Aerodynamic Measurements

L. Battistie-mail: [email protected]

L. Zanne

S. Dell’Anna

DIMS, Faculty of Engineering,

Universita degli Studi di Trento,

Via Mesiano 77, I-38050 Povo (TN), Italy

V. Dossenae-mail: [email protected]

G. Persico

B. Paradiso

Laboratorio di Fluidodinamica delle Macchine,

Dipartimento di Energia, Politecnico di Milano,

Via Lambruschini 4, I-20158, Milano, Italy

Aerodynamic Measurementson a Vertical Axis Wind Turbinein a Large Scale Wind TunnelThis paper presents the first results of a wide experimental investigation on the aerody-namics of a vertical axis wind turbine. Vertical axis wind turbines have recently receivedparticular attention, as interesting alternative for small and micro generation applica-tions. However, the complex fluid dynamic mechanisms occurring in these machinesmake the aerodynamic optimization of the rotors still an open issue and detailed experi-mental analyses are now highly recommended to convert improved flow field comprehen-sions into novel design techniques. The experiments were performed in the large-scalewind tunnel of the Politecnico di Milano (Italy), where real-scale wind turbines for microgeneration can be tested in full similarity conditions. Open and closed wind tunnel con-figurations are considered in such a way to quantify the influence of model blockage forseveral operational conditions. Integral torque and thrust measurements, as well asdetailed aerodynamic measurements were carried out to characterize the 3D flow fielddownstream of the turbine. The local unsteady flow field and the streamwise turbulentcomponent, both resolved in phase with the rotor position, were derived by hot wiremeasurements. The paper critically analyses the models and the correlations usuallyapplied to correct the wind tunnel blockage effects. Results highlight that the presentlyavailable theoretical correction models do not provide accurate estimates of the blockageeffect in the case of vertical axis wind turbines. The tip aerodynamic phenomena, in par-ticular, seem to play a key role for the prediction of the turbine performance; large-scaleunsteadiness is observed in that region and a simple flow model is used here to explainthe different flow features with respect to horizontal axis wind turbines.[DOI: 10.1115/1.4004360]

Keywords: VAWT, blockage, wind Tunnel, wind turbine, aerodynamic measurements,unsteady flows

1 Introduction

In recent years a renewed interest has arisen on vertical axisconcept in wind turbines (VAWT). In the urban environment,affected by highly turbulent flows and strong vertical velocity gra-dients, VAWT claims for several advantages: insensitivity to yaw,ability to withstand rapid changes of wind direction and to providegood performances also in skewed flows [1], low noise emissiondue to low tip speed ratios [2]. Finally the better integration in ar-chitectural projects also represents a key advantage.

VAWT concept, however, still presents a lot of challenges to besolved, principally due to its intrinsic flow complexity. With its fullythree-dimensional geometry and flow structure, the blades elaboratetwice the streamtube in a revolution and interact with the wake shedfrom upstream blades. Furthermore, the blade profiles work with anoscillating angle of attack leading to blade loading unsteadinessand possibly to dynamic stall, affecting both the wake and the tipvortex development. The variety of geometries of rotor design, fromclassical Darrieus troposkein geometry to the V and the H designs,complicates the definition of a general model [3].

At the moment, the VAWT flow field is still far to be com-pletely understood. In particular, there is a strong need of detailedexperimental analyses to convert an improved flow field compre-hension into better aerodynamic models suitable to support noveldesigns. In this way, a more refined aerodynamic optimization ofthe rotor shape and performance could be achieved, and thematching with aeroelastic codes, used to predict extreme and fa-tigue loads, could be improved.

At the present time, the most used aerodynamic model is the dou-ble-multiple streamtubes (DMS) [4], a blade element momentummodel (BEM) applied to two actuator disks, one representingthe upwind side of the rotor and the other one the downwind side,each of them subdivided into multiple streamtubes. Takinginto account dynamic stall and secondary effects (e.g., parasitic armsdrag, tip losses, turbulent wake state correction, wake expansion,and tower deficit) with dedicated submodels, the results are stillfar from providing a sufficiently reliable performance estimation.

More recently, the so-called vortex methods have been devel-oped with the aim to get a more realistic simulation of theunsteady behavior of the wake, the tip vortex, and the blade loads.These methods allow computing the flow field from the blade cir-culation, and the trailing and shed vorticity. The wake can be ei-ther prescribed or free, the latter giving much better results butbeing computationally slower and affected by numerical instabil-ities. Normally these methods use a database for airfoils perform-ances—as DMS does—to calculate the blade circulation. Toavoid the use of the database an unsteady panel method allowscalculating the airfoil performances [5], once coupled with thesolution of the unsteady boundary layer [6].

CFD analysis represents the most recent class of aerodynamicperformance prediction methods. In these codes, the flow field iscomputed by replacing the blades elements with body forcesderived from airfoil look-up tables [7,8]. As a more complex alter-native they can resolve the entire flow field including the bladeprofile boundary layer.

In this uncertain scenario there is a lack of experimental datarequired to validate new models of the flow behavior.

Throughout the VAWT research, Sandia Laboratories focusedthe attention on free field testing [4], and only a few tests wereperformed in wind tunnels (i.e., [9]). In this way, full scale

Contributed by the Advanced Energy Systems Division of ASME for publicationin the JOURNAL OF ENERGY RESOURCES TECHNOLOGY. Manuscript received September 9,2010; final manuscript received May 13, 2011; published online July 22, 2011.Assoc. Editor: Andrea Lazzaretto.

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analyses (i.e., conserving Reynolds number, tip speed ratio, andreduced frequency) can be carried out at the expense of anincreased complexity in the test management. Loading data fromopen air testing are actually available [4] but a rigorous attemptfor the wake characteristics determination has not been done tillrelatively recent years.

The dynamic stall phenomena was investigated by Fujisawaand Shibuya [10] and Simao Ferreira et al. [11], that performedPIV measurements in a wind turbine to clarify the vortex genera-tion and shedding during the blade revolution. Hofemann et al.[12] analyzed tip vortex evolution with a 3D stereo PIV technique.A study of the rotor performance and wake shedding in non-skewed and skewed flow was performed with flow visualizationand hot wire measurements by Simao Ferreira et al. [13].

Unfortunately, tests in tunnels introduce further sources ofuncertainty due to the effect of blockage. Sørensen et al. [14,15]used the one-dimensional momentum approach to derive a correc-tion for a HAWT in closed tunnels, and validated it by numericalsimulations. They also elaborated a simulation to get the correc-tion for a special case of open wind tunnel. Nevertheless, the one-dimensional approach still shows some lacks and it can be usedonly till the vortex system behind the turbine does not break up inthe turbulent wake. The open question in literature is whether it ispossible to use the same models in the case of a VAWT, or betterusing the Maskell theory [16] as required by the Measnet associa-tion for cup anemometer calibration.

Moreover, in the case of an open tunnel, the blockage factorpresented in Ref. [14] is considered too high and possibly othercorrections can be used, as that developed by Mercker and Wiede-mann in Ref. [17].

In many cases the data are collected in free field with no con-trolled conditions, or in closed wind tunnel with a high blockage(exceeding 5–10%) without any correction for blockage and forthe three-dimensional shape of the wake. For open-jet tunnel theblockage effects are less evident but this normally leads to testexcessively large models characterized by high-blockage effect, of-ten resulting in the application of even more doubtful corrections.

This lack of accuracy of design and analysis tools are themotivation of the present work, which presents an experimentalstudy on the aerodynamics and performance of a VAWT. Meas-urements were performed in the large-scale wind tunnel of thePolitecnico di Milano, where real-scale wind turbines for microgeneration can be tested in full similarity conditions. The research

covers several fields of activity. At first a critical evaluation ofcorrelations for wind tunnel blockage correction is carried out; toquantify the blockage effect, both open and closed wind tunnelconfigurations are considered. Moreover, a number of differentoperating conditions are investigated to evaluate the effect ofloading on the turbine performance for both wind tunnel configu-rations. The turbine tip aerodynamic and its implications on theoverall aerodynamic performance are finally discussed with thesupport of unsteady flow data and flow schematics.

2 The Politecnico di Milano Large Scale Wind Tunnel

The Politecnico di Milano wind tunnel is a large scale closedloop facility, which develops along two floors of a dedicatedbuilding. Within the facility two different test sections are avail-able: a low speed test section (14 m� 3.84 m) allowing a maxi-mum air velocity of about 15 m=s and a high speed section(4 m� 3.84 m) allowing a maximum velocity of 55 m=s. Thewind generator is composed by two rows of 7 fans, each of themdriven by an inverter controlled electric motor. The overall in-stalled electric power is approximately 1.4 MW.

Tests were performed in the high speed test section (Fig. 1),located immediately upstream of the diffuser leading to the fans,and characterized by a very low turbulence level (<1%).

Tests can be carried out in a closed configuration by positioningthe models inside of a removable test room of 4 m width� 3.84 mheight, or in a “free jet” (open) configuration by removing the testroom and installing the models directly facing the upstream tunneltube (Fig. 2). The same VAWT was tested in both the closed andthe open configurations in order to analyze the influence of themodel blockage for different operational conditions.

Figure 1 reports a sketch of the experimental set-up in the free-jet configuration together with the location of the aerodynamicmeasurement plane and a dashed draw showing the closed testroom trace when operating in a closed environment. The modelwas centered in both configurations. A blockage ratio B¼ 0.10results in the closed room, where the frontal section of the modelis given by AD ¼ 2H � D.

3 Turbine Model and Instrumentation

The physical model of the turbine is a small VAWT character-ized by three straight blades as represented in the picture of Fig. 2.The machine was designed and built for research purposes by thecompany Tozzi Nord Wind Turbines sited in Trento (Italy) andinstrumented by the turbomachinery laboratory of the universityof Trento. The main turbine geometrical data are reported in Ta-ble 1. The wind turbine was equipped with strain gauge bridgesinstalled on the supporting mast 1.2 m below the rotor midsection, to measure the streamwise and lateral aerodynamic

Fig. 1 Experimental set-up. The measuring section is located1.5 D downstream of the rotor axis

Fig. 2 Picture of the wind turbine and traversing system in theopen tunnel configuration

031201-2 / Vol. 133, SEPTEMBER 2011 Transactions of the ASME


forces; a torque meter was installed to measure the shaftmechanical torque. The rotational speed and the rotor phase wereprovided by a 13 bit encoder.

4 Description of the Instrumentation

Aerodynamic measurements were performed on a plane normalto the upstream wind direction located 1.5 rotor diameters down-stream of the VAWT shaft, as previously shown in Fig. 1.

Two aerodynamic probes were used to define the time andphase averaged flow field on the whole measuring plane:

• Directional pneumatic five hole probe: The probe was cali-brated on a low speed jet over an angular range of 6 24 degboth in pitch and yaw direction. This probe was used in orderto obtain the local total pressure, static pressure and the 3Dvelocity vector. Uncertainty in total and static pressure meas-urements can be assumed to be lower than 5 Pa, while forangular measurement an uncertainty of 0.2 deg has beenevaluated.

• Single sensor hot wire anemometer: The probe head, character-ized by a sensor wire diameter of 5 lm normal to the probestem, was operated in constant temperature mode. The probewas mounted with the wire in vertical direction, to minimizethe effects due to vertical velocity components. So operating,no specific directional calibration of the hot wire probe isrequired. Hot wire data allowed to define, for each point of themeasurement grid, the time averaged component of the flowvelocity and the periodic velocity component, as a function ofthe rotor angular position. An estimation of the turbulence in-tensity was also provided by the hot wire measurements.

Because of the higher accuracy of the hot wire technique inevaluating the velocity magnitude, the time averaged value of thelocal kinetic head derived from hot wire measurements was usedto improve the accuracy in evaluating the velocity vector and thestatic pressure field from the 5 hole probe measurements.

Probes were traversed on a measurement grid of 3700mm� 900 mm defined by 21 points along horizontal transversedirection (Y) and 9 points along vertical spanwise direction (Z),extending from rotor midspan (Z=H¼ 0.0) to 170 mm above therotor tip (Z=H¼ 1.24). Test carried out on the whole plane aredenoted as “3D”, while the ones performed only at the mid-spansection are denoted as “2D”.

The operating conditions considered for the current tests repro-duced tip speed ratios k reported in Table 2, obtained by varyingthe rotational speed in the range from 400 rpm to 500 rpm andoperating the tunnel at wind speeds ranging from 10 to 20 m=s.The k¼ 1.6 condition is in the ascending branch of the CP-k curve

where the blades stall for a nonnegligible part of the rotor revolu-tion, while for k¼ 2.5 this rotor works near the maximum CP con-dition. The maximum rotational speed was limited to avoidstructural collapse of the wind turbine rotor, while the minimumwind speed was limited by the accuracy in pressure measure-ments. Table 2 reports the main specifications of the tests consid-ered in the following.

5 3D Wake Profile

Figures 3 and 4 show the general shape of the velocity field onthe measurement plane for k¼ 1.6 and k¼ 2.5, in both open andclosed wind tunnel tests. A frame representing the model locationis also superimposed.

The wake appears to be not symmetric and deformed turnwise,according to the rotational direction of the wind turbine, i.e., counterclockwise when viewed from above. A similar characteristic wasalso found by Simao Ferreira et al. in Ref. [13]. These phenomenaare due to the blade tip vortex interactions, that are stronger in theupcoming blade region (on the left of the figures), and weaker in theretreating blade one (on the right in the front view of the rotor). Thisasymmetry reduces as the tip speed ratio increases, as the variationof fluid dynamic conditions experienced by the blades (in terms ofrelative velocity, blade angle of attack, and vortex convection veloc-ity) also reduces during a full revolution, consequently reducing thefluctuation of blade bound vorticity. At low tip speed ratios theblade normally stalls for a nonnegligible portion of the revolution,producing a more intense bound circulation in the upcoming regionand a stronger tip vortex [4].

This conclusion was also drawn from the simulations presentedby Dixon et al. [5] that predicted such a behavior with a 3Dunsteady panel method. The present experimental results confirmthe capability of vortex methods to capture the general 3D flowfield. At low tip speed ratios the core flow is characterized byweak velocity gradients for the open and the closed tunnel config-urations. At high tip speed ratio the more intense work extraction(and CT) determines a higher velocity drop in the core flow. Fur-thermore stronger gradients are observed in the closed chambertests due to blockage effects.

6 Blockage Modeling

Since the wind tunnel test section has a confined volume, theaerodynamic measurements obtained from the wind tunnel testsdo not resemble those obtained in infinitely spaced boundaries,such as the case of in open field.

The test of wind turbines in wind tunnels is based on fluid-dynamic similarity rules, which scale the actual sizes to the modelones according to the Reynolds and Strouhal numbers (the Machnumber is not of concern because the process can be consideredas incompressible). Tests are usually devoted to assess rotorglobal performances (power curve) and to evaluate improvedmodels for the turbulent wake. Nevertheless both tasks have todeal with the accurate correction of the tunnel blockage effect.When rotating devices as wind turbine rotors are actually tested,additional complexities arise from the influence of test conditionson the wake shape. Highly loaded rotors have a more expandedwake compared to lightly loaded rotors. Moreover, blade passingevents cause turbulent structures to be shed downstream and leadto periodic fluctuations in the velocity and pressure fields.

A number of approaches are available to study the actual airflowaround the wind turbine rotor and to estimate the influence of block-age [18–22] for an unshrouded propeller or a wind turbine. Glauert[18], developed the basic theory for thrusting propellers, which, bylimited extension, can be adapted to HAWT wind turbines. Glauertused a momentum-balance=actuator-disk model to provide a proce-dure for setting the wind tunnel inlet flow speed so as to generatethe same thrust as the one experienced in unbounded conditions.

The resulting simple relationship has been used throughout thepropulsion industry but encounters difficulty for wind turbines, as

Table 1 Main characteristics of the three-blade VAWT

Blade height (2H) 1.457 mRotor diameter (D) 1.030 mBlade profile NACA0021Solidity (Nc=D) 0.250

Table 2 Summary of the test conditions

Open tunnel tests k [-] CT [-] V0 [m=s]

2D 1.62 0.46 16.143D 1.56 0.46 13.143D 2.50 0.68 10.50

Closed tunnel tests2D 1.64 0.50 16.053D 1.63 0.49 13.303D 2.54 0.78 10.55

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a singularity occurs when the thrust coefficient equals unity. Mik-kelsen and Sørensen [15] revisited the issue and, still using amomentum-balance=actuator-disk model, provided a new velocityadjustment relation that avoids Glauert’s model singularity. Com-parison to Navier Stokes CFD studies confirmed the predictions ofthe simple actuator-disc model for blockage levels of practical in-terest. To date though, no method has been put forward for esti-mating the blockage correction factors for unshrouded VAWT.

Regardless of the tunnel configuration, a unique model can beadopted to compute the one-dimensional streamwise thrust stress-ing the turbine. With reference to Fig. 5, the flow field within thetest chamber is shown. Five main sections are identified: section0, upstream; sections 1 and 2, wind turbine inlet and outlet respec-tively; section D, wind turbine (disk section); section 3, far down-stream (measuring section). The stream is entering the test sectionwith a uniform velocity V0 over the whole wind tunnel sectionand divides itself into a core fractional mass flow ( _mcore) and anouter fractional mass flow ( _mouter). The former defines a streamtube which passes through the upstream area A0, the turbine areaAD—delivering the work and providing the thrust—and the down-stream area A3 (wake area). The outer air mass flow entering thevolume outside of the central streamtube mixes with a fractionalmass ( _mR), possibly exchanged with the external environment.This fraction vanishes when a closed wind tunnel is of concern.

Considering the core streamtube, the thrust operated by theflow stream on the disk can be easily evaluated by the static pres-sure drop across the disk itself as

p1 � p2ð ÞAD ¼ T (1)

The total pressure can be assumed constant from section 0 tosection 1 and from section 2 to section 3; considering a uniform

velocity distribution on section 3, and being sections 1 and 2 infin-itely close compared to the wake streamwise extension,V1¼V2¼VD and Eq. (1) can be rewritten as

T ¼ AD p0 þ1

2qV2

0 �1

2qV2

D

� �� p3 þ

1

2qV2

3 �1

2qV2

D

� �� (2)

leading to

T ¼ p0 � p3ð ÞAD þ1

2qðV2

0 � V23ÞAD (3)

Since pressures and velocities are locally measured in sections 0and 3, thrust T can be directly deduced by the Eq. (3) and theresult can be compared with the streamwise thrust measured bythe strain gauges installed on the supporting mast of the model.

Since on the downstream measuring plane the actual measuredvelocity distribution is not uniform in both directions, a methodol-ogy for the evaluation of an equivalent velocity V3 and of a corre-sponding section A3 is described in the following.

As previously discussed, the blockage model requires thedownstream flow field to be divided in a “wake region” (coreflow) and in a “free-stream region” (outer flow). To identify theedges of the wake region, an analysis of the nondimensional totalpressure drop distribution (Cpt) at measurement traverses has beenperformed for an each spanwise position. Thanks to the sharpedges of the wake, its boundaries can be easily identified for allturbine operational conditions. An example of typical wake pro-files in terms of Cpt distributions at several spanwise coordinatesis reported in Fig. 6 (closed tunnel, k¼ 2.5) which shows how thewake boundaries are sharply defined. The total pressure drop

Fig. 3 Contours of non dimensional velocity V=V0 on the measurement plane at k 5 1.6 for open and closed configurations



profile is constant until almost the tip of the blade (Z=H¼ 0.8),and starts deviating consistently only in the tip region. The drop isreabsorbed almost completely just above the turbine maximumheight (Z=H¼ 1.2), except for the nonsymmetrical local peakinduced by the blade load variation throughout the full revolution.A combined analysis of Figs. 3, 4, and 6, indicates that the 3Deffects vanish quite rapidly as the tip of the blade is reached.

Figure 7 shows a comparison of the nondimensional velocitydistribution in the wake for the closed and open tunnel tests. It isto be noted that the highest nondimensional velocity rate meas-ured in the closed wind tunnel contributes to the increase of thrust

coefficient (Eq. (3)). The fairly complete superposition of resultsfor the open or for the closed configuration at constant k provesthe measurement reliability.

Once the wake edges were defined, a mass averaging procedurewas performed on the velocity and total pressure distributions atfixed spanwise coordinate, to finally determine the “equivalentwake velocity” and the outer equivalent velocity (see Fig. 5). Thecorresponding extension of the wake region, for a fixed spanwisecoordinate, was further evaluated on the basis of the mass flowrate measured within the wake streamtube ( _mcore). The same pro-cedure was also extended to the whole measuring plane, leading

Fig. 4 Contours of non dimensional velocity V=V0 on the measurement plane at k 5 2.5 for open and closed configurations

Fig. 5 Simplified flow field model upstream and downstream of a wind turbine



to the definition of an “over-all wake velocity” (V3) and of a“wake streamtube section”.

According to Eq. (3), the actual streamwise thrust acting oneach spanwise section of the machine can be easily computed onthe basis of the above defined aerodynamic quantities. Finally, theintegration of the spanwise thrust distribution derived from aero-dynamic measurements allows the evaluation of the bending loadacting on the model mast. This is then compared with the thrustmeasurement performed by the strain gauges. In Fig. 8, straingauge measured thrust coefficients Ct,m are compared to the thrustcoefficient obtained by the data reduction procedure, Ct,aer.Results show a very good agreement witnessing the good accu-racy of both measurement techniques, and, in particular, of theapplied data reduction methodology.

To generalize wind tunnel results to unbounded conditions, themeasurements need to be corrected for wind tunnel wall effects.Due to the wind tunnel walls constraining the flow velocity out-side of the wake is increased compared to the unbounded condi-tion (V03 > V0), as also shown by the nondimensional velocitydistributions reported in Fig. 7. Consequently, the static pressuredownstream of the model, outside of the wake, is reduced and in

general p < p0. Far downstream of the model, the static pressurebecomes constant across the whole tunnel section, and no furtherexpansion occurs along the wake streamtube.

The static pressure in the wake of closed configuration tests ishence reduced compared to the unbounded conditions, namelyp3 < p0. Thus the static pressure difference p3 � p0ð Þ generates athrust T higher than that observed in unbounded conditions (seeEq. (3)).

The absence of physical walls in the test section of an open jetwind tunnel (such as the case of the open configuration) stronglyreduces the blockage effects, leading to an almost complete recoveryof the undisturbed pressure in the downstream far field (see Fig. 5).

Measurements at station 3 reveal that the static pressure differ-ence between inside and outside the core streamtube is small,leading to a maximum 1–2% contribution in the thrust coefficientfor both open and closed test chambers. This evidence allows stat-ing that, at station 3, the wake can be considered completelydeveloped.

Glauert [18] proposed to determine an equivalent undisturbedwind speed V0 that in the unbounded flow condition gives thesame thrust of the undisturbed wind speed V0 in the wind tunnelapplication. Thus, by denoting with the apex prime the equivalentunbounded conditions, we have

VD ¼ V0D (4)

CTV20 ¼ C0TV020 (5)

The one-dimensional momentum theory for the equivalentunbounded conditions yields

T0 ¼ 1

2qADC0TV020 ¼ 2V0DqAD V00 � V0D

� �(6)

By combining Eqs. (4)–(6), it is found

V00V0

¼ VD

V0

þ CT

4 VD

V0

(7)

Since the velocity and the area of the wake in section 3 are com-puted by the aerodynamic measurements, VD can be easily esti-mated as

VD ¼ V3

A3

AD(8)

Fig. 6 Wake shape in terms of Cpt distribution at differentspanwise locations (test closed, 3D, k 5 2.5)

Fig. 7 Comparison of the wake shape at rotor midspan inclosed and open tunnel tests at k 5 1.6 (2D and 3D tests)

Fig. 8 Comparison of thrust coefficients CT obtained by straingauge measurement and by aerodynamic measurement datareduction for the open and closed configurations



Since the CT coefficient determined in the wind tunnel tests is alsoknown, the ratio V00

�V0 can be easily computed by Eq. (7).

In Fig. 9, the ratio V00�

V0 is shown as a function of the thrustcoefficient and compared to the original Glauert correlation [18],to the correlations by Mikkelsen and Sørensen [15] and Maskell[16] for closed tunnel and Merker-Wiedemann [17] for open testsection. The former has been plotted for two limiting cases: thefirst case, for B¼ 0.1, is based on the actual geometrical ratio; thesecond case, B¼ 0.25, corresponds to a purely 2D blockage i.e.. itconsiders the tunnel height not influencing the wake evolution.

Figure 9 shows that the actual aerodynamic behavior of theVAWT is different from that predicted by simple actuator disk, orsimple blockage models, also in the limiting case of B¼ 0.25. Atfirst the experimental corrections for the closed tunnel are by farmore remarkable compared to the ones predicted by usual correc-tions. Moreover the measurements show that the magnitude of thecorrection reduces as CT increases. This result is quite surprisingbecause the trend is opposite to that given by the aforementionedmodels, but indicates that the complex fluid-dynamics of aVAWT cannot be modeled by a simple 1D momentum approach,but rather by an “actuator cylinder” approach.

A possible explanation of this trend is that, as CT increases,both the k coefficient and the blade rotor loading enhance; so, ahigher bound shed vorticity is generated by the blades, but withweaker blade bound vorticity and trailing tip vortex. It can be fig-ured out that the vortices shed by the tip of the upstream running

Fig. 9 Wind speed correction factors as function of the thrustcoefficient. Experiments and theoretical predictions with theGlauert, Maskell, and M-W (Merker-Wiedemann) methods

Fig. 10 (a) Phase-resolved velocity magnitude at k 5 2.5, midspan, (b) Phase-resolved turbulence intensity at k 5 2.5, mid-span, (c) Phase-resolved velocity magnitude at k 5 2.5, Z=H 5 1.1, and (d) Phase-resolved turbulence intensity at k 5 2.5,Z=H 5 1.1



blades entrain some mass flow into the core flow from the virtualbases of the rotor defined by the blade tips. The effect is toincrease the mass flow through the wind turbine and, as a conse-quence, in the wake, thus modifying the magnitude of the velocitycorrection. This hypothesis is confirmed by the simulation ofDixon [5] showing a roll-up phenomenon of the upwind tip vorti-ces toward the inner part of the rotor. These phenomena will befurther discussed and modeled throughout the unsteady flowanalysis.

The results of these experiments also raise a further questionconcerning the rotor wind tunnel blockage study, i.e., the applic-ability of the actuator disk model for stalled conditions. Maskell[16] showed that the blockage corrections for stalled wing andbluff bodies are about five times the corrections for attached flow.

7 Unsteady Flow Field

The analysis reported in the previous sections has shown thatthe aerodynamic of VAWT is complex, with significant deviationswith respect to the most known horizontal axis machines.

In this section, the unsteady-periodic component of the flowfield measured by the hot wire probe is presented, with the aim toimprove the level of comprehension of the flow field. The periodiccomponent is meant to be the unsteady fluctuation of flow velocityresolved in phase with the rotor position.

The passing of the rotor blades induces a periodic unsteadinessin the flow field; its amplitude can be used as a proper marker oflarge-scale viscous structures released by the blades, commonlyneglected by classical theories.

To perform such a data-reduction, the velocity measurementswere first triggered on the rotor instantaneous position (phase-locking); then, they were phase-averaged over the blade passingperiods available in each acquisition time-span (about 25 periodswere available for each acquisition) to obtain the periodic flowcomponent over the blade passing period. Correspondingly, theRMS of the instantaneous-to-periodic flow velocity was also eval-uated for each phase of the period, from which a phase-resolvedturbulence intensity was computed.

In Fig. 10 the phase-resolved velocity magnitude and turbu-lence intensity are reported for k¼ 2.5 (open configuration) attwo different spans, i.e., the midspan section and a sectionplaced just above the blade tip. These plots are arranged as two-dimensional fields in which the abscissa is the nondimensionalY=D position along the measurement traverse, and the ordinaterepresents the phase s, connected to the rotor angular position (awhole revolution period is reported, i.e., three blade passingperiods).

In these maps the mean spatial gradients (the turbine wake pro-file, for example) appear as vertical bands, while unsteady effectscan be recognized as perturbation of the main horizontal gradients.

In Fig. 11 a sketch of the vortex structures released by theblades at the two sections are depicted. At midspan the vortex pat-tern is dominated by the shed vorticity, made unsteady by theblade circulation variation throughout a revolution. Since the axisof the unsteady shed vorticity is parallel to the rotor shaft, veloc-ities in a plane perpendicular to the rotor axis are induced, in ac-cordance to the application of the Biot–Savart law.

A different vorticity configuration is sketched at the tip sec-tion, where the tip vortices show an axis perpendicular to therotor shaft. The tip vortices can induce velocities both in thehorizontal and the vertical directions, so they are responsibleboth for a part of the horizontal wake velocity induction and forthe entrainment of air in the rotor and in the wake from their topand bottom sides.

Figure 10(a) clearly shows that, in the midspan section, the roleof the periodic unsteadiness is negligible; this means that the vis-cous wakes (shed vorticity) of the single profiles are almost com-pletely mixed out in the measurement plane. The distribution ofthe turbulence intensity (Fig. 10(b)) at midspan is slightly moresensitive to the rotor blade unsteadiness, but the structure of theturbulent field is space-dominated, with high peaks in the marginsof the wake of the machine (especially on the right side). Thishigh turbulence region is connected to the shed vorticity, which isdue especially to dynamic stall as it was already shown by the ex-perimental results of Fujisawa and Shibuya [14]. In particular, asthe blade starts the downwind passage, it interacts with this shedvorticity which is higher at the rotor margin.

The situation at the tip section of the wake (Figs. 10(c) and10(d)) is very different: the velocity defect in the wake is muchlower than that at midspan, and a distinct unsteady modulation ofthe velocity field on the right side of the wake is now clearly visi-ble. The higher relevance of unsteadiness at this section is furtherdemonstrated by the turbulence field, which is now characterizedby very high values of turbulence intensity in the whole wakeregion (and not only on the wake margins). The structure of theturbulence field inside the wake is now dominated by an unsteadyperturbation, where the maximum peaks of turbulence are found,linked to the blade passing events. The maps in Fig. 10(c) andespecially Fig. 10(d) indicate that a large-scale viscous structureis generated by the tip of the rotor blades—the tip vortex of theprofiles; the relevance of this structure, measurable in a sectionplaced relatively far from the blades, is much higher than the oneof the single wakes of the profiles.

The tip region is, therefore, effectively affected by fully 3D andunsteady flow phenomena that play a relevant role on the defini-tion of the streamtube passing through the machine and measureddownstream. Hence, it strongly influences all the integral quanti-ties that determine the performance of the wind turbine. The mod-eling of such a tip effect is therefore crucial for the setting up of

Fig. 11 Schematic of the vortex structures released by theblades at mispan and tip sections of the turbine



accurate models for the prediction of vertical axis wind turbineperformance.

8 Conclusions

In this paper the first results of a wide experimental investiga-tion on the aerodynamics and performance of a VAWT have beenpresented.

The measurement technique and the data reduction methodol-ogy employed for the present investigation were shown to beaccurate and reliable, on the basis of thrust measurement compari-son and data repeatability.

The wake analysis shows congruent behavior at different tipspeed ratios and thrust coefficients, and indicates a wake dimen-sion comparable to the wind turbine swept area. The flow ismostly two-dimensional almost up to the tip, where large scaleturbulent structures appear in a very confined region. This sug-gests that modeling the tip effects is crucial for the prediction ofVAWT performances.

Experimental results evidence that the upstream velocity cor-rections, commonly applied to account for blockage effects inwind tunnels, are of doubtful validity in the case of VAWT,leading probably to underestimated correction rates for all theconsidered cases. Furthermore upstream velocity correctionsbased on experimental results decrease as the thrust coefficientincreases.

The key-problem of the existence of velocity components paral-lel to the VAWT shaft, responsible for the entrainment of an unex-pected flow rate, is discussed with the support of tip unsteady dataand of a tip vortex evolution model.

Acknowledgment

The authors thank the Company Tozzi-Nord Wind Turbines(Italy) for partial funding the research. The authors also acknowl-edge Mr. Claudio De Ponti for his valuable contribution to the ex-perimental set-up.

Nomenclature

A¼ area (m2)B¼ blockage ratio (B ¼ AD=AT)c¼ blade chord

Cpt¼ pt;0 � pt;3

� �=0; 5 � q � V2

0 non dimensional total pressuredrop

CT¼ thrust coefficientD¼ turbine diameter (m)H¼ half blade height (m)_m¼ mass flow (kg=s)N¼ number of bladesp¼ static pressure (Pa)

Tu¼ turbulence intensityT¼ thrust (N)V¼ velocity (m=s)V0 ¼ corrected velocity (m=s)

X, Y, Z¼ streamwise, transversal, spanwise coordinatesX¼ streamwise non-dimensional distance from rotor axisq¼ air density (kg=m3)k¼ tip speed ratio (x D=2=V0)s¼ rotor phase (time divided by the blade passing period)

x¼ rotational speed (rad=s)C¼ vorticity

Subscripts

0¼ wind tunnel upstream1¼ model upstream2¼ model downstream3¼ measurement sectionD¼ disk section

aer¼ from aerodynamic measurementsm¼ measured with strain gaugest¼ total

T¼ tunnel

References[1] Mertens, S., van Kuik, G., and van Bussel, G., 2003, “Performance of a H-Darrieus

in the Skewed Flow on a Roof,” ASME J. Sol. Energy Eng., 125, pp. 433–440.[2] van Bussel, G. J. W., Mertens, S., Polinder, H., and Sidler, H. F. A., 2004,

“TURBYVR : Concept and Realisation of a Small VAWT for the Built Environ-ment,” The Science of making Torque from Wind, 19–21 April, Delft.

[3] Battisti, L., Brighenti, A., and Zanne, L., 2009, “Analisi dell’effetto della sceltadell’architettura palare sulle prestazioni di turbine eoliche ad asse verticale,”Atti del 64� Congresso Nazionale ATI.

[4] Paraschivoiu, I., 2002, Wind Turbine Design—With Emphasis on DarrieusConcept (Polytechnic International Press, Montreal, 2002).

[5] Dixon, K., Simao Ferreira, C. J., Hofemann, C., Van Bussel, G. J. W., and VanKuik, G. A. M., 2008, “A 3D Unsteady Panel Method for Vertical Axis WindTurbines,” Proceedings of EWEC Brussels.

[6] Oler, J. W., Strickland, J. H., Im, B. J., and Graham, G. H., 1983, “DynamicStall Regulation of the Darrieus Turbine,” Sandia National Laboratories, Albu-querque, New Mexico, SAND83–7029.

[7] Allet, A., and Paraschivoiu, I., 1995, “Viscous Flow and Dynamic Stall Effectson Vertical-Axis Wind Turbines,” Int. J. Rotating Mach., 2(1), pp. 1–14.

[8] Shen, W. Z., Zhang, J. H., and Sørensen, J. N., 2009, “The Actuator SurfaceModel: A New Navier–Stokes Based Model for Rotor Computations,” ASME J.Sol. Energy Eng., 131, p. 011002.

[9] Blackwell, B. F., Sheldal, R. E., and Feltz, L. V., 1976, “Wind Tunnel Perform-ance Data for the Darrieus Wind Turbine With NACA0012 Blades,” SandiaNational Laboratories, Albuquerque, New Mexico, SAND76–0130.

[10] Fujisawa, N., and Shibuya, S., 2001, “Observations of Dynamic Stall on Dar-rieus Wind Turbine Blades,” J. Wind. Eng. Ind. Aerodyn., 89(2), pp. 201–214.

[11] Simao Ferreira, C. J., van Bussel, G. J. W., Scarano, F., and van Kuik, G., 2007,“2D PIV Visualization of Dynamic Stall on a Vertical Axis Wind Turbine” Pro-ceedings of the AIAA=ASME Wind Energy Symposium.

[12] Hofemann, C., Simao Ferreira, C. J., Van Bussel, G. J. W., van Kuik, G. A. M.,Scarano, F., and Dixon, K. R., 2008, “3D Stereo PIV Study of Tip Vortex Evo-lution on a VAWT,” Proceedings of EWEC, Brussels.

[13] Simao Ferreira, C. J., van Kuik, G., and van Bussel, G., 2006, “Wind tunnelhotwire measurements, flow visualization and thrust measurement of a VAWTin skew,” Proceedings of the AIAA=ASME Wind Energy Symposium.

[14] Sørensen, J. N, Shen, W. Z., and Mikkelsen, R., 2006, “Wall Correction Modelfor Wind Tunnels With Open Test Section,” AIAA J., 44(8), pp. 1890–1894.

[15] Mikkelsen, R., and Sørensen, J. N., 2002, “Modelling of Wind Tunnel Block-age,” Proceedings of the Global Windpower Conference and Exhibition.

[16] Maskell, E. C., 1963, “A Theory of the Blockage Effects on Bluff Bodies andStalled Wings in an Enclosed Wind Tunnel,” ARC=R & M–3400.

[17] Mercker, E., and Wiedemann, J., 1996, “On the Correction of the InterferenceEffects in Open Jet Wind Tunnels,” SAE Paper No. 960671.

[18] Glauert, H., 1947, The Elements of Aerofoil and Airscrew Theory, 2nd ed.,Cambridge University, Cambridge, England.

[19] Loeffler, A. L., Jr., and Steinhoff, J. S., 1985, “Computation of Wind TunnelWall Effects in Ducted Rotor Experiments,” J. Aircr., 22(3), pp 188–192.

[20] Barlow, J. B., Rae, W. H., and Pope, A., 1999, Low Speed Wind Tunnel Testing,3rd ed., John Wiley and Sons Inc., New York.

[21] Fitzgerald, R. E., 2007, “Wind Tunnel Blockage Corrections for Propellers,”MS thesis, University of Maryland, College Park, MD.

[22] Hansen, M. O. L., 2000, Aerodynamics of Wind Turbines, James and JamesPublishing, London.



http://dx.doi.org/10.1115/1.1629309

http://dx.doi.org/10.1155/S1023621X95000157

http://dx.doi.org/10.1115/1.3027502

http://dx.doi.org/10.1115/1.3027502

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Aerodynamic Measurements

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