aerodynamic models for wind turbine design codes can be improved, developed and validated. Although the emphasis of the IEA Annex XIV/XVIII activities

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1 J.G. Schepers 1, R. van Rooij 2, A. Bruining 2 1 Energy Research Centre of the Netherlands, Westerduinweg 3, Petten, NL-1755 ZG, The Netherlands, tel: , Fax: , schepers@ecn.nl 2 Institute for Wind Energy, Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlands, tel: , fax: , R.vanRooij@CiTG.TUDelft.nl; A.Bruining@CiTG.TUDelft.NL In the last decade, several institutes performed aerodynamic field measurements on wind turbines. In such test programs, local aerodynamic quantities (pressures, forces, inflow data) are measured at several radial positions along the blade. Information on local aerodynamic characteristics is a major step forward in understanding the very complicated flow behaviour on a rotating wind turbine blade. In conventional measurement programs only integrated, total loads are supplied. The aerodynamic test programs were coordinated within two IEA Annexes: IEA Annex XIV and IEA Annex XVIII. In these IEA Annexes, 7 measurement programs were involved and the measurements were stored into a database (see Until now, most attention in these aerodynamic test programs was focussed on the experimental set-up and data recording, where the logical second step, i.e. the analysis of data and validation got too little attention. For this reason, the so-called Annexlyse project is carried out. Participants in this project are the Netherlands Energy Research Foundation ECN and the Delft University of Technology, DUT. The project is sponsored by the Netherlands Agency for Energy and the Environment, NOVEM. The project started in May 2002 and will end in November The objective of the Annexlyse project is to carry out a thorough analysis of the aerodynamic measurement data in order to improve and validate aerodynamic models. In the paper, a summary will be given of an inventory of analyses, which have already been performed on the IEA Annex XIV/XVIII data. Conclusions, uncertainties and recommendations from the analyses will be presented. In addition to the inventory of previous analyses, a comparison will be shown between measured sectional airfoil data with calculated data, using different 3D aerodynamic models. Deficiencies in the 3D models will be highlighted. More detailed information, including a full graphical presentation of results, can be found on the following internet site: Keywords: Aerodynamic modelling, wind turbines, experiments 1 INTRODUCTION Within the Annexlyse project, the aerodynamic measurements, which have been collected in IEA Annex XVIII are analysed. In IEA Annex XVIII (and its predecessor project IEA Annex XIV), 7 organisations participated in performing aerodynamic experimental research on full scale horizontal axis wind turbines at field conditions. The organisations, which participated were: - Netherlands Energy Research Foundation, ECN (NL); - Delft University of Technology, DUT (NL); - RISØ, The Test Station for Wind Turbines (DK); - Imperial College together with Rutherford Appleton Laboratory (UK); - National Renewable Energy Laboratory, NREL (USA); - Mie University, The Department of Mechanical Engineering (JP); - Centre for Renewable Energy Systems, CRES (Gr) The result of IEA Annex XIV/XVIII was a database of aerodynamic properties (i.e. pressure distributions along the blade, inflow data etc), collected on the facilities from the participants. A more detailed description of the facilities, the experimental programs and the database can be found in [1]. The database is publicly available on an Internet site, see under the condition that users of the database supply feedback to the IEA Annex XIV/XVIII participants about experiences gained with the database. In this way the Annex XIV/XVIII participants are informed on the research which is carried out with the measurements. The ultimate goal of the IEA Annexes was obviously to provide a better understanding of the aerodynamic behaviour of a wind turbine, such that aerodynamic wind turbine models can be developed and validated. Nevertheless the emphasis in IEA Annex XIV/XVIII has been on the creation of the database and the analyses of the measurement results was not part of the IEA Annexes. For this reason the Dutch Annexlyse project is carried out: It s aim is to perform a thorough analysis of the IEA Annex XIV/XVIII measurements, such that 1

2 aerodynamic models for wind turbine design codes can be improved, developed and validated. Although the emphasis of the IEA Annex XIV/XVIII activities has been on the creation of the database, a limited number of external database users did already perform analyses on the aerodynamic measurements. As stated before these users were requested to report their analyses to the Annex XIV/XVIII participants. These previous analyses are studied in the first task of the Annexlyse project, see the sections 3 and 4. The inventory of previous analyses will prevent an overlap between the Annexlyse activities and the work already performed by other institutes. Furthermore, the already existing analyses will guide and facilitate the future analyses which ECN and DUT intend to carry out. An example of such ECN/DUT analysis (i.e. a comparison between calculated and measured airfoil characteristics) is given in section 5. The present paper only gives a very short summary of the project results. More information, including a full graphical presentation of results, can be found on the following internet site: 2 USE OF DATABASE In August 2002, permission to use the database has been granted to approximately 35 institutes and also to some research groups (i.e. EU Joule project groups) As explained in the previous chapter, access to the database was only granted, on the promise to inform the IEA Annex XIV/XVIII members on the experiences with the database. This promise was made on basis of a Gentlemen s (or Lady s) agreement and not all of the above mentioned institutes fullfilled their promise. Some informed that they only had a very brief look. Nevertheless a considerable amount of institutes did provide some feedback. 3 INVESTIGATIONS ON THE IEA ANNEX XVIII DATA In this chapter a selection is made of projects and investigations, which made at least to some extent use of the IEA Annex XVIII data. Thereto investigations were selected in which different aerodynamic models were validated and developed. A detailed description of the analyses is considered to be outside the scope of this paper. In the sequel only a very short summary is given of the analyses. For more details the reader is referred to the task report which can be found on EU-Joule project Dynamic Stall and Three Dimensional Effect The EU-Joule project Dynamic Stall and Three Dimensional Effect, JOU2-CT , is described in [2] and [3]. The aim of this project was to develop and validate engineering models for dynamic stall and three dimensional effects. In particular the investigation of dynamic stall effects turned out to be a problem. This is mainly due to the following reasons: The definition of angle of attack is a point of discussion; The most appropriate definition would be to define the angle of attack as the angle between the chord line and the effective wind velocity, where the effective wind velocity is the vector sum of (almost) all velocity components: - Incoming wind speed; - Rotational speed; - Induced velocities without the induction from the bound vortex; This leaves the problem that the physical inflow velocity also contains the induction from the bound vortex. This induction (also called the upwash) should then be subtracted, but the velocity induced by the bound vortex is unknown in the 3D environment. Furthermore the location where the effective velocity should be determined, is arbitrary; In field measurements, the incoming wind speed (which in turn effects the angle of attack) is unknown: Usually the incoming wind speed is derived from measurements on meteorological masts. However these meteorological masts are placed some distance away from the turbine. Generally speaking such measurements only give an indication for the incoming wind speed on a time averaged basis. Furthermore the meteorological masts only have a limited number of anemometers, which makes the spatial distribution of the incoming wind field uncertain. In the sequel of the Annexlyse project a validation of dynamic stall models on basis of statistical data (i.e. ranges, standard deviations of c n, angle of attack etc.) is performed instead of a validation on chaotic dynamic stall loops. 3.2 EU project Viscel The EU project VISCEL is described in [4] and [5]; In this project a large number of European 3D viscous CFD codes were validated. At low wind speeds the agreement with measured pressure distributions was good, but at high wind speeds the calculations did nog converge. It should be noted that the project was carried out at the end of the nineties. Nowadays the codes would be able to simulate higher wind speeds. 3.2 Investigation from Rzeszow University 2

3 The investigations carried out by Rzeszow University, are described in [6] and [7]. The results from a vortex line model have been compared with the IEA Annex XIV/XVIII experimental data, where the attention was focussed on a comparison between calculated and measured angles of attack and power. Generally speaking a reasonable agreement is found. 3.3 Investigation from Carlos III university The investigations carried out by Carlos III university, are described in [8]; Carlos III University validated a 3D unsteady potential free wake model. A reasonable agreement between calculated and measured pressure distributions is found. However, stalled conditions were not considered. 3.4 Investigation on yawed condtions An investigations on yawed measurements was carried out by RISØ, see [9]; In [9] a comparison is made between the RISØ measurements and calculations under yawed conditions. Two yaw models have been applied: The model which is implemented in the standard RISØ aeroelastic code HawC. This model does not account for the variation in induction at yawed conditions; HawC-3D, i.e. the aeroelastic code HawC is coupled with a 3D actuator disc model for the calculation of induction. The latter, coupled model accounts for the variation in the induction factor over the rotor plane at yawed conditions. Although the induction was not measured directly, an indication for it could be derived from the inflow angles which were measured with a five hole pitot probe. The measurements and calculations are compared on basis of azimuthally binned averages. It appears that under yawed conditions the HAWC-3D model provides more realistic results, not only for the inflow quantities but also for the flap-wise moments. Furthermore, the HAWC-3D model predicts a restoring yawing moment, which is generally believed to be more realistic than the zero yawing moment predicted by the standard HAWC code. 3.5 EU project ROTOW The EU project ROTOW is described in [10]; In this project, the rotor tower interaction model in the ECN code (PHATAS, using a potential dipole tower interaction model) is assessed by comparing calculated and measured azimuthally binned averages of the normal force coefficients and flat-wise moments, In general, it was found that the potential dipole model over-predicts the tower shadow effects 3.6 Investigation from Delft University of Technology The investigations carried out by DUT are described in [11]; As starting point, a comparison is made between calculated and measured 2D airfoil performance. The calculations were performed using RFOIL. RFOIL is an extension of XFOIL, which includes rotational effects. Thereto wind tunnel measurements and RFOIL calculations have been carried out with the actual blade coordinates. In general a good agreement is found; Furthermore the measurement accuracies and corrections have been evaluated. In particular the determination of the dynamic pressure is extremely important since the segment characteristics are made non-dimensional with this dynamic pressure. Thereto it should be noted that most IEA Annex XVIII facilities measure differential pressures relative to a reference pressure. A possible uncertainty in the reference pressure does not effect the dimensional values of the aerodynamic loads, but it may effect the dynamic pressure (as maximum in the pressure distribution) and as such the non-dimensionalisation. A possible off-set in dynamic pressure level has been evaluated by investigating the accompanying pressure distribution. In particular the trailing edge pressure is assessed. In attached flow, the trailing edge pressure under rotating conditons is expected to be almost similar to the trailing edge pressure at 2D conditions. However, this was was not always true. After correction for this apparent off-set, a very good agreement between the rotating and the 2D pressure distributions was found; A comparison is made between the RFOIL calculations and the stationary field measurements. In some cases a good agreement was found, but in other cases the agreement was poor, in particular at stalled conditions. 3.7 NREL measurements/baseline selection In the USA (and Canada), very many analyses have been performed on the NREL measurements. Among others the following analyses were found: In [12] and [13], the baseline selection technique is also introduced. This technique aims to select measurements which are taken at conditions which are as steady as possible. Thereto data sets are selected at which the variations in wind speed and yaw error are limited over three subsequent rotations, representing steady conditions. The middle cycle data has been averaged for all the 3

4 azimuth positions and is the final baseline result. The baseline data generally show less scatter than the data from the full base of measurements. It must be noted that the criterium relies on conditions, which are measured at a meteorological mast which is 1.2 or 1.5 D away from the turbine. Hence some time lag may be expected; A comparison is made between airfoil characteristics on a twisted and an untwisted blade. In general the differences are small [14]; Furthermore, local blade loads (i.e. section normal forces) are correlated with blade root loads and hub loads In the low wind speed regime the agreement between the measurements and the determined values was excellent. The agreement becomes poorer for increased wind speeds [12]; CFD calculations are compared with measurements in [15], [16] and [17]. 4 CONCLUSIONS/UNCERTAINTIES/ RECOMMENDATIONS FROM PREVIOUS ANALYSES ON IEA ANNEX XVIII DATA 4.1 Conclusions from previous analyses The main conclusions which were drawn from the inventory of analyses on the Annex XVIII data are: The IEA Annex XVIII measurements have already been used for several analyses; Aerodynamic field measurements are difficult to use for a detailed validation of dynamic stall models. However a more global validation on statistical data is believed to be possible; The most successful activities seemed to have been: - Comparison of rotating (mean) airfoil data with 2D airfoil data. Several differences are found, among others the well known stall delay at the inner sections, i.e. the aerodynamic lift forces are considerable higher compared to 2D wind tunnel experiments. - Validation of CFD codes (RANS codes, free wake panel methods and RFOIL) on basis of pressure distributions. It must be noted that little validation of blade element momentum methods is reported; - Validation of yaw models on basis of azimuthally binned averaged normal forces and inflow data (i.e. angles of attack); The IEA Annex XVIII measurements have mainly been used for validation of existing models. Development of new aerodynamic models on basis of the measurements has hardly been reported; Most comparisons between calculations and measurements show a good to reasonable agreement below stall, but above stall the agreement becomes poor. 4.2 Uncertainties The main uncertainties in the interpretation of the measurements are associated to the above mentioned problems of the angle of attack, the dynamic pressure and the fact that the incoming inflow conditions are nonstationary and partly unknown. To some extent the latter problem is overcome by selecting data according to the baseline criterion. Obviously the measurements which recently have been taken by NREL in the wind tunnel (the well known NASA Ames experiment) donot suffer from these uncertainties in the inflow. For validation of some CFD methods the modelling of transition turns out to be a problem. Unfortunately no experiments with fixed transition are included in the IEA Annex XVIII database. When comparing calculational with measured results, another very important uncertainty was found in the fact that the model descriptions of the IEA Annex XVIII facilities (i.e. the geometric data, the mass and stiffness data etc) were not always complete. Hence differences between calculations and measurements may be a result of uncertainties in the aero-elastic model descriptions. 4.3 Recommendations from previous analyses For the remaining Annexlyse tasks it is recommended to: Investigate alternative definitions for the angle of attack, and try, where possible, to avoid the use of an angle of attack and the non-dimensionalisation with dynamic pressure. Thereto a comparison will be made on basis of dimensional values of the aerodynamic forces; Validate dynamic stall models on basis of statistical quantities instead of (chaotic) dynamic stall loops; Derive, where possible, baseline data for all facilities, The baseline criteria as defined by NREL seem appropriate, although the inclusion of the time delay between the mast and turbine may give an improvement. For some facilities it is known that no proper indication of the inflow is available. Then it is recommended to define selection criteria from the turbine behaviour; 4

5 Determine the c n -α and c t -α curves for the IEA Annex XVIII facilities where also span wise pressure distributions are available. In particular the focus should be on high angle of attack to distinguish stall delay effects for e.g. different blade pitch angles, see section 5; Investigate the yaw behaviour for the turbines by binning c n and the inflow quantities against azimuth angle. From this data, dependencies in the inflow distributions should be determined and thereafter an empirical formula for inflow variation in yaw should be studied. It is expected that the measurement unertainties play a limited role in such investigation, since it is mainly the variation of the aerodynamic quantities which is of importance instead of the absolute level. Hardly any investigations are performed on standstill aerodynamics, nor the correlation between the different measurement sections along the blade span has been investigated. It is recommended to include such investigations. 5 COMPARISON BETWEEN MEASURED AND CALCULATED AIRFOIL DATA Within the Annexlyse project the measured sectional data (c n, c t c l and c d as function of angle of attack) are compared with calculated data. Many models exist to describe the aerodynamic coefficinents under rotation. At the time of writing the paper, the IEA Annex XVIII measurements are compared with data which rely on 3D models from ECN (denoted by ECN1 and ECN2, see [18] and [21]), CRES, see [19] and the University of Illinois, see [20]. The models all assume that the c l -α and (possibly) the c d -α curves can be derived from the 2D aerodynamic coefficients. Usually (not for the ECN2 model) a fraction of the difference between the potential c l -α curve (c l,pot ) and the actual 2D c l -α curve ( c l,pot )is added to the 2D c l -α curve. Hence the expression for the rotating c l -α is usually written as: c l,3d = c l,2d + f (c l,pot - c l,2d ) (1) For the drag curve a fraction of the difference between the minimum c d and the actual 2D c d -α curve is considered. As explained below, the function f depends on the local solidity c/r. In some models it also depends on the tip speed ratio and the pitch angle. In the ECN2 model the additional term is proportional to the fraction of the chord which is separated (which initially is expected to have some correlation with c l ) Some remarks on the models are: - The ECN models only take the 3D effects on the lift coefficient into account, the models from CRES and Illinois also consider 3D effects on the drag coefficient; - The ECN1 model assumes a (c/r) 2 dependancy, where the ECN2 and CRES model assume a linear c/r dependancy. The Illinois model assumes a more complicated c/r dependancy, which is among others a function of the tip speed ratio ; - The CRES model takes a dependancy on the pitch angle (+twist) into account, i.e. the angle between the Coriolis force (which is an important source for 3D effects ) and the actual airfoil; - The Illinois model takes a dependancy on the tip speed ratio into account. At the time of writing this paper, the comparison is not completed yet and a detailed discussion of the results will not be made. Only a few results on the NREL and RISØ c n -α measurements are shown in the figures 1 to 4. The interested reader is again referred to the Annexlyse internet site: This site will regularly be updated with new graphical comparisons from different models and measurements. It is noted that the 2D data are calculated from the Aerodynamic Table Generator [22], which generates airfoil data at the appropriate thickness and Reynolds number using a database of 2D wind tunnel measurements. Furthermore the NREL measured angle of attack got a constant shift in order to match the 2D values. At the time of writing this paper, the following preliminary conclusions and suggestions for further investigations were drawn: - The agreement between the calculations and measurements, in general terms, is only moderate. This holds for all investigated models. A model may perform well for one particular turbine, but for another turbine the agreement may be poor; 5

6 - The relative poor agreement can partly be explained by the observation from the figures 5 and 6. In these figures the term f from equation 1 is derived from the NREL and RISØ measurements. It can be observed that f is a function of the angle of attack: In both figures, f increases gradually to a maximum level near α 20 degrees, where it remains approximately constant. However, the ECN1, the CRES and the Illinois models assume no dependancy on the angle of attack at all, i.e. f is constant for a particular c/r and/or pitch angle and/or tip speed ratio. Although the tip speed ratio is obviously varying with the angle of attack, this parameter is expected to decrease with angle of attack and hence f would be expected to decrease as well. Possibly a 3D model like the ECN2 model, which is proportional to the fraction of the chord, which is separated, could form a better basis for a 3D model, but such fraction could be difficult to estimate, also because it is determined by 3D effects itself! In summary, a factor f, which is a function of c/r, tip speed ratio, pitch angle and angle of attack, is believed to form the most realistic basis for a 3D model; - It can also be noted that the term f for the RISØ measurements is higher (the maximum value is above 0.6) compared to the NREL measurements (where the maximum value is between 0.4 and 0.5). The difference may possibly be caused by a higher tip speed ratio in the RISØ measurements, since the values of c/r and the pitch angle are almost similar; - The dependancy on pitch angle which is included in the CRES model is also visible in c n - α measurements from ECN and NREL (The only facilities which supplied c n -α curvers for different pitch angles), as an example see figure 2; values of c/r. Due to the fact that c l,3d (and c d,3d) depend on more factors than c/r alone, such dependancy may be difficult to extract; - It will be attempted to bin the measurements on the rotor speed, so that the dependancy on the rotor speed can be investigated. NREL, 47% span, c n -α curve at different pitch angles, comparison between measurements and calculations from ECN and Illinois NREL, 47% span, c n -α curve at different pitch angles, comparison between measurements and calculations from CRES - In stall, most of the models seem to overpredict the value of c t see as an example figure 4. The underlying cause may be an underprediction of c d in stall; On the RISØ and NREL facility, measurements at the very tip were made (95% or even 98% blade span). At these sections the c n -α and c t -α curves show a considerable overprediction (in terms of maximum c n and c n as well as in terms of dc n /dα and dc t /dα), see figure 3. It must be noted that the well known Prandtl tip correction is only a finite blade correction, by which the angle of attack and as such the lift is reduced. An explicit tip loss, due to the finite length of the blade is not taken into account; - The dependancy on c/r still needs to be investigated by comparing the airfoil characteristics at different 6

7 RISØ, 98% span, c n -α curve, comparison between calculations and measurements RISØ, 37% span, (c/r =0.28, pitch(+twist) angle is 11.3 degrees, Ωr 17.7 m/s): Factor f, derived from measured c l -α curve : RISØ, 63% span, c t -α curve, comparison between calculations and measurements NREL, 30% span, (c/r =0.30, pitch(+twist) angle is 12 degrees, Ωr 11.4 m/s): Factor f, derived from measured c l -α curve ACKNOWLEDGEMENT The project has been sponsored by NOVEM, the Netherlands Agency for Energy and the Environment, under contract

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