Bechmann, Andreas

Koblitz, Tilman

Sogachev, Andrey

Sørensen, Niels N.

For wind resource assessment, the wind industry is increasingly relying on Computational Fluid Dynamics (CFD) models that focus primarily on modeling the airflow in a neutrally stratified surface layer. So far, physical processes that are specific to the atmospheric boundary layer (ABL), for example the Coriolis force, buoyancy forces and heat transport, are mostly ignored in state-of-the-art CFD models. In order to decrease the uncertainty of wind resource assessment, especially in complex terrain, the effect of thermal stratification on the ABL should be included in such models.The present work examines the influence of stability on the whole ABL using the modified in-house CFD code (DTU Wind Energy) EllipSys3D. Typical diurnal cycles are simulated and compared against previous simulations and measurements from the GABLS II model intercomparison [6]

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Downloaded from orbit.dtu.dk on: Dec 20, 2017Atmospheric stability in CFD &NDASH; Representation of the diurnal cycle in theatmospheric boundary layerKoblitz, Tilman; Bechmann, Andreas; Sogachev, Andrey; Sørensen, Niels N.Publication date:2012Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Koblitz, T., Bechmann, A., Sogachev, A., & Sørensen, N. N. (2012). Atmospheric stability in CFD &NDASH;Representation of the diurnal cycle in the atmospheric boundary layer. Poster session presented at EWEA 2012- European Wind Energy Conference & Exhibition, Copenhagen, Denmark.Atmospheric Stability in CFD    the diurnal cycle in the atmospheric boundary layer        T Koblitz A Bechmann A Sogachev N Sørensen. , . , . , . DTU Wi d E n  nergyAbstractFor wind resource assessment, the wind industry is increasingly relying onComputational Fluid Dynamics (CFD) models that focus primarily on modeling theairflow in a neutrally stratified surface layer So far physical processes that are. ,ifi t th t h i b d l (ABL) f l th C i li fspec c o e a mosp er c oun ary ayer , or examp e e or o s orce,buoyancy forces and heat transport are mostly ignored in state of the art CFD, - - -models. In order to decrease the uncertainty of wind resource assessment,especially in complex terrain the effect of thermal stratification on the ABL should,be included in such models.The present work examines the influence of stability on the whole ABL using themodified in-house CFD code (DTU Wind Energy) EllipSys3D Typical diurnal.l i l t d d d i t i i l ti dcyc es are s mu a e an compare aga ns prev ous s mu a ons anmeasurements from the GABLS II model intercomparison [6].Obj tiec vesT dif th i ti Elli S 3D CFD t t i t d i ti f• o mo y e ex s ng p ys o ge a more appropr a e escr p on othe wind flow in the ABL during non-neutral conditions• To validate the model against previous simulations and measurementsT l th i fl f di l t t i ti th ABL• o ana yze e n uence o urna empera ure var a ons on eTo set the starting point for non neutral simulations over complex terrain• -Stable stratification Figure 2: (a) surface temperature (b) geostrophic wind (c) time series of wind speed at 10m400 m a.g.l. (d) time series of turbulent kinetic energy at 55m a.g.l. (e) vertical profiles of wind speedat 1400 23 October (f) vertical profiles of potential temperature at 1400 23 October, , .10U t bl t tifi tiu (m/s)ns a e s ra ca on 0 The above modifications of the 3D version of EllipSys enable us to apply thedeveloped model for flows over complex terrain Figure 1 shows the horizontalFigure 1: non-neutral simulation over complex terrain: horizontal wind speed for (a) stable and .(b) t bl diti th B k h lli it i I di F d t il b t th wind speed for non-neutral conditions over the Benakanahalli site in Indiauns a e con ons over e ena ana a s e n n a. or e a s a ou e .Benakanahalli measurement campaign see poster ID 107.C l ionc us onsMethods The general features of the typical diurnal cycle and its representation by theThe present study considers the simulation of the diurnal cycle in the ABL The model are presented. Furthermore the sensitivity of the numerical results on the.f i fl fl i (h i ll h fl ) bj d f i d th i iti l diti i d Th h th d l tocus s on ow over at terra n or zonta y omogeneous ow , su ecte to orc ng an e n a con ons are exam ne . e c osen me o o ogy otemporally varying surface temperatures To model the ABL more appropriately the implement stability effects into the CFD code EllipSys3D represents a promising.h d i fi t t t t d th li ti f th d l t t tifi d fleffect of the Coriolis forcing and buoyancy are included in the CFD code approac an s a rs s ep o ex en e app ca on o e mo e o s ra e owElli S 3D Th f ti f th i t f th t ti l over complex terrainp ys . ere ore an equa on or e energy n erms o e po en a .temperature is solved in addition to the RANS equations To close the given set of The developed model is able to reproduce the general flow pattern of a non-neutral.i difi d i f h k b l d l i d i h ABL fl bj t d t di ll i f t tequat ons a mo e vers on o t e -ε tur u ence mo e s use : n contrast to t e ow su ec e o a urna y vary ng sur ace empera ure.standard formulation we use a limiter on the resulting length scale [1 2] and- , , The present simulations and the measurements from [6] show rather goodadditional buoyancy terms [3 4] Also ambient floor values for the turbulence t f l t ft til l i D i th i t iti, .i bl i d i d t id i l i d t t b l lagreemen rom a e a ernoon un ear y morn ng. ur ng e morn ng rans on,var a es are mpose n or er o avo numer ca ssues ue o ur u ence va ues however the growth of the convective ABL and the turbulence level are too weakclose to zero [5] With these modifications the model is capable of representing, ,hi h i l i ibl i th d ti t d l l l i d d N f th.l di i d h difi d i f h b lw c s a so v s e n e un eres ma e ow- eve w n spee . one o enon-neutra con t ons, an t e two mo e transport equat ons or t e tur u ent models intercompared in [6] was able to capture the morning transition Modellingkinetic energy k and the dissipation ε read: .the diurnal cycle presents a big challenge, and the present model shows thebi t d i ti ft th i t iti d th b t t d i l t(1) gges ev a ons a er e morn ng rans on, an e es agreemen ur ng a eafternoon and early evening.C i i t b ti i th i f i iti l d b d(2) ompar son aga ns o serva ons ra ses e ssue o n a an oun aryconditions of numerical experiments because perfect test cases do not occur in,lit Th d ll d lt iti t th i iti l t t fil Al(3) rea y. e mo e e resu s are sens ve o e n a empera ure pro e. solarge scale atmospheric variations influence measured statistics as for exampleh B i th d ti ( l) f t b l t ki ti b b,w ere s e pro uc on or remova o ur u en ne c energy y uoyancy apparent in the non-constant geostrophic wind during the GABLS test case (seeforces and depends upon the local temperature gradient The coefficient C in (2) fi 2b) Th i l t d i d fi ld i i fl d i ifi tl b th h i f th, . ε1i l d b (3) i d li i h i i l h [1 2] All h ffi igure . e s mu a e w n e s n uence s gn can y y e c o ce o es rep ace y n or er to m t t e m x ng engt , . ot er coe c ents are geostrophic wind as the model’s forcingchosen according to [3 4]., .ReferencesResults1. Apsley, D.D., Castro I.P., 1997. A limited-length-scale k–e model for the neutral and stably-stratified atmospheric boundary layer. Boundary-Layer Meteorol, 83:75–98.Surface winds temperature stratifications and TKE values resulting from a non- 2. Blackadar, A.K., 1962. The Vertical Distribution of Wind and Turbulent Exchange in a Neutral Atmosphere. J. Geophys. Res, 97:,f f 3095–3102..neutrally strati ied ABL low are compared against modelled and observed data3. Sogachev, A., Panferov O., 2006. Modification of two-equation models to account for plant drag. Boundary- Layer Meteorol., from the GABLS II experiment from Kansas USA [6] shown in figure 2 The 121:229–266., , .4. Sogachev, A., 2009. A note on two-equation closure modelling of canopy flow. Bound.-Lay. Meteorol., 130: 423–435.development of the ABL throughout the day is driven by a prescribed surface5. Spalart, P.R., Rumsey, C.L., 2007. Effective Inflow Conditions for Turbulence Models in Aerodynamic Calculations. AIAA Journal, t t th t i i d ith ti d t t t hi i d (fi 2 b) Vol. 45, No. 10, pp. 2544-2553.empera ure a s var e w me an a cons an geos rop c w n gure a, .6. Svensson, G. et al., 2011. Evaluation of the Diurnal Cycle in the Atmospheric Boundary Layer Over Land as Represented by a Details concerning model set-up and forcing are given in [6] Variety of Single-Column Models: The Second GABLS Experiment. Boundary-Layer Meteorology 140, no. 2: 177-206..EWEA 2012 Copenhagen Denmark: Europe’s Premier Wind Energy Event , ,      0000 0600 1200 1800 0000 0600 1200 1800 000002468101214161820Geostrophic wind speed (m/s)Observations from soundings (3-4 km)0000 0600 1200 1800 0000 0600 1200 1800 0000−5051015202523 Oct 24 Oct 23 Oct 24 OctModel forcing00000 0600 1200 1800 0000 060002468Local timeWind speed (m/s)00.511.522.530000 0600 1200 1800 0000 0600Local time0 2 4 6 8 10 12 1400.510Wind speed (m/s)1.5z (km)285 287 289 291 293 29500.511.5Potential temperature (K)z (km)tke (m2 s-2 )Surface temperature (C°)(b)(a)(c) (d)(e) (f)EllipSys3DLES [6]1D ABL models [6]central tower [6]Models:Observations:nearby tower [6]outer network [6]Model forcingPO. ID 182 

Atmospheric stability in CFD &amp;NDASH; Representation of the diurnal cycle in the atmospheric boundary layer

                       General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.  • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal   If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.     Downloaded from orbit.dtu.dk on: Dec 20, 2017Atmospheric stability in CFD &NDASH; Representation of the diurnal cycle in theatmospheric boundary layerKoblitz, Tilman; Bechmann, Andreas; Sogachev, Andrey; Sørensen, Niels N.Published in:ProceedingsPublication date:2012Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Koblitz, T., Bechmann, A., Sogachev, A., & Sørensen, N. N. (2012). Atmospheric stability in CFD &NDASH;Representation of the diurnal cycle in the atmospheric boundary layer. In Proceedings European Wind EnergyAssociation (EWEA).ABSTRACT ID:  188Theme: RESOURCE ASSESSMENT  Topic:  Wind resource variationsATMOSPHERIC STABILITY IN CFD &AMP;NDASH; REPRESENTATION OF THE DIURNAL CYCLE IN THE ATMOSPHERIC BOUNDARY LAYERTilman Koblitz(1)  (F)  (P) Andreas Bechmann(1) Andrey Sogachev(1) Niels Sørensen(1) (1) DTU Wind Energy, Roskilde, DenmarkIntroductionFor wind resource assessment, the wind industry is increasingly relying on Computational Fluid Dynamics (CFD) models that focus primarily onmodeling the airflow in a neutrally stratified surface layer. So far, physical processes that are peculiar to the atmospheric boundary layer (ABL), forexample the Coriolis force, buoyancy forces and heat transport, are mostly ignored in state-of-the-art CFD models. In order to decrease theuncertainty of wind resource assessment, especially in complex terrain, the effect of thermal stratification on the ABL should be included in suchmodels.ApproachThe starting point for the present study is the existing in-house CFD code EllipSys3D (Sørensen, N.N., 1995. General purpose flow solver applied toflow over hills. Technical Report Risø-R-827(EN), Risø National Lab., Roskilde, Denmark). This general purpose CFD solver has been initiallydeveloped for flow over terrain and is used for a wide range of wind energy applications. To model the flow within the ABL more appropriately, thefinite-volume code is modified.Main body of abstractThe present study considers the simulation of the diurnal cycle in the ABL. The focus is on flow over flat terrain (horizontally homogeneous flow),subjected to temporally varying surface temperatures. To account for non-neutral stratifications, the evolution equation for potential temperature isadded and the effect of buoyancy is included in the turbulence model: additional terms are added to the equations for the turbulent kinetic energy(TKE) and the dissipation rate of TKE (Sogachev, A., 2009. A note on two-equation closure modeling of canopy flow. Bound.-Lay. Meteorol., 130:423–435). Coriolis forcing is also implemented. The resulting surface winds, temperature stratifications and TKE values are compared against observations from the GEWEX (Global Energy andWater cycle EXperiment) Atmospheric Boundary Layer Study (GABLS) taken in Kansas, USA (Svensson, G. et al., 2011. Evaluation of the DiurnalCycle in the Atmospheric Boundary Layer Over Land as Represented by a Variety of Single-Column Models: The Second GABLS Experiment.Bound.-Lay. Meteorol., 140(2): 177-206), and against simulations of similar diurnal cycles from other atmospheric models. The evolution of the flowfield during the day is satisfactorily reproduced . Nevertheless, comparison against observations raises the issue of initial and boundary conditions ofnumerical experiments, because perfect test cases do not occur in reality. Large scale atmospheric variations influence measured statistics, as forexample apparent in the non-constant geostrophic wind during the GABLS test case.ConclusionThe general features of the typical diurnal cycle and its representation in the model are presented and discussed. Furthermore the sensitivity of thenumerical results on the forcing and the initial conditions are examined. The chosen methodology to implement stability effects into the CFD codeEllipSys3D represents a promising approach and is a first step in order to extend the application of the model to stratified flow over complex terrain.

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Atmospheric stability in CFD &NDASH; Representation of the diurnal cycle in the atmospheric boundary layer

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Atmospheric stability in CFD &amp;NDASH; Representation of the diurnal cycle in the atmospheric boundary layer

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Atmospheric stability in CFD &NDASH; Representation of the diurnal cycle in the atmospheric boundary layer