Local Dynamic Pressure

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NREL/-2574654 UC Category: 261 DE92001200

Recent Results m Data Analsisof Dynamic Stal Wnd TurbieBlades

P N w Eg L

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Presented at the International Enegy AgencyExperts Meeting on Wind TurbineAerodynamics, Stttgart, Gerany,December

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Naonal Renewable Energy Laboratory(foerly the Sol Energy Research Institute)1617 Cole BoulevardGolden, Colorado 804013393 Division of Midwest Research InstituteOperated for the U.S Department of Energyunder Conact No DEAC0283CH1003

Janury 192



On September , , the Solar Energy Researc Istitut was designated a natia laboratory, ad ts name as can to te Natinal Reeable Eergy Laboratry.

NTC

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Rct Rut fm Dt Ayi f mic t Wi Tubi

C.P BuHeield (NREL)Dr S uyer (University of Coloa

D Simms (NREL)

Ntional Renewable Energy Lborato EL),formerly the Solar Energy Research Instit (SER)

Presented at the Intenational Energy AnyExpes Meeing on Wind Tubine Aeryais

StuHga, GermanyDecembe 4, 99

Abstact

id turbines are subjected to dynamic loading om a variety of dieret sources id shear and turbulencecuse timevaryig infow that results in unsteady arloads Tower shadow upwid turbine wakes adyaw angles also itroduce unsteady nflow to wind

turbine rotors id turbie desigers ust predictthese loads accurately i order to adequately design blades hubs ad the remaiing support structure to achieve a 30-year life Structural analysts ave not been able to predict mean or dyamic loads accuratelyenough to predict the fatigue life of major wnd turbinecomponets with coence Part of the problem isdue to uncertaity in the stochastic wind environment as metioed earlier Aother important prt of thepobem is the lack of basic kowledge of rotary wig

arfol stall performace There is mounting evidencetat dynamic stall may be related to dynamc loads that

are greater than predictions This paper descrbes someresults of investigations of unsteady aerodynmic loadsmeasured on a wid turbine blade The obectve of theinvestigation is to uderstad the steady and unsteadystall behavior of wind turbie blades

ntroution

The helicopter idustry has invested significant rearchtime in udertading dyamic stall on elicopter blades but has gored steady stal because elicopters

avoid operatig contions that would result i lrgescae steady stall nd turbines commoly operate isteady stall as a eans of regulatig peak power andloads It has been discovered by Buttereld et a! [1 ad may others 23 that airfoils do ot stall onrotatig wigs as tey do in wid tuel testsButtereld et a aso diovered that dyamic sllc exist o wind turbne blades during ormal opera

tg codtns Tere is moung evidece tatdyamic stall may be related dynamic loads that aregreater than predictons right 5] and Hansen 6]show improved accuracy n predicted dynamic loads and yaw loads wen dynac stall is introduced intotheir dynamic anayses

I order to deveop statc and dynamic stall models for wind turbines a a base of easured dynaic stallcharacteristics must est for comparison ad validationof �w codes Helicopter eperience and codes may be applicable but ust be vaidated using measurementsom operating wnd turbine measuremets The

National Renewabe nergy aboratory (NRE)supported

b te S epartment of ergy (DO) hasco ucted a series of eperimets that will supply this basc data The eperiment is called the CobnedExperimet and is escrbed by Butterfield et al in tworefereces [78] is paper describes dynamic stallmeasurements at four bade spawise staons of arotatig wind turbne bade oads are correlated withthe measured aroas Timevaryig pressuredistribuos are ened i detail



Test Desciption

A 10-m, three bladed, dowwid horizotal axis widturbie was used as a test platform. Molds were madeto high toleraces so that airfoil coordiates woul e

accurately trasferred to the test blades he E S80 airfoil was used because exsive wid tuel

data were available for this airfoil his arfoil is oe of a faily of airfoils desiged specically for widturbie u ager [] describes the airfoil as a 21%thick, lamiar flow arfoil with low roughesssesitivity

wo blades were made with o istrumetatio; a third was costructed with 124 pressure taps istalled isidethe blade. Butterfield [7] deribes the istallatiotechique ad the pressure measuremet istrumetatio for the first phase of this test program Phase Itestig required four chordwise pressure distributioslocated at 30%, 47%, 63%, ad 80% blade spas Pres

sure taps were located at 4% chord ad 36% chord othe suctio side of the airfoil, for six additioalspawise locatios

he pressure tap locatios ad istrumetati wereused i wid tuel tests at the Ohio State Uiversity OSU) ad Colorado State Uiversity CSU) widtuels as described by Buttereld et al [12 Bykeepig the istrumetatio, pressure tap, ad airfoilidetical betwee wid tuel tests ad rotatig blade

wid turbie tests, diereces i the results would bemore likely attributable to real difreces i airfoilperformace caused b three-dimesioal ad rotatig

blade eects.he dyamic pressure ad loca ow agle weremeasured at each of the four pressure distributios.yamic pressure was measured usig a total pressureprobe with a iteral agle of 45 deg. his probe wastested i the CSU wid tuel ad foud to give

accurate total pressure measuremets for agularmisaligmets up to 40 deg. he flow agle probe was

also ested i the wid tuel while mouted o the airfoil Upwash caused by circuatio effects causlocal flow agles to eviate om the geometric agle of

attack. this test the deviatios were measured adused to correct the rotag blade measured aglesButterfeld [4] describes these correctios as well asdyamic respo tes performed o the probe. otalpressure dyamic pressure) measured by the probe was ot aected by upwash.

Data Case Desciptions

wo data cases were chose for aalysis of resulshow i gures 1 through 22. he first case spaed

20 s of time durig 30deg yaw agle operatio ad wid speeds of 135 m/ s he ompass yaw agle of theturbie was 300 deg while the wid directio was20 eg he turbie rotates clockwise whe viewedo a dowwid ocatio lookig ito the wid eisumeted blade is poitig up whe i the 0-deg aimuth positio.

he secod case saed 10 s of time durig zero yawerror i wid speeds of 15 m/s hese two cases arecompared i able 1 ad were chose to illustrate thecoditios that cause dyamic stall

Tb C C 30-deg Yaw

CaseNo Yaw Cse

Vertica Plane Aay

(VPA) Aveage WndSpeed (m/s)

13.67 1539

Average TubineIntensity

014 006

Shear (m/s) 124 072

Pitch Angle (deg) 113 11.54

Pitch StandadDeviation deg)

033 021

Rotor Torque (N-m) 139 1976

Data Pocessin and Pessue CoeicientNomaization

All the pressure data were digiized at a 520-Hz samperate. ata were ater filtered at 10-Hz ad preaveraged

by a factor of 10 to obta a al 10Hz badwidth ad52-Hz sample rate. hese data were the sorted ito

bis usig the measured agle of attack AOA) as theidepedet variable his approac was used forresults preseted i figures 1 trough 22.

o obtai ormaized pressure coefficiets, dimesioalpressure data were divided by loca dyamic pressureper equatio 1). yamic pressure was establishe itwo ways. First, atmospheric pressure was subtractedom measured tota pressure to get a ocal dyamicpressure Q he cod method derived the local

value of Q by usig equatio 2) plus the diskaveraged wid speed measured from the vertical plae array),the rotor agular speed, ad the radius to the pressuretap.



C P

= ( Psuac e- P t) I Q (1)

Q = 1/2 (density) ( ( wind speed) + (r * omega) ) (2)

Both methods gave smlar results or small yaw agl s.

Ths was demostrated by Buttereld et al [12 whe

both ormazato techques compared well wth wd uel data at low AOAs At hgh AOAs (greatertha 25 deg), the measured Q method gave values o pressure coecet (Cp) greater tha oe or the 30% blade spa pressure dstrbuto

Most wd tuel dyamc stall data are ormalzed bylocal dyamc pressure Ths s smple becausedyamc pressure the wd tuel s steady Thedata are the used by structural dyamc codes, usgusteady local dyamc pressure to calculate bladeorces Fgure 1 shows measured ad calculateddyamc pressure durg yawed operato Themeasured dyamc pressure

Qshows the tower

shado at approxmately 80 deg The calculatoscluded the vector sum o the ee stream velocty adthe -plae blade velocty compoet, whch cludedthe ects o the advacg ad retreatg blade yawed ow Fgure 2 shows lt coecet dyamcstall wth costat Q (ucorrected), alculated perequato 2, ad tme varyg Q (corrected) as show Fgure 2 The corrected cve s the approprate curveor structural dyamc codes, whch calculate local,tme-varyg Ths dcates that the eects odyamc stall a appear to be more vere or lesssevere depedg o the ormalzato used

For spawse arloads t s approprate to ormalze bythe dyamc pressure calculated at the blade tp Ths approach shows a more realstc ull blade loaddstrbuo ths report all data are ormalzed byocal dyamc pressure based o the reestream velocty (as equato 2) except spawse pressuredstrbutos preted the pressure dstrbutossecto These are ormalzed by blade tp dyamcpressure

Azimth-Averaged Results

Fgures 3 through 6 llustrate AOA ad lt coecet

(CL) varatos azmuth averaged over 25 revolutosdurg 30deg yawed operato or 80%, 63%, 7%, ad30% blade spas The board statos cearly reachhgh values o CL whle the blade s rsg at

azmuth agles o 270 deg Mmum values o CLoccur ater the arol has stalled, whe the blade azmuth agle s betwee 0 ad 0 deg, o thedowwd sde o the rotor Statc stall wd tueltests results values o CL equal to 05 the

arol dd ot stall ad was learly related to AOA, the

CL woud ollow the AOA the cose shape show the same gues The derece lt that exsts omthe upwd sde o the yawed rotor (200 to 300 deg

azmuth agle) to the dowwd sde o the rotor (0 to00 deg azmuth agle) causes hgh yaw momets adlow speed shat (SS) cyclc loads

Peakto-peak values o AOA vary om 11 deg at the80% spa to 26 deg at the 30% spa Ths mples that al horzotal-axs rotors would experece AOA cyclc ampludes large eough to cau dyamc stall oreve modest yaw agles Both xed-ptch rotors adpaally eathered ptch-cool rotors woud experecemaxmum AOAs great eough to cause local stallg

gures 7 through 10 show smlar plots o CL ad AOA vaatos durg 0-deg yaw error operato As ca bee or ths case, AOA varatos ae small bycomparso because asymmetrcal low s due oly to

wd shear o 07 m/s across the rotor Tower shadows the major cotrbutor to low dsturbaces Ths sobvous rom the rapd chage AOA ad CL at180deg azmuth agle Ths dstubace s largeeough o cause dyamc stall as mpled by the sudderse CL above statc stall CL o 05

ynamic ta eslts

gures 11 through 1 llustrate azmuthaveraged CL vesus AOA compared to statc lt cves measured the CSU wd tuel tests [12] The dyamc stall

behavor s evdet the large hysteress loops thatsuroud the stc curves At the 80% spa therotatg blade CL values do ot exceed statc valuessgcatly Coversely, Fgures 12 through 1 showstatos 63%, 7%, ad 30%, whch show CL valuesexceedg statc vaues by 32% to 110%, respectvelyThe labeled ceter symbols dcate blade azmuth agle The derece lt at azmuth agles o 0 deg ad 270 deg s aga obvous these cures As thehysteress loos grow larger, the dereces crea

ad the resultg yaw momets crease

Pressre Distribtions

Fgure 15 shows the upper surace pressure dstrbutoo a rotatg turbe blade wth zero yaw take at 30%spa at a wd veocty o 17 m/s over a sglerotatoal cycle At 0deg azmuth, the turbe blade spotg up ad at 180 deg the blade s the ceter othe wake behd the tower Three-dmesoa suraceplots llustrate the chordwse pressure dstrbuto adts varao throughout the rotato cycle At staosdowstream alog the chord, a delay suctopressure maxma occurred relatve to the rotato cycleRobso [13] used ow vsualzato cojucto



with surface pressure data to correlate dynmic stal ortex formation with unsteady surface pressuresignatures. He osered a temporal delay in maximumsuction pressure at downseam locations along thechord which corresponded to passage of a dynamicstall ortex. Present results seem consistent with ortexformation and conection oer te suction surface of

wind turbine blades This pressure signture becomesmore obious for a yawed horiontal axis wind turbineHWT) as will be demonstrated later.

Figure 16 shows a comparison of measured and thereical AOA The agreement is generally good but there are discrepancies at high AOAs and the transducercannot respond fast enough to characterize the towershadow. The discrepancy at high angles may be due toinduced elocity ects or fow anomalies that are notincluded in the theoretical cre. The theoretical modelonly includes the ector addition of freestrem

elocity local in-plain elocity the effect of the yawed

turbine and a simple tower shadow model. Thiscomparison demonsates that the measured AOAyields reasonable results and has sufficient frequencyresponse to gie accurate dynamic stall characteriationfor yawed flow conditions but not for tower shadowcomparisons.

Figure 1 shows the theoretical AOA ariation of ayawed downstrem W for a yaw angle of 30 deg and wind elocity of 167 m/s. Figure 18 shows a bin aeraged upper surface pressure distribution at 30%span for 30-deg yaw. This may be contrasted to singlecycle data shown in Figure 1 for similar testconditions. In both cas two suction pressure peaks were en. ata taken for a single cycle howeershows that the suction pressue peaks are larger andmore transient in nature. Conection of a ortexstructure may be inferred by the delay in mximumsuction pressure at downstream stations long thehord which is consistent with results seen for pitching airfoils

Figure 20 shows the upper surface pressure distributionfor a turbine blade situated at -33deg yaw oer onecycle. Notice that the suction pressure peaks are

approximately 180 deg out of phase compared with thepositie yaw ca. Four direte suction pressure peaks

were obsered implying the formation of a number of ortices throughout the rotation cycle Conection ofthese ortices may be inferred at both 75-deg and270-deg azimuth. The upper surface pressuredistribution at 63% span is shown in Figure 21. Asuction pressure spike was obred at approximately120-deg azimuth and a smaller spike at 210 deg Noticethat the suction pressure pe persists oer a largerportion of the cycle (0-120 de) d conection of this

ortex can be inferred in the suction pressure ridge butoy u to 50% chord. This is in contrast to welldefined ortex conection to the trailing edge at 30%spa.

Figres 22 and 23 show span wise pressure distributionstaen at the 4% chord location for 0-deg yaw and bin

aerage results. All spanwise pressues renormalied with respect to the tip elocity using /2 * V + R ) where R 5.05 m. As can be seenin Figure 22 for low wind elocities of 7.7 m/s surfacepressure remains approximately constant along thespan There does ppear a suction pressure mximum

at approximately 47% span with suction pressuredecreasing modestly inboard and outboard. This may

be contrasted with the high wind elocity case of187 m/s seen in Figure 23 For this case suctionpressure decreases slghtly om 30% to 47% span andlitte eidence of a tower shadow induced ortex can beseen From 50% span outboard howeer oerallsuction pressure increses appreciably. In addition atower shadow nduced ortex signature becomes more

well defined at 180-deg azimuth for outboard stations.

Figures 24 and 25 show surface pressure distributionstaken at 4% chord for negatie and positie yaw cas and bin aeraged results Again al surface pressuredata were normalized with respect to the tip elocity.Figure 24 shows a -265deg yaw case. In generalmaximum suction presses occurred as the bladereached maximum height (0-deg and 360deg aimuth) At 80-deg azimuth minimum suction pressures wereencountered. These data exibit an interestingspanwise distribution howeer. From 30%- 47% spn

there is an appreciable decrease in suction pressure.From 47% - 63% span there was an increse in suctionpressure that leeled off om 63% - 80% sp In

addition at 30% span there appeared no eidence of atower induced ortex (which would likely occur near180deg azimuth) but this ortex did appear in thepressure signature near 63% span and outboardlocations Figure 25 shows a spanwise pressuredistribuon for the 30deg yaw case. As expectedmaximum suction pressures occurred approximately180 deg out of phas compared with the negatie yawca There is no eidence of a tower induced ortex inthe pressure profile except at 80% sp A constant

pressure ridge does persist at approximately 00-deg aimuth along the entire sp possibly indicatingunform ortex initiation. Howeer the suctionpressre peak persists for a relatiely short while at30% span and persists longer at outboard stations At80% span in fact the suction pressure mxima persistoer 45 deg of the rotation cycle.



Fgure 26 s dentcal to Fgure 25 except sngle cyclerather than bnaeraged results are presented. Ingeneral, the trends are dentcal to the bnaeragedresults wth a ew exceptons. The oerall suctonpressures are hgher or sngle cycle results Turbulenceleels, exhbted by rndom pressure uctuatons,

appeared greater and ths turbulence ncread atoutboard statons.

Surace pressure dstrbuons deonstrated theprence o dscrete ortex structures Conecton othese ortex structures was edenced by the teporaldelay n maxmum sucton pressures downstream alongthe chord and were consstent wth pressure sgnaturesproduced by ortex ormatons on ptchng arolsFuture analyss s requred, howeer, to ore accuratelypredct the ormaton o ortex structures on rotatng

wnd turbnes Ths wll allow or better modelng ostructural loadng and power generaton

Conclusions

Dynamc stall was shown to exst on a H W oper atng at a 3deg yaw angle. Dynac stall also occursor low yaw error operaton when tower shadow, wndshear, or nflow turbulece cause large AOA excursons.The ncreased aerodynamc loads cause creasedstructural loadng It was shown that dynac stallorces, caused by tower shadow, are short n duraton

and lower n agntude compared to those caused byyawed flow. It s mportant to odel dynac stall oryawed rotors or ths reason. The data presented n thsreport plus uture data wll prode the bascnoraton needed to deelop dyamc stall odels

Futue Wok

Blade geoetry appears to aect arol perormanceTo understand the eect o blade twst d taper on arol stll perormance, a tapered and twsted blade wll be deeloped and tested. Results wll be comparedto the exstng blade, whch has no twst or taper.

The flow condtons adjacent to the blade but o thesurace outer low condton) ay reeal the cau o

arol perormance abnoralty These flow states can be tested by obserng soke low patterns as the blades rotatng through the smoke Vdeo caeras wll beused to record the soke patterns Vdeo ageprocessng wll be used to correlate the patters wthpressure dstrbutons and other operatng codtonsThs normaton wll be used to proe stall odelsor wnd turbnes

References

1 Buttereld, C.P., DA. Smms, WP. Musal,G Scott, Spanwis A rodyamic oads on a Rotating

Blad SERI/TP2573983, UC Category 261,DE91211, Wnd Power '9 October 199)

2. Musal, W, and CP. Buttereld, A Copariso o Two and Three Densonal S89 Arol Prop rti s

or Rough and Sooth HWT Rotor Op ration

SRI/TP257363, C Category: 261, DE899512, Amercan Socety o Mechancal Egneers anuary199).

3. Vterna, A., and D. Corrgan, Fxed Ptch RotorPerorance o Larg Horzontal Axs Wnd Turbi s

DOE/ASA Workshop on arge Horzontal Axs WndTurbnes, Cleeland, Oho July 283, 1981)

4 Buttereld, C.P., ThreeDensonal ArolPerormance Measureents on a Rotating Win,

SERI/TP217355, Solar nergy Rearch Isttute,Golden, CO June 1989).

5. Wrght, A, and C.P. Butterfeld, The SERI T t ring

ub Rotor Code: Fnal Results and Conclusios, WndPower 91, Palm Sprngs, CA Septeber 1991).

6. Hann, A.C, and C.P. Buttereld, "Yaw oads and Motons o a Horzontal Axs Wnd Turbne," ASMEJournal o Solar En r Vol. 12 Noeber 199)

7. Buttereld, C.P., and . Nelsen, A rodyaic

T sting o a Rotating Wnd Turbne Blad SERI/TP257349, Solar nergy Rearch Insttute, Golden, COanuary 199).

8. Buttereld, CP, M. Jenks, D Smms, ad W. Musal,A rodyamic Pressure Measurements on a Rotatig

Wnd Turbne Blad , SRI/TP2573695, Solar nergyRearch Isttute, Goldn, CO May 199).

9. Tanger, J., and D Soers, Status o theSp cial

Purpos Arol Faili s SERI/TP2173264, SolarEnergy Research Insttute, Golden, CO Deceber 1987).

1. Somers, DM, D sign ad Exp ri ntal Results orthe S89 Airfoil, March 1989 to be publshed)

11. Gregorek, GM unpublshed report o OSU testngo S89, Noember 1989)

12. Buttereld, CP, G. Scott, nd W Musal,Comparison o Wnd Tunel Arol Perorance Data

wth Wd Turbne ladeData, SERI/TP2543799, SolarEnergy Research Insttute, Golden, CO uly 199).



13 Robnson M C D v lopm nt o Vortiity andVortes rom Fored Unst ady Flow S paration

hDThess Unersty o Colorado Boulder CO ly 198)



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iue 22 Spanwise pessue distibution vesusazimuth anle 77 s wind speed 4% chod tipvelocity nomaization binaveaed data

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SPANWISE PRESSURE DSTRIBUTON

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iue 23 Spanwise pessue distibution vesusazimuth ane 187 m/s wind speed 4% chod tipvelocity noaization binaveaed data

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SP ANWSE PRESSURE DISTRIBUTON 4% CHORD LOC ATION

iue 24. Spanwise pessue distiution vesusazimuth ane 18 s wind speed 4% cd tipvelocity nomalization binaveaed data 25deyaw



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ANWE EUE TIBUTON

%CHO LOCAON

T V e lo c N o rma zao 4 V = 14 mjs ;

deg yaw 3 � 8

Fgue 25. Spanwise pessue disibuion vesusauh ange, 14 s wnd speed, 4% chod, pveociy noaaion, bnaveaged daa, 30deg yaw

ANWE EUE TBUON

4%CHO OCAON

One cyce o V = j

de yaw

T p Velocty normaizato �

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gue 26 Spanwise pessue disbuon vesusuh ange, 67 s wnd speed, 4% cod pelociy noaiaon, bnaveaged daa, 30deg yaw

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