent types of cup anemometers havebeen found (Fig. 1).

As all compared cup anemometers havebeen calibrated in the same wind tunnelaccording to the latest standards [1] thisobservation must be linked to the turbu-lent nature of free air flow (in contrast tothe low turbulence present in wind tun-nels). The results have been verified atvarious sites in flat and complex terrainup to heights of 83 m above ground (seereference [2] and [3] for details). The effects on cup anemometers have been found to be site depen-dent. However, even in flat terrain conditions 83 m above ground significant deviations between themeasurements with different types of cup anemometers in the order of typically 2 % have been obser-ved [3]. This finding is of major interest for the wind energy industry because it implies that wind turbi-ne power performance tests are comparable onlywhen they are performed with the same type of cupanemometer and under comparable turbulence cha-racteristics. Furthermore, additional uncertainties forsite assessments are present when the cup anemo-meter in use for the site assessment does not matchthe type of cup anemometer used for the determina-tion of the power curve of the projected wind turbinetype. Since the effect has been discovered thebodies occupied with the standardisation of powerperformance testing aim to overcome the problem.Basically three different solutions are under discus-sion [2]:

• use of a standardised cup anemometer,• a classification of cup anemometers,• a correction of cup anemometers.

Open Field Cup Anemometry

Axel Albers, DEWI

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0 5 10 15 20 25v-Thies 4.3303.22.000 [m/s]

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es [%

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Risoe P2445b, 30mRisoe P2445b, 8mVector A100, 30mVector A100, 8mMetone 010C-1, 30mMierij, 8mVaisala WAA151Friedrichs 4033.1100x, 30mFriedrichs 4033.1100x, 8mThies Compact, 8m

Fig. 1: Deviations between measurements with various cup anemome-ters in flat terrain 8 m and 30 m above gorund

Fig. 2: Test rig for outdoor cup anemometer comparison.The distance between the cup anemometers is2.5 m.


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As has been outlined in [3] until lately it was not clear which flow and turbulence parameters andanemometer properties have to be considered for a proper solution of the problem. Major progress inthis respect has been gained recently from a new large data base of high resolution time series byDEWI [4], which is subject of this presentation.

2. Methodology

Two cup anemometers of type Thies 4.3303.22.000 and Risø P2445B have been mounted in parallelposition on a special test rig on top of a 30 m high meteorological mast in flat terrain (Fig. 2). An ultrasonic anemometer (USA) has been placed in middle of the test rig in order to evaluate turbulence cha-racteristics as well as the vertical flow inclination. The whole test rig is aligned perpendicular to theactual wind direction by a micro processor controlled drive system. Before anemometers of differenttype were compared, detailed tests with two anemometers of the same type mounted on the test righave been performed in order to ensure that the two cup anemometer positions are exposed to thesame wind conditions. Furthermore, before the outdoor tests, all anemometers including the USA havebeen calibrated in the same wind tunnel according to the MEASNET procedure [1].

A large data base of time series sampled with a frequency of 10 Hz (output rate of USA 4 Hz) has beencollected over a period of 2440.5 h. The aim of this intense data collection was to study which turbu-lence and flow parameters are significant to explain the differences between the cup anemometer mea-surements. From the time series about 100 turbulence and flow parameters have been calculated foreach 10 minute period. The ratios between the 10 minute averages of the anemometer measurementshave been analysed in detail on their dependence on turbulence and flow parameters.

The Risø P2445B and Thies 4.3303.22.000 anemometer have been selectedfor this investigation because theseanemometers have formerly shownlarge deviations, both anemometers arewidely used in Germany and Denmarkrespectively and because there is hopethat the anemometers may representtwo groups of anemometers consistingof the faster anemometers Friedrichs4033.1100X, the Thies 4.3303.22.000and the Thies compact series and theslower anemometers Vector A100, RisøP2445b and Metone 010C-1.

3. Results3.1 Comparison between Cup Ane-

mometers and Ultra Sonic Ane-mometer

A key finding is that the Thies 4.3303.22.000 anemometer clearly overestimateseven the magnitude of the full threedimensional wind speed vector (in shortvector wind speed v-vector) in highly tur-bulent air flow as it is present in wakes ofwind turbines (Fig. 3). Under the sameconditions the Risø P2445 B anemome-ter still tracks well the horizontal windspeed component (Fig. 4). Furthermore,in unperturbed air flow the wind speedratio of the Thies anemometer and theUSA is more dependent on site specificflow parameters than the Risø anemo-meter (compare Fig 3 and Fig 4 for thesector 110°-350°).

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0 40 80 120 160 200 240 280 320 360

Wind Direction [°]

Win

d Sp

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io [-

] v-Risoe/Uv-Risoe/v-horizontalv-Risoe/v-vector

wake sector free sector

Fig. 4: Ratio of measurements with a Risø P2445B cup anemometer andthe longitudinal wind speed component (U), the horizontal windspeed component (v-horizontal) and the magnitude of the full 3D-wind speed vector (v-vector) as evaluated with a calibrated ultrasonic anemometer.

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v-Thies/Uv-Thies/v-horizontalv-Thies/v-Vector


Fig. 3: Ratio of measurements with a Thies 4.3303.22.000 cup anemo-meter and the longitudinal wind speed component (U), the hori-zontal wind speed component (v-horizontal) and the magnitude ofthe full 3D-wind speed vector (v-vector) as evaluated with a cali-brated ultra sonic anemometer.


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3.2 Correlation of Time Series

The correlation coefficient between the cup anemometers and the ultra sonic anemometer within 10minute periods has been investigated. Key findings of this analysis are:

• The Risø P2445 B anemometer correlates clearly better with the horizontal flow componentthan with v-vector or the longitudinal wind speed component.

• The Thies 4.3303.22.000 anemometer has a small tendency to correlate better with v-vectorthan with the horizontal flow component. It correlates clearly better with v-vector or the hori-zontal flow component than with the longitudinal wind speed component.

3.3 Influence of Flow and TurbulenceVariables on Cup AnemometerMeasurements

The influence of flow and turbulenceparameters on the wind speed ratios ofthe two cup anemometers and the windspeed ratios of cup anemometers and thelongitudinal wind speed component, thehorizontal wind speed component and v-vector has been investigated followingthree different procedures:

• The correlation coefficient bet-ween the wind speed ratiosbased on 10 minute averagesand the flow and turbulencevariables has been evaluated.

• Multi variate regression analy-sis has been performed follo-wing the technique described in[5] with the wind speed ratiosbased on 10 minute averagesas dependent variables and theflow and turbulence variablesas independent variables.

• A multi classification techniqueas described in [6] has beenapplied in order to care for theco-linearity of the flow and tur-bulence parameters.

The following results can be drawn in fullconsistence from all three evaluationtechniques:

• The wind speed ratios with theThies 4.3303.22.000 anemo-meter correlate significantlywith different turbulence varia-bles. In very turbulent air flowthe anemometer measures amuch higher wind speed thanthe vector wind speed.


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• The wind speed ratios of the Risø P2445 B anemometer and the horizontal wind speed com-ponent shows only a very low correlation with all considered flow and turbulence parameters.Turbulence effects on this anemometer are much smaller than in case of the Thies4.3303.22.000 anemometer (Fig. 5)

• The vertical turbulence intensity is the most significant variable for the explanation betweenthe differences in the cup anemometer measurements. It has the largest contribution to thevariation of the wind speed ratios (Fig. 5, Fig. 6).

• Other significant variables are the flow inclination, the correlation between the lateral windspeed component and the other two flow components and the integral turbulence lengthscale of the vertical flow component (Fig. 5).

• The influence of the longitudinal turbulence intensity is much smaller than the influence of thevertical turbulence intensity.

• The most significant flow and turbulence variables are the same for both types of anemome-ters (Fig. 5).

• The air density, air temperature (considered range 5-20°C), air pressure and the wind speedhave shown no significant influence on the ratios between the cup anemometers measure-ments.

4. Correction of Cup Anemometer Measurements

A correction of cup anemometers is needed to make wind turbine power curve measurements perfor-med with different cup anemometers comparable and to match existing power curve measurements tothe cup anemometer in use for site specific wind speed measurements. The results given in the pre-vious chapter imply that a cup anemometer correction should consider at least the vertical turbulenceintensity. The simplest correction would be a linear correction according to the vertical turbulence inten-sity. Fig. 7 demonstrates how the variation of the wind speed ratios with the wind direction is reducedby the wind speed correction.

5. Classification of Cup Anemometers

A classification of cup anemometers should take into account the sensitivity of the cup anemometersto flow and turbulence parameters. As a first attempt the sensitivity of cup anemometers on only themost significant turbulence parameter has been used for the classification of cup anemometers on thebases of field tests. The sensitivity of the cup anemometers on the vertical turbulence intensity isexpressed by the slope of the regression through the ratios of the cup anemometer measurements andthe horizontal flow component (or v-vector) versus the vertical turbulence intensity. From the slope the(simplified) measurement error can be calculated by a multiplication with the vertical turbulence inten-sity. Then the maximum allowable vertical turbulence intensity can be calculated for each class whenthe class index is defined as the maximum measurement error in percent. The resulting classificationscheme in Tab. 1 should be considered only as a first attempt to classify cup anemometers by meansof open field measurements. Furthermore, it must be pointed out that except of the Thies

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std(

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,W)

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tens

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v-Thies/v-horizontal, 110°-350° (free air flow)

v-Thies/v-horizontal, 350°-110°, (wake situation)

v-Risoe/v-horizontal,110°-350° (free air flow)

v-Risoe/v-horizontal,350°-110°, (wake situation)

std(x): standard deviation of flow component x within 10 minutesR(x,y): corellation coefficient between flow component x and y within 10 minutesL(x): integrale turbulence length scale of flow component xtensor(xy): turbulent stress between flow component x and y (Reynolds tensor element)inclination: vertical flow inclination angle averaged over 10 minutes

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v-Risoe/v-horizontalv-Thies/v-horizontal

Fig. 5: Influence of flow and turbulence variables on windspeed ratios of cup anemometer measurements andthe horizontal wind speed component. The significan-ce coefficient as gained from multi variate regressionanalysis describes the relative change of the windspeed ratio when the respective flow or turbulencevariable is increased by one standard deviation.

Fig. 6: Influence of the vertical turbulence intensity on windspeed ratios of cup anemometer measurements andthe horizontal wind speed component.


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4.3303.22.000 and the Risø P2445B anemometerthe presented slopes of the regressions have beenevaluated from measurements with some shortco-mings (see [4]). However, the ranking of the anemo-meters according to the regression slopes coinci-des very well with the differences in the cup anemo-meter measurements as observed in flat terrain(compare Tab. 1 and Fig. 1). An exception is theMetone 010C-1 anemometer which has been veryclose to the Risø P2445B and the Vector A100anemometer in the outdoor comparison while itsregression slope is relatively large.

6. Conclusions

• Large deviations between measurements withcup anemometers of different type occur even inflat terrain and when the anemometers are cali-brated uniformly in the same wind tunnel. Thedeviations are in the range of some percent.

• Wind turbine power curve measurements are strongly effected by the choice of the cup anemome-ter.

• The observed differences in wind speed measurements are dependent on the site specific flow andturbulence regime.

• The vertical turbulence intensity seems to bethe flow variable with the largest influenceon the differences in the cup anemometermeasurements. Among other important flowvariables seem to be the flow inclination, thecorrelation between the lateral and the otherflow components and the integral turbulencelength scale of the vertical flow component.

• Classifications of cup anemometers and cor-rections of wind speed measurementsshould account for the identified importantflow and turbulence parameters.

• The Risø P2445B cup anemometer seemsto be nearly insensitive against turbulenceeffects, at least in flat terrain conditions.

• The Thies 4.3303.22.000 cup anemometerseems to be seriously influenced by the ver-tical turbulence intensity. In very turbulentflow the anemometer overestimates eventhe vector wind speed significantly.

• A simple correction algorithm for cupanemometer measurements in flat terrainconditions has been proposed. This correc-tion, however, requires a good knowledgeabout the vertical turbulence intensity.Although shortcomings of the correction areevident, its application can lead to moreaccurate results of wind speed measure-ments at least in flat terrain when anemome-ters with a high sensitivity on turbulenceeffects are in use.

• A first attempt to classify cup anemometersaccording to their sensitivity on the verticalturbulence intensity, as it is determined byoutdoor measurements, coincides well with

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linear correction for std(W)/U from free sector


Fig. 7: Ratios of measurements with a Thies 4.3303.22.000and a Risø P4225B cup anemometer for the not cor-rected Thies anemometer and the Thies anemome-ter corrected for the effect of the vertical turbulenceintensity. The correction has been established fromthe free sector 110°-350°.

Instrument for Horizontal Wind Speed Instrument for Vector Wind SpeedSlope Maximum Vertical

Turbulence IntensityClass Slope Maximum Vertical

Turbulence IntensityClass

Thies 4.3303.22.000 Thies 4.3303.22.0000,315 0,032 1 0,2215 0,045 1

0,063 2 0,090 20,095 3 0,135 30,127 4 0,181 40,159 5 0,226 5

Risoe P2445b Risoe P2445b0,0511 0,196 1 -0,039 -0,255 1

0,392 2 -0,511 20,588 3 -0,766 30,784 4 -1,021 40,979 5 -1,276 5

Metone 010C-1 (limited validity)0,2387 0,042 1

0,084 20,126 30,168 40,209 5

Results from 8m mast (limited validity)Vector A100 (limited validity)0,0632 0,158 1

0,317 20,475 30,633 40,792 5

Mierij (limited validity)0,1743 0,057 1

0,115 20,172 30,230 40,287 5

Vaisala WAA151 (limited validity)0,2792 0,036 1

0,072 20,107 30,143 40,179 5

Friedrichs 4033.1100x (limited validity)0,3699 0,027 1

0,054 20,081 30,108 40,135 5

Tab. 1: Classification of cup anemometers according to theirsensitivity on the vertical turbulence intensity


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1. Einleitung

Windenergieanlagen (WEA) sind im Betriebszustand unterschiedlichen Belastungen ausgesetzt, diedie Lebensdauer der Anlagen, vor allem durch die wechselnden (instationären) Belastungen, herab-setzen. Neben den deterministischen Anteilen (wie z.B. Schwer- und Kreiselkräfte) sind es insbeson-dere stochastische Lastanteile, die die mechanische Auslegung von Windkraftanlagen sehr komplexwerden lassen. [1] Die stochastischen mechanischen Belastungen haben ihre Ursache in der schwan-kenden Windgeschwindigkeit. Sie werden in der Regel in Windturbulenzen und in Windböen unter-gliedert. Der Einfluß der Windturbulenz wird oftmals durch den Turbulenzgrad

wiedergegeben, der das Verhältnis der Standardabweichung des Windgeschwindigkeitsfeldes zu sei-ner mittleren Geschwindigkeit beschreibt. Die Frage nach der Natur von Windböen ist trotz ihrerscheinbaren Trivialität bis heute nicht allgemein geklärt und eine strenge Definition des Begriffes findetsich in der Literatur nicht [3]. Dabei ist die Kenntnis der maximal zu erwartenen Geschwindigkeit einerWindböe besonders hinsichtlich der Abschätzung von Extrembelastungen von großem Interesse. Ähn-lich relevant und auch schwierig ist die Einschätzung der Häufigkeit von Böenereignissen und derenzeitliche Abfolge [2]. Die Unterscheidung von Böen und Windturbulenzen wird in der Regel folgender-maßen formuliert. Während die Windturbulenz u´(t) als permanente, fluktuierende Überlagerung dermittleren Geschwindigkeit u(t) aufgefaßt werden kann, stellen Böen nach [3] eine “erhebliche Abwei-chung von der mittleren Windgeschwindigkeit im Bereich von einigen bis einigen zehn Sekunden" dar.Letztere werden demnach im Gegensatz zur Windturbulenz als einzelne und extreme Ereignisse cha-rakterisiert.

Um zu untersuchen wie Windböen vom statistischen Standpunkt aus erfaßt werden können, werdenwir im folgenden zwei Geschwindigkeitszeitreihen auf ihre statistischen Eigenschaften untersuchen:Einen Freifeld-Windgeschwindigkeits- und einen Labor-Geschwindigkeitsdatensatz. In diesem Artikel

the observed differences between the cup anemometer measurements.• Future research should emphasise on the verification of the findings regarding the turbulence para-

meter analysis with other types of cup anemometers and at other locations, especially in complexterrain. Simulations of turbulence effects on cup anemometers in wind tunnels require a good con-trol over the here identified important turbulence measures.

7. Acknowledgements

The work has been partially supported by the European Commission under contract number JOR3-CT98-0257.

8. References

[1] MEASNET: Measurement Procedure Cup Anemometer Calibrations, 1997[2] A. Albers, H. Klug, D. Westermann; Outdoor Comparison of Cup Anemometers; Pro-ceedings of

DEWEK 2000[3] A. Albers, H. Klug, D. Westermann; Cup Anemometry in Wind Engineering, Struggle for Improvement,

DEWI Magazine no. 18, February 2001[4] A. Albers; Identification of Variables for Site Calibration and Power Curve Assessment in Complex

Terrain, JOR3-CT98-0257, Project Task 6, Relative Power Curve Measurements in Flat terrain, FinalReport, Report-No. JOR3-CT98-0257-0601-DEWI07, June 2001

[5] F. Mouzakis, E. Morfiadakis, P Dellaportas; Parameter Identification on Power Performance of WindTurbines Operating at Complex Terrain, 2nd European & African Conference on Wind EngineeringGeneva, Italy, 1997

[6] A. Albers; Comments on Techniques for Multi-Parameter Site Calibration, DEWI Report-No.: JOR3-CT98-0257-1198-DEWI01, 1998

Problematik der Windböen

Frank Böttcher, Christoph Renner, Hans-Peter Waldl und Joachim PeinkeFachbereich Physik, Carl-von-Ossietzky Universität Oldenburg

uS

σ= (1)

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Open Field Cup Anemometry