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CFD methods for wind turbine aerodynamics

Sørensen, Niels N.

Published in:Extended abstracts

Publication date:2008

Link back to DTU Orbit

Citation (APA):Sørensen, N. N. (2008). CFD methods for wind turbine aerodynamics. In T. Kvamsdal, K. M. Mathisen, & B.Pettersen (Eds.), Extended abstracts (pp. 39-42). International Center of Numerical Methods in Engineering.

https://orbit.dtu.dk/en/publications/cd8bbebd-31e4-4839-b58f-a11660472a20

CFD Methods For Wind Turbine Aerodynamics21st Nordic Seminar on Computational Mechanics21st Nordic Seminar on Computational MechanicsOctober 16-17, 2008, Trondheim

Niels N. SørensenWind Energy Department, Risø DTUNational Laboratory for Sustainable Energy

Risø’s history in briefRisø’s history in brief

• 1956 Peaceful utilisation of nuclear energy• 1976 Nuclear energy and other energy sources• 1976 Nuclear energy and other energy sources• 1986 Energy research in general• 1990 R&D with energy as the primary area• 1994 State owned enterprise• 1994 State-owned enterprise• 2000 The last nuclear reactor is decommissioned• 2005 Impact within

1 T h l f t titi 1. Technology for greater competitiveness 2. Sustainable energy supply3. Health technology

• 2007 Merger with DTU National Laboratory for • 2007 Merger with DTU, National Laboratory for Sustainable Energy

Risø DTU, Technical University of Denmark 17.10.200821st Nordic Seminar on Computational Mechanics2

Outline of talkOutline of talk

• Aerodynamics of wind tubines• Aerodynamics of wind tubines• CFD in Wind Turbine Aerodynamics

– General OverviewThe EllipSys2D/3D solvers– The EllipSys2D/3D solvers

• Applications– Airfoil Aerodynamics

T iti d li– Transition modeling– Dynamic stall– Deep stall aerodynamics

d– Rotor aerodynamics– Aeroelasticity– Flow over terrain


Aerodynamics for wind turbinesAerodynamics for wind turbines

• Flow over complex terrain• Flow over complex terrain• Rotor aerodynamics• Rotor/Tower interaction

Wake aerodynamics• Wake aerodynamics• Airfoil Flows• Laminar/turbulent transition

H t i h d i t ll• Hysteresis phenomena, dynamic stall• Damping and stability• Aeroelasticity


Development and origin of CFD for Wind Development and origin of CFD for Wind Energy• The application of numerical methods to fixed wing and rotor • The application of numerical methods to fixed wing and rotor

aerodynamics dates back to the late seventies in the aerospace community, solving steady Potential and Euler equations.

• The first unsteady solution to the Euler equations were seen through the eighties.

• With the continuous increase in computer power in the late eighties and early nineties the first Reynolds Averaged Navier-Stokes codes for helicopter applications appeared.

• With the possibility of handling viscous flow, the first applications of Navier-Stokes CFD solvers appeared in the wind turbine community in th l t i tithe late nineties.

• In Europe a series of EU-financed projects were providing the basis for many of these activities


many of these activities.

Components of a CFD methodComponents of a CFD method

The basic idea is to take the partial differential equations describing the The basic idea is to take the partial differential equations describing the fluid flow, transform them into a set of algebraic equations, and solve these using a numerical method on a computer.

Typical components of a CFD code are listed below:

• Mathematical Model• Mathematical Model– Compressible/Incompressible– Potential/Euler/Navier-Stokes– Turbulence ModelingTurbulence Modeling

• Coordinate and basis vector systems• Discretisation method, space and time

– Finite Difference/ Finite Volume/ Finite Elements– Finite Difference/ Finite Volume/ Finite Elements• Solution Method• Computational Grid


Turbulence ModelingTurbulence Modeling

• Direct Numerical Simulation (DNS)• Direct Numerical Simulation (DNS)

• Filtered EquationsLarge Eddy Simulation (LES)– Large Eddy Simulation (LES)

• Time Averaged Equations, Reynolds Averaged Navier-Stokes(RANS)Al b i M d l ( B ld i L )– Algebraic Models (e.g. Baldwin-Lomax)

– One Equations Models (e.g. Spalart-Allmaras, Baldwin-Barth)

– Two Equation Models (e.g. k-ω, k-ε)

ld S d l– Reynolds Stress Models

• Hybrid Models– Detached Eddy Simulation


Laminar to Turbulent TransitionLaminar to Turbulent Transition

• Michel Model• Michel Model• e^n-model (Data base)• Bypass transition

Correlation based transition• Correlation based transition


Computational Grid

• Structured

Computational Grid

• Structured• Unstructured• Multi-block

Different type of conforming grids– Different type of conforming grids– Overlapping or Chimera

• Hybrid Meshes (Structured/ Unstructured)C t i C t C ll• Cartesian Cut Cells


Ellipsys2D/3DThe choices in our flow solverThe choices in our flow solver

• Incompressible Navier Stokes• Incompressible Navier-Stokes• Finite volume code (non-staggered)• Cartesian or polar velocity components

Patched multi block grids (new overset option)• Patched multi-block grids (new overset option)• Pressure/Velocity formulation • Steady/Unsteady algorithm

M i M h/M i F• Moving Mesh/Moving Frame• Turbulence Modelling RANS/DES• Acceleration techniques: grid sequence/multi-grid

ll ll d f d b d• Parallellized using MPI for distributed computers


Airfoil Aerodynamics2D applications2D applications

• Basic studies• Basic studies• Cl/Cd/Cm for BEM computations• Airfoil catalog

Airfoil design and optimization• Airfoil design and optimization• Planning and conduction of measurements• Laminar/turbulent transition

D i t ll t ti • Dynamic stall computations


Airfoil AerodynamicsLaminar/Turbulent transition

NACA 64 618 Re=6 0 millionNACA 64-618, Re=6.0 million• C-mesh with 512x128 cells• Wall normal distance: y+<1

Maximum expansion rate: < 5%• Maximum expansion rate: < 5%• Inflow turbulence intensity 0.009%


Airfoil AerodynamicsDynamic stall

• Dynamic stall• Dynamic stall• Stall characteristics• Aerodynamic damping


Deep Stall Aerodynamics3D blade section in static stall

RANS simulation DES simulationDES simulation

Unsteady lift time series

The development of a stochasticstall-model based on time seriesfrom DES-simulations is ongoing


Rotor AerodynamicsNREL/Nasa Ames testNREL/Nasa Ames test

Risø DTU, Technical University of Denmark 17.10.200821st Nordic Seminar on Computational Mechanics15 NASA Ames Tunnel (24.4x36.6 m)NREL Phase-VI Wind Turbine

Rotor AerodynamicsBlind ComparisonBlind Comparison

Upwind Configuration, Zero Yawp g ,

3500

4000

2500

3000

Torq

ue (N

m)

measurements♦

1500

2000

Spee

d Sh

aft T

Risø comp.

500

1000

Low

-S

05 10 15 20 25

Wind Speed (m/s)


Rotor AerodynamicsNREL Phase VI rotor Cp distribution NREL Phase-VI rotor, Cp. distribution

V=7m/s


Rotor AerodynamicsNREL Phase VI rotor Cp distributionNREL Phase-VI rotor, Cp. distribution

V=20m/s


Rotor AerodynamicsNREL Phase-VI rotor, Laminar/turbulent transition

The transition model can improve the results in some casesThe transition model can improve the results in some cases


Rotor AerodynamicsNREL Phase-VI rotor, Laminar/turbulent transition

• Limiting streamlines• Limiting streamlines


Rotor AerodynamicsYaw computations (60 degrees)

Azimuth variation of normal force

4

6

8

10

12

CN

S1500600, r/R=0.30

MeasurementsComputed

0.7

0.8

0.9

1

1.1

CN

S1500600, r/R=0.95


1.4

1.6

1.8

2

2.2

2.4

2.6

CN

S1500600, r/R=0.63


-2

0

2

4

0 50 100 150 200 250 300 350

Azimuth Angle [deg.]

0.3

0.4

0.5

0.6

0 50 100 150 200 250 300 350


0.4

0.6

0.8

1

1.2

0 50 100 150 200 250 300 350


Top view


Rotor AerodynamicsRotors in atmospheric shearRotors in atmospheric shear

• Rotors in shear flow has been studied• Rotors in shear flow has been studiedusing CFD to quantify hysteresis effects

Azimuthal hysteresis of tangential force


Rotor AerodynamicsDetailled design analysisDetailled design analysis

Enercon E-70 design Tip-design


Rotor AerodynamicsDrag values for parked wind turbine bladesDrag values for parked wind turbine blades

Definition of aspect ratioL/w=L2/Area

Blade L/w Cd comp.

LM8.2 11.4 1.23

LM19.1 18.0 1.32


Modern 21.0 1.16

Rotor AerodynamicsRotor tower interactionRotor tower interaction

Details of blade tower interaction investigated in order to:Details of blade-tower interaction investigated in order to:- study lock-in phenomina- develop semi-emperical tower shadow model and noise model


Site AnalysisComplex TerrainComplex Terrain

Terrains where WAsP is not suitable

Determining Speed-up, and flow inclination

Evaluation of turbine positions, from levels of turbulent kinetic energy


ConclusionConclusion

• A CFD methodology suited for wind • A CFD methodology suited for wind energy has been introduced

• A series of applications of CFD to wind • A series of applications of CFD to wind energy, including airfoil and rotor aerodynamics has been given, illustrating the possibilities.


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