CEID Annual Seminar 2012 - LUT · − CFD model of an operating wind farm, coupled with a local...

CFD Modelling on WindCFD Modelling on Wind FlowsFlows

Ashvinkumar Chaudhari

Centre of Computational Engineering and Integrated Design (CEID)Lappeenranta University of Technology (LUT)P.O.Box 20, FI‐53851 Lappeenranta, Finland

21 March 2012 www.lut.fi/ceid

I t d tiIntroduction

− Modelling of a wind flow in environment is of great interest in terms of windenergy applications, such as it helps to locate and construct as well as to

t l th i d fcontrol the wind farms.

− CFD model of an operating wind farm coupled with a local wind forecastCFD model of an operating wind farm, coupled with a local wind forecast,can increase accuracy of electric power generation, providing valuableinformation to wind farm operators.

− Several theoretical, experimental and numerical studies have been reportedabout modelling of a wind flow in complex terrains contain hills, forest, lake,g p , , ,etc.

21 March 2012 2www.lut.fi/ceid

RENEWTECH j t i LUT/CEIDRENEWTECH project in LUT/CEID

− RENEWTECH: Development of wind power technology and business in South Finland 2011-2013

− Coordinated by Cursor Oy and funded by− Coordinated by Cursor Oy and funded by ERDF

− Building a fluid dynamics research on wind energy by combining expertise on energy CFD and industrial mathematicsenergy, CFD and industrial mathematics.

− 5 Doctoral dissertations are connected to RENEWTECH project in LUT


RENEWTECH project: ObjectivesRENEWTECH project: Objectives

− CFD modelling from meteorological scale and atmospheric boundary layer to wind farm and turbine blade simulations. Large Eddy Simulation for atmospheric boundary layer in complex terrain Large-Eddy Simulation for atmospheric boundary layer in complex terrain CFD for wind park with forests, lakes and hills Efficient wind analysis tools (field measurements, WASP modelling) Aerodynamics of wind turbine blades with some defects Multi-objective optimization of wind turbines w.r.t. energy, economy, noise,

mechanicsmechanics− International networking within ERCOFTAC

21 March 2012 www.lut.fi/ceid 4

T b l d lTurbulence models

− Most of the research currently taking place in the field of CFD concerns thestudy of turbulent flows. Almost any naturally occurring flow is turbulent, andhence it is important to be able to model turbulent flows accuratelyhence it is important to be able to model turbulent flows accurately.

− Different approaches to make turbulence computationally tractable: Reynolds-Averaging Navier-Stokes Equations (RANS)

• Gives a prediction of the mean velocity and the mean level ofturbulent quantitiesturbulent quantities

Direct Numerical Simulations (DNS)• It captures all of the relevant scales of the turbulent motion. But this

approach is extremely expensiveapproach is extremely expensive,• Computational cost requires for DNS is proportional to (Turbulent

Reynolds number) L Edd Si l ti (LES)

4/9ReT

Large Eddy Simulations (LES) Detached Eddy Simulations (DES)


L Edd Si l ti (LES)Large Eddy Simulation (LES)

─ Large Eddy Simulation (LES) is model to simulate the turbulent flows in CFDwhere the smaller eddies are filtered and are modeled using a sub-grid scale(SGS) models, while the larger energy carrying eddies are simulated.

─ It was initially proposed in 1963 by Joseph Smagorinsky to simulateatmospheric air currents.

─ First real applications were made by Deardorff in 1970, where he simulated theconvective ABL.

─ Why LES ??─ Some applications need explicit computation of accurate unsteady fields.

Bl ff b d d i h th fl i d b l t b l t l− Bluff body aerodynamics, where the flow is governed by large turbulent scales− Aerodynamically generated noise (sound)− Atmospheric boundary layer (ABL)− Mixing

Figure courtesy of Ansys Fluent

Mixing− Combustion − Examples:


U t d flUnsteady flow

− Turbulent flow over a complex terrain contains complex flow characteristics suchas, separation, reattachment, aerodynamic instabilities, etc. Therefore, anadvance approach should be appliedadvance approach should be applied.

− Due to inherent unsteady phenomena of this flow, it is difficult to model byRANS approach. Thus, time dependent computations such as DNS or LES arerequired.

− Relative to RANS, LES can be computationally expensive, requiring about 1000times greater computational resources however it yields fidelity solutions fortimes greater computational resources, however, it yields fidelity solutions forflow configurations where RANS fails.

Figure courtesy of Ansys Fluent

DNS

3D, unsteadyRANS

2D or 3D,steady or unsteady

LES

3D, unsteady


unsteady

Modeling of complex terrainModeling of complex terrain Forest data to CFD code

− Measurement form real forest, using laser scanning− Find the porosity and permeability of forest canopy − Shape of the forest canopy giving “roughness” to large-scale modelling− Modelling of flow through forest using porous medium approach in CFD

codecode

laser scanned data from forest

velocity profile

find the porosity, CFD d liCFD modeling

apply CFD(RANS or LES)( )


At h i B d L (ABL)Atmospheric Boundary Layer (ABL)

− What is ABL?− Lowest region of the atmosphere, directly affected by the Earth’s surface

Th ABL l i t t l i fi ld i l di i ll ti d th− The ABL plays an important role in many fields, including air pollution and the dispersal of pollutants, aeronautical, meteorology, weather forecasting, and climate studies, etc.

− Very high Re (~107 to 109 ) turbulent flow, over rough wall, strongly affected by buoyancy forcesOver the past four decades the atmospheric community has done much work− Over the past four decades, the atmospheric community has done much work using LES to accurately simulate the ABL

Free Atmosphere

Turbulent Eddies

Free Atmosphere

Atmospheric pBoundary Layer : 1‐2 km

Courtesy of Marcelo Chamecki


9

Test case: LES for Channel flowTest case: LES for Channel flow (Re_tau=180)

− Domain size: 2πδ X 2δ X πδ, where δ is the channel half height (=1).− Mesh resolution: (100 X 101 X 100) = 10 10 000( )− Flow time=21.78 s, with time step (dt)= 0.0015 sec

u‐ instantaneous


LES of fully developed channel flowLES of fully developed channel flow(Re_tau=180)

Grid: 100 X 101 X 100

− LES has a reasonably good agreements with DNS data provided by Moser y g g p yet al. (1999)

− Values are normalized with ut.

O l l d R ld t− Only resolved Reynolds stress21 March 2012 www.lut.fi/ceid 11

LES f i fl 2D th hillLES for air flow over 2D smooth hill

Full Domain

− Grid nodes: 292 X 120 X 58 (=2032320)− Log law velocity profile with artificial velocity fluctuation

P i di BC i i (Z) di i− Periodic BC in span-wize (Z) direction− FVM based commercial code ANSYS Fluent 13.0 − Reynolds number based on hill height and free stream velocity Re =3120Reynolds number based on hill height and free stream velocity, ReH=3120.


Results visualizations RMS l iResults visualizations Contours of U-velocity instantaneous

RMS u‐ velocity

instantaneous

48.5 sec

Streamlines of U-velocity instantaneous


Results visualizations RMS l iResults visualizations RMS u‐ velocity

Contours of U-mean velocityU-mean velocity

Contours of U-RMS velocity


LES i tiLES-animation− Instantaneous stream-wise velocity (mmean + fluctuation)Instantaneous stream wise velocity (mmean fluctuation)− This shows the flow separation and reattachment behind hill


Comparison with measurementComparison with measurement [Khurshudyan et al. (1981)]

− Mean velocity profile at several locations before and after hill.− Results are normalized by free stream velocity, uinf =4.1 (m/s)

− Even with the different Reynolds number (ReH), the overall mean velocity


y ( H) yprofile has a quantitatively good agreement at several locations.

CFD code:CFD code: from commercial to open source

− For the present parallel LES case:− ReD : 2.33 X 104 ,Total no. of nodes: 2x106 , − CPU time: 400 Hours (~ 16.5 days), cores:12

Wh t if th t h i R ld b 108?− What if we use the atmospheric Reynolds number, say 108?− mesh nodes (assume): 500x106 >> which should required 1000 cores

(i.e. 500k nodes per core) >> CPU time: ??( p )− using commercial CFD code, the cost: 1000 € X 1000 (licenses) − already1 M € (just for licenses), we still need 1000 processors, cost ??

− LES and DNS cost a lot CPU time >> require massive parallel computing and they are still difficult in a recent timethey are still difficult in a recent time.

− Therefore the use of open source code is really necessary for LES and DNS, at least we can get the CPU speed and save some money.


R fReferences

− ANSYS FLUENT 12.0 Theory Guide, April 2009− ANSYS FLUENT 12.0 User’s Guide, April 2009

CFD Wiki: www cfd online com/Wiki/− CFD-Wiki: www.cfd-online.com/Wiki/− Moser, Kim and Mansour (1999): DNS of Turbulent Channel Flow up to Re_tau=590,

Physics of Fluids, vol. 11, 943-945.− Fredrik Carlsson PPT slides: “LES With FLUENT“ LES DES hybrid LES/RANS and− Fredrik Carlsson, PPT slides: LES With FLUENT . LES, DES, hybrid LES/RANS and

URANS, Chalmers University of Technology, Göteborg, Sweden, 2006.− Marcelo Chamecki, Lecture notes: ”Large Eddy Simulation Applications to Meteorology”,

(2010).− Davidson L., (2009): Large Eddy Simulations: How to evalute resolution, Int. Jou. Of Heat

and Fluid Flow, vol.-30, 1016-1025.− Allen, T. and Brown, A.R., (2002): Large-eddy simulation of turbulent separated flow over

rough hills. Boundary-Layer Meteorol. 102, 177-198.− Kim, J. J., Baik, J. J., and H. Y. Chun, (2001): Two-dimensional numerical modeling of flow

and dispersion in the presence of hill and buildings. Journal of Wind Engineering andI d t i l A d i 89 947 966Industrial Aerodynamics. 89, 947-966.


R fReferences

− Castro, I. P., Apsley, D. D., (1997): Flow and dispersion over topography: a comparisonbetween numerical and laboratory data for two-dimensional flows. AtmosphericEnvironment. 31, 839-850.Environment. 31, 839 850.

− Khurshudyan, L. H., Snyder, W. H. and Nekrasov 1. V. (1981): Flow and dispersion ofpollutants over two-dimensional hills United States Environmental Protection Agencypollutants over two dimensional hills. United States Environmental Protection AgencyReport EPA-600/4-8 I-067.

− Tamura T Cao S Okuno A (2007): LES study of turbulent boundary layer over aTamura, T., Cao, S., Okuno, A., (2007): LES study of turbulent boundary layer over asmooth and a rough 2D hill model. Flow, Turbulence and Combustion, 79, 405-432.

− Stangroom P (2004): CFD modelling CFD modelling of wind flow over terrain PhD− Stangroom P., (2004): CFD modelling CFD modelling of wind flow over terrain, PhDdissertation at University of Nottingham.


Th k f tt ti !!!− Thank you for your attentions !!!

− Comments are appreciated !!!


Inland wind farms - Modelling ofInland wind farms Modelling of forests and hills

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CEID Annual Seminar 2012 - LUT · − CFD model of an operating wind farm, coupled with a local...

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