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Atmospheric stability in CFD – Representation of the diurnal cycle in theatmospheric boundary layer

Koblitz, Tilman; Bechmann, Andreas; Sogachev, Andrey; Sørensen, Niels N.

Publication date:2012

Document VersionPublisher's PDF, also known as Version of record

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Citation (APA):Koblitz, T., Bechmann, A., Sogachev, A., & Sørensen, N. N. (2012). Atmospheric stability in CFD –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 CFDAtmospheric Stability in CFDAtmospheric Stability in CFD the diurnal cycle in the atmospheric boundary layerthe diurnal cycle in the atmospheric boundary layerthe diurnal cycle in the atmospheric boundary layer

T Koblitz A Bechmann A Sogachev N SørensenT. Koblitz, A. Bechmann, A. Sogachev, N. SørensenDTU Wi d EDTU Wind Energygy

AbstractAbstract

For wind resource assessment, the wind industry is increasingly relying onFor wind resource assessment, the wind industry is increasingly relying onComputational Fluid Dynamics (CFD) models that focus primarily on modeling theComputational Fluid Dynamics (CFD) models that focus primarily on modeling theairflow in a neutrally stratified surface layer So far physical processes that areairflow 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 fspecific to the atmospheric boundary layer (ABL), for example the Coriolis force,p p y y ( ), p ,buoyancy forces and heat transport are mostly ignored in state of the art CFDbuoyancy forces and heat transport, are mostly ignored in state-of-the-art CFDmodels. In order to decrease the uncertainty of wind resource assessment,models. In order to decrease the uncertainty of wind resource assessment,especially in complex terrain the effect of thermal stratification on the ABL shouldespecially in complex terrain, the effect of thermal stratification on the ABL shouldbe included in such modelsbe included in such models.

The present work examines the influence of stability on the whole ABL using theThe present work examines the influence of stability on the whole ABL using themodified in-house CFD code (DTU Wind Energy) EllipSys3D Typical diurnalmodified in house CFD code (DTU Wind Energy) EllipSys3D. Typical diurnal

l i l t d d d i t i i l ti dcycles are simulated and compared against previous simulations andy p g pmeasurements from the GABLS II model intercomparison [6]measurements from the GABLS II model intercomparison [6].

Obj tiObjectivesObjectives

T dif th i ti Elli S 3D CFD t t i t d i ti f• To modify the existing EllipSys3D CFD to get a more appropriate description ofy g p y g pp p pthe wind flow in the ABL during non-neutral conditionsthe wind flow in the ABL during non-neutral conditions

• To validate the model against previous simulations and measurementsTo validate the model against previous simulations and measurementsT l th i fl f di l t t i ti th ABL• To analyze the influence of diurnal temperature variations on the ABLy pTo set the starting point for non neutral simulations over complex terrain• To set the starting point for non-neutral simulations over complex terrain

Stable stratificationStable stratification

Figure 2: (a) surface temperature (b) geostrophic wind (c) time series of wind speed at 10m400 m 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 speedg ( ) gy g ( ) p pat 1400 23 October (f) vertical profiles of potential temperature at 1400 23 Octoberat 1400, 23 October (f) vertical profiles of potential temperature at 1400, 23 October.

10

U t bl t tifi tiu (m/s)

Unstable stratification 0 The above modifications of the 3D version of EllipSys enable us to apply theThe above modifications of the 3D version of EllipSys enable us to apply thedeveloped model for flows over complex terrain Figure 1 shows the horizontal

Figure 1: non-neutral simulation over complex terrain: horizontal wind speed for (a) stable anddeveloped model for flows over complex terrain. Figure 1 shows the horizontal

Figure 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 India(b) unstable conditions over the Benakanahalli site in India. For details about the wind speed for non neutral conditions over the Benakanahalli site in India.Benakanahalli measurement campaign see poster ID 107.

C l iBenakanahalli measurement campaign see poster ID 107.

ConclusionsMethodsMethods

The general features of the typical diurnal cycle and its representation by theThe 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 theThe present study considers the simulation of the diurnal cycle in the ABL. Thef i fl fl i (h i ll h fl ) bj d

model are presented. Furthermore the sensitivity of the numerical results on thef i d th i iti l diti i d Th h th d l tfocus is on flow over flat terrain (horizontally homogeneous flow), subjected to forcing and the initial conditions are examined. The chosen methodology to( y g ), j

temporally varying surface temperatures To model the ABL more appropriately theg gy

implement stability effects into the CFD code EllipSys3D represents a promisingtemporally varying surface temperatures. To model the ABL more appropriately the implement stability effects into the CFD code EllipSys3D represents a promisingh 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 approach and is a first step to extend the application of the model to stratified floweffect of the Coriolis forcing and buoyancy are included in the CFD code

Elli S 3D Th f ti f th i t f th t ti lpp p pp

over complex terrainEllipSys3D. Therefore an equation for the energy in terms of the potential over complex terrain.p y q gy ptemperature 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-neutraltemperature is solved in addition to the RANS equations. To close the given set of

i difi d i f h k b l d l i d i hThe developed model is able to reproduce the general flow pattern of a non neutralABL fl bj t d t di ll i f t tequations a modified version of the k-ε turbulence model is used: in contrast to the ABL flow subjected to a diurnally varying surface temperature.q

standard formulation we use a limiter on the resulting length scale [1 2] andj y y g

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

The present simulations and the measurements from [6] show rather goodt f l t ft til l i D i th i t itiadditional buoyancy terms [3,4]. Also ambient floor values for the turbulence

i bl i d i d t id i l i d t t b l lagreement from late afternoon until early morning. During the morning transition,

variables are imposed in order to avoid numerical issues due to turbulence valuesg y g g g

however the growth of the convective ABL and the turbulence level are too weakpclose to zero [5] With these modifications the model is capable of representing

however, the growth of the convective ABL and the turbulence level are too weak,hi h i l i ibl i th d ti t d l l l i d d N f thclose to zero [5]. With these modifications the model is capable of representing

l di i d h difi d i f h b lwhich is also visible in the underestimated low-level wind speed. None of the

non-neutral conditions, and the two modified transport equations for the turbulentp

models intercompared in [6] was able to capture the morning transition Modelling, p qkinetic energy k and the dissipation ε read:

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 thethe diurnal cycle presents a big challenge, and the present model shows the

bi t d i ti ft th i t iti d th b t t d i l t(1)

biggest deviations after the morning transition, and the best agreement during late(1)

gg g g gafternoon and early eveningafternoon and early evening.

C i i t b ti i th i f i iti l d b d(2) Comparison against observations raises the issue of initial and boundary(2) g yconditions of numerical experiments because perfect test cases do not occur inconditions 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) reality. The modelled results are sensitive to the initial temperature profile. Also(3) y p plarge scale atmospheric variations influence measured statistics as for example

h B i th d ti ( l) f t b l t ki ti b blarge scale atmospheric variations influence measured statistics, as for example

where B is the production (or removal) of turbulent kinetic energy by buoyancy apparent in the non-constant geostrophic wind during the GABLS test case (seep ( ) gy y y yforces and depends upon the local temperature gradient The coefficient C in (2)

apparent in the non constant geostrophic wind during the GABLS test case (seefi 2b) Th i l t d i d fi ld i i fl d i ifi tl b th h i f thforces, and depends upon the local temperature gradient. The coefficient Cε1 in (2)

i l d b (3) i d li i h i i l h [1 2] All h ffi ifigure 2b). The simulated wind field is influenced significantly by the choice of the

is replaced by (3) in order to limit the mixing length [1,2]. All other coefficients areg ) g y y

geostrophic wind as the model’s forcingp y ( ) g g [ , ]chosen according to [3 4]

geostrophic wind as the model s forcing.chosen according to [3,4].

ReferencesReferencesResultsResults

1. Apsley, D.D., Castro I.P., 1997. A limited-length-scale k–e model for the neutral and stably-stratified atmospheric boundary layer. y g y y yBoundary-Layer Meteorol, 83:75–98.Surface winds temperature stratifications and TKE values resulting from a non- y y

2. Blackadar, A.K., 1962. The Vertical Distribution of Wind and Turbulent Exchange in a Neutral Atmosphere. J. Geophys. Res, 97:Surface winds, temperature stratifications and TKE values resulting from a non-f f

g y3095–3102..neutrally stratified ABL flow are compared against modelled and observed data

3. Sogachev, A., Panferov O., 2006. Modification of two-equation models to account for plant drag. Boundary- Layer Meteorol., y p g

from the GABLS II experiment from Kansas USA [6] shown in figure 2 The g g y y121:229–266.from the GABLS II experiment from Kansas, USA [6], shown in figure 2. The

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 surface g g y y5. Spalart, P.R., Rumsey, C.L., 2007. Effective Inflow Conditions for Turbulence Models in Aerodynamic Calculations. AIAA Journal,

development of the ABL throughout the day is driven by a prescribed surfacet t th t i i d ith ti d t t t hi i d (fi 2 b)

y yVol. 45, No. 10, pp. 2544-2553.temperature that is varied with time and a constant geostrophic wind (figure 2a,b).

6. Svensson, G. et al., 2011. Evaluation of the Diurnal Cycle in the Atmospheric Boundary Layer Over Land as Represented by a p g p ( g )

Details concerning model set-up and forcing are given in [6] y y y yVariety of Single-Column Models: The Second GABLS Experiment. Boundary-Layer Meteorology 140, no. 2: 177-206.Details concerning model set-up and forcing are given in [6]. y g y y gy

EWEA 2012 Copenhagen Denmark: Europe’s Premier Wind Energy EventEWEA 2012, Copenhagen, Denmark: Europe s Premier Wind Energy Event

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PO. ID 182