Available online at ScienceDirect. Energy Procedia 59 (2014 )

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1 Aville online t ScienceDirect Energy Procedi 59 (2014 ) Europen Geosciences Union Generl Assemly 2014, EGU 2014 Turulence chrcteristics in offshore wind frms from LES simultions of Lillgrund wind frm Wolf-Gerrit Früh *, Angus C.W. Creech, A. Eoghn Mguire c School of Engineering nd Physicl Sciences, Heriot-Wtt University, Edinurgh, Scotlnd, UK School of Engineering, University of Edinurgh, Edinurgh EH9 3JL, Scotlnd, UK c Vttenfll United Kingdom, The Tun, Holyrood Rod, Edinurgh, Scotlnd, UK Astrct The effect of wind turine wkes in lrge offshore wind energy rrys cn e sustntil fctor in ffecting the performnce of turines inside the rry. Turulent mixing plys key role in the wke recovery, hving significnt effect on the length over which the wke is strong enough to ffect the performnce of other turines significntly. We highlight how turulence ffects wind turine wkes using results from LES simultions of Lillgrund offshore wind frm in the context of SCADA dt selected to mirror the wind conditions simulted. The nlysis here concentrted on temporl spectr of wind velocities mesured y the turine's ncelle nemometer nd clculted t the turine loctions in the computtionl model. The effect of the wind turine rotor on the downstrem flow is quntified y nlysing the chnge in spectrl fetures of turines within the wind frm compred to turines t the side of the frm exposed to the wind The Authors. Pulished y Elsevier Ltd. This is n open ccess rticle under the CC BY-NC-ND license 2014 The Authors. Pulished y Elsevier Ltd. ( Peer-review under responsiility of the Austrin Acdemy of Sciences. Peer-review under responsiility of the Austrin Acdemy of Sciences Keywords: wind turine ; wke ; turulence. 1. Introduction With new offshore wind frms eing uilt t n incresing rte nd scle, the finncil implictions of losing out on potentil electricity production due to turine wkes dversely ffecting downstrem turines in the wind frm re significnt. It hs een shown tht turines within the wind frm my produce s little s 30% of their potentil * Corresponding uthor. Tel.: ; fx: E-mil ddress: w.g.fruh@hw.c.uk The Authors. Pulished y Elsevier Ltd. This is n open ccess rticle under the CC BY-NC-ND license ( Peer-review under responsiility of the Austrin Acdemy of Sciences doi: /j.egypro

2 Wolf-Gerrit Frü h et l. / Energy Procedi 59 ( 2014 ) when they re in the wke of nother turine [1] nd tht the free-strem turulence ffects the wke recovery sustntilly [2]. One oservtion from the wind frm investigted here ws tht the second turine in row ws performing worst of ll [1,3] which suggested tht complex interction of the free-strem turulence nd the turulence creted y the turine rotors my led to slow wke recovery fter the front turine, ut fster recovery further into the rry. The im of this pper is to investigte some turulence chrcteristics of the ir flow t the loctions of the turines using mesured nd simulted ncelle wind speeds dt Lillgrund wind frm Lillgrund offshore wind frm is locted 7 km south of the Øresund ridge etween Copenhgen in Denmrk nd Mlmö in Sweden nd hs een operted y Vttenfll Vindkrft AB since Decemer 2007 [4]. The rry consists of 48 turines, ech with rotor dimeter of D = 93 m nd hu height of 65 m, in regulr lttice-type rry s shown in Fig. 1 () where ech turine is given numer s well s grid-nme using column letters A to H nd row numers 1 to 8. The turines re close to ech other, with spcing of 4.3D = 400 m in the previling wind direction, 223 ), nd 3.3D = 307 m in the trnsverse. Overll, the extent of the wind frm is up to 2.9 km in the previling wind direction nd 2.25 km cross, covering totl re of round 6 km 2. The nlysis dt set ws derived from the output of turine dignostics from the SCADA (supervisory control nd dt cquisition) system t n intervl of 1 minute covering period of 323 dys, strting in Jnury 2008 when ll turines were connected to the system. 2. Methodology 2.1. The computtionl model The Computtionl Fluid Dynmics (CFD) solver used in these simultions ws the hr-dptive finite-element solver Fluidity, with the Wll-dpting Locl Eddy (WALE) sugrid turulence model [5], vrint of Lrge Eddy Simultion (LES). The three-dimensionl computtionl domin hd squre re of 8.1 km y 8.1 km, nd height of 600 m, s shown in Fig. 1. The wind frm ws positioned such tht the first turine ws 2 km from the inlet, llowing turulence to develop fully efore encountering the wind frm. The orienttion of the domin ws kept constnt such tht one side ws lwys specified with prescried inlet velocity profile consistent with neutrl tmosphere frequently found t Lillgrund [6], superimposed on which tmospheric turulence ws introduced using the Synthetic Eddy method [7].

3 184 Wolf-Gerrit Frü h et l. / Energy Procedi 59 ( 2014 ) Fig. 1. () Lyout of Lillgrund wind frm nd () Illustrtion of the computtionl domin. Fig. 2. () Velocity mesured t turines in row C nd () effect of turine wkes on performnce of downstrem turines, oth for wind direction of 223. The turines were incorported into the CFD domin using vrint of the ctutor disc method, which fetured oth, torque-controlled genertor nd ctive lde pitching. The interction etween the fluid nd the turine ws chieved through the lift nd drg coefficients from the turine ldes, with lde solidity distriuted uniformly in the zimuthl direction, nd spred with Gussin function in the stremwise direction within the turine volume. Additionlly, some turulence ws generted, especilly in the tip region [2]. Fr wy from the turines the resolution ws 75 m in the horizontl nd t lest 25 m in the verticl, whilst nerer the turines, the resolution ws mximum of 5 m oth horizontlly nd verticlly. To simulte different wind directions, the entire wind frm ws rotted within the fixed fluid domin.

4 Wolf-Gerrit Frü h et l. / Energy Procedi 59 ( 2014 ) Model configurtion Bsed on previous ssessment of the wind frm performnce [1, 8], the SCADA dt nlysis ws restricted to wind speed nd well within the cut-in wind speed nd the rted wind speed, nmely etween 5.5 nd 11 m/s, nd the CFD model used nominl wind speed of 10 m/s t hu height to specify the inflow conditions with verticl profile consistent with neutrl tmosphere. This ensured tht the turines would not rech their rted power ut tht even those within strong wke would still e operting. After n initil spin-up of the model without ctive turines lsting for 2000 s simultion time, the turines were ctivted nd llowed to rech stle operting conditions, s monitored y the power output from the turines. Typiclly, the turine models hd reched tht level fter round 400 to 600 s of model time. The ctul model results were then otined from continuing the simultion for further 600 s. The set of simultions covered eight wind directions from 198 to 236. The mteril presented here is primrily for the wind direction of 223, when the wind direction is fully ligned with the turine column, nd 229 when the second turine is prtilly shded y the front turine. 3. Results In this section, we first illustrte the oservtions from the SCADA system to provide the frmework for the computtionl results. After presenting the results in terms of turulence intensity, the findings re refined y spectrl nlysis of the stremwise velocity component SCADA results Fig. 3. Snpshot of the stremwise velocity field t hu height t the end of the simultion for the wind direction of 198. Fig. 2 shows the rnge of wind speeds t the turines in row C in the centre of the wind frm when the wind direction is fully ligned with tht row in form of oxes showing the qurtiles of the oservtions from the SCADA system. The second turine hs the lowest medin velocity lthough turines further downstrem my hve lrger rnge. This ehviour is mirrored y the power output from those turines in Fig. 2 where the shded ckground corresponds to the oservtions from the SCADA system nd the ox-nd-whisker plots show the CFD results. One cn lso see in Fig. 2 tht the velocity rnge is smllest for the front turines nd increses deeper into the turine rry.

5 186 Wolf-Gerrit Frü h et l. / Energy Procedi 59 ( 2014 ) Computtionl results A typicl instntneous snpshot of the stremwise velocity mgnitude in horizontl slice is shown in Fig. 3 where the left side is the inlet with the prescried velocity profile nd synthetic eddies to led to typicl turulence intensity t the wind frm loction. The first 1000 m re n entrnce section where the generted eddies rek up into typicl spectrum nd re then dvected through the domin. One cn see few eddies nd streks persisting throughout the domin, with some leding to jetting within the domin ut one cn lso see the wkes ehind individul turines (with lue nd turquoise colours) nd n extended wind frm wke persisting up to the outlet on the right Turulence intensity From the velocity time series of the equilirted finl 10 minutes of integrtion, smpled every 0.5 s, the turulence intensity ws clculted s /U 0 where U 0 is the upstrem wind speed t hu height. These computtionl results, shown s ox-nd-whiskers plot for ll eight wind directions investigted in Fig. 4, re lrgely consistent with the SCADA dt in tht the turulence intensity is smllest t the front turine, reches mximum within the wind frm nd then slightly reduces towrds the rer of the of the rry. It hs to e orn in mind tht this includes cses where the turine C07 is fully exposed to the free strem s well s those where it is fully shded y C08, which explins the lrge rnge of turulence intensities found for C07 cross this wind direction sector. To resolve the vrition in the turulence intensity for ech wind direction, we represent the turulence intensity for ech turine in row C (in the columns) ginst the wind directions simulted (in the rows) in Fig. 4. There we see firly uniformly low turulence level in the front turine, ut lso TI mximum in second row when this turine is prtilly shded nd low TI in tht turine when it is either fully exposed or fully shded. This suggests tht turulence genertion t the lde tips nd susequent mixing t the edge of the expnding wke with the surrounding ir is key process in orgnising the evolution of the wke. In prticulr, the wke recovery of turine s wke nd consequent power output t downstrem turine is strongly ffected y the inflow conditions into the upstrem turine. The wke of turine C08 recovers slowly s its rotor is exposed to lowturulence flow, nd the turulence is concentrted t the perimeter of the wke, only grdully mixing towrds the wke s centre. Hence, turine C07, when fully shded y C08, experiences reltively deep wke with only slightly elevted turulence. The comintion of the wke from C07 within C08 then provides sufficient turulence levels nd turulent mixing to recover tht comined wke fster, leding to higher power output t C06 s well s higher turulence intensity t tht turine. In contrst, when C07 is only prtilly shded, its rotor intersects with the perimeter of C08 s wke with the result tht it experiences high turulence intensity s well s sptilly highlyvrile wind speeds, leding to power output etween tht of the fully shded nd fully exposed cses. In ddition to the higher turulence level, the turine ldes will lso experience lterlly vrying loding Spectr The turulence intensity provides good ut firly lunt mesure of the flow chrcteristics. To understnd the processes etter one cn use spectr of the velocity components or of the kinetic energy to identify length scles or time scles which ply n ctive role in the flow dynmics. With n ville smpling rte of 1 min 1, the SCADA dt re insufficiently smpled for spectrl nlysis ut the CFD results, smpled t 2 Hz re sufficiently smpled to ccess the time scles of interest for wind turines. Fig. 5 overlys the temporl spectr of the stremwise velocity for the first four turines in row C (C08 to C05) in () for the wind direction where they ll re fully shded y the front turine (223 ) nd, in (), for the cse where C07 is prtilly shded y C08. On the whole they ll show similr spectr, with reltively flt prt t the low frequencies elow 0.1 Hz, decy lrgely consistent with f 0.01 Hz, nd high-frequency decy consistent with etween the four spectr with no cler difference etween those for C08 nd C07, ut slightly extended lowfrequency shelf (from 0.01 to 0.02 Hz) nd very slightly enhnced vriility for C06 nd C05 t frequencies

6 Wolf-Gerrit Frü h et l. / Energy Procedi 59 ( 2014 ) round 0.05 Hz nd for C05 lone t round Hz. In contrst, when the turines re not fully ligned, the lowfrequency plteu is lso extended for C07, nd C06 nd C05 show enhnced vriility clerly in the frequency rnge from 0.02 to 0.05 Hz. Fig. 4. () Stndrd devition of velocity in row C; () Level plot of turulence intensity t row C for ll wind directions. Fig. 5. Spectrum of stremwise velocity t the first four turines, () t wind direction of 223 when ll turines re fully shded y the front turine the front turine, nd () t wind direction of 229 when the second turine is only prtilly shded y the front turine.

7 188 Wolf-Gerrit Frü h et l. / Energy Procedi 59 ( 2014 ) Fig. 6. Rtio of the Spectr of second to forth turines over tht of the front turine, with the horizontl xis rescled to equivlent length scles, () t wind direction of 223 when the ll other turines re fully shded y the front turine, nd () t wind direction of $229 when the second turine is only prtilly shded y the front turine. To drw out these differences, the rtio of the spectrl mplitude for turine t ech frequency over tht t the front turine ws clculted. To lso get feeling for the length scles of flow with men velocity trnsporting the fluctutions, the frequencies were trnsformed to equivlent length scles y L= U 0 /f. The results of this comprison nd trnsformtion re shown in Fig. 6 for the sme cses s in Fig. 5. They highlight for 223 in Fig. 6() tht the second turine (C07, solid red line) experiences little chnge in the spectrl chrcteristics of the flow while the third (C06, dsh-doule-dotted green line) sees enhncement t the length scles round 200 nd 500 m nd further enhncement in fourth row (C05, purple dsh-dotted line) t round 300 m. At the wind direction of 229 in Fig. 6, the second turine experiences full enhncement t the length scle of 500 m which then extends down to 200 m for the third turine with no ovious further chnge for the fourth turine. 4. Conclusions We hve demonstrted tht high-resolution lrge-eddy simultion of wind frms using pproprite turine representtions cn e used to investigte the link etween tmospheric nd turine-generted turulence nd their effect on wke recovery nd turine performnce. The model results highlight the link etween turulence intensity, wke recovery nd wind frm performnce, suggesting tht the intersection or interction of successive turine wkes is key fctor in determining the wke decy. A etter quntittive understnding of this process will help to refine engineering wke models to improve the representtion of multiple wkes in lrge turine rry. The spectrl nlysis of the wind speed time series t the turine loction hs shown tht the incresed turulence intensity cn e ssocited with distinct time or length scles which re t length scles of the turine spcing. To sustntite this, the next nlysis step is sptil spectrl nlysis of the full flow field. Acknowledgements The uthors re grteful to Vttenfll for providing the SCADA dt, nd to the Edinurgh Prllel Computing Centre for providing time on their HECToR supercomputing resource. Angus Creech is grteful to Vttenfll for finncil support of this work. References [1] Creech, ACW, Früh, W-G, Mguire, AE. Full-scle simultions of wind frm using lrge eddy simultion nd torque-controlled ctutor disc model. Surveys in Geophysics sumitted.

8 Wolf-Gerrit Frü h et l. / Energy Procedi 59 ( 2014 ) [2] Creech, ACW, Früh, W-G, Clive, P. Actutor volumes nd hr-dptive methods for 3D simultion of wind turine wkes nd performnce. Wind Energy 2012;15(6): [3] Creech, ACW, Früh, W-G, Mguire, AE. High-resolution CFD modelling of Lillgrund wind frm. In: Interntionl Conference on Renewle Energies nd Power Qulity (ICREPQ 13); vol. 11. Renewle Energy nd Power Qulity Journl; [4] Jeppsson, J, Lrsen, PE, Lrsson, Å. Technicl description Lillgrund wind power plnt. Tech. Rep. 2.1 Lillgrund Pilot Project; Vttenfll Vind-krft AB; [5] Nicoud, F, Ducros, F. Sugrid-scle stress modelling sed on the squre of the velocity grdient tensor. Flow, Turulence nd Comustion 1999;62: [6] Bergström, H.. Meteorologicl conditions t Lillgrund. Tech. Rep. 6 2 LG Pilot Report ; Vttenfll Vindkrft AB; [7] Jrrin, N, Benhmdouche, S, Lurence, D, Prosser, R. A synthetic- eddy-method for generting inflow conditions for lrge-eddy simultions. Int Journl of Het Fluid Flows 2006;27: [8] Dhlerg, JÅ. Assesment of the Lillgrund windfrm: Power performnce, Wke effects. Tech. Rep. 6.1 Lillgrund Pilot Project; Vttenfll Vindkrft AB; 2009.