simulation of a 5mw wind turbine in an atmospheric boundary layer

University of Stuttgart The Science of Making Torque from Wind, K. Meister, 10.10.2012

Institute of Aerodynamics und Gas Dynamics Email: [email protected]

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Simulation of a 5MW wind turbine in an atmospheric

boundary layer

K. Meister, Th. Lutz, E. Krämer

Institute of Aerodynamics and Gas Dynamics



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� Motivation

� Process chain for wind turbine simulation

� Atmospheric boundary layer (ABL) simulation

� Setup

� Turbulence propagation

� Simulation of the REpower 5M (Alpha Ventus)

� Setup

� Load evaluation

� Conclusion & Outlook

Outline



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� Efficient wind power production depends on reliability and economic feasibility

� Turbine loads are affected by unsteady aerodynamic effects

� Pitch oscillations (Theodorsen’s Theory)

� Gusts (Sears’ Problem, Miles’ Problem)

� …

� CFD methods are able to consider unsteady aerodynamic effects physically correct

� Better load prediction

– Load fluctuations

– Peak loads

� Accurate near wake prediction

– Far wake prediction (wind farms)

Motivation

[1] J. Gordon Leishman, Principles of Helicopter Aerodynamics

[1]

Pitch oscillations



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� Finite volume based 2nd order flow solver (provided by DLR)

� Structured multi block code

� (U)RANS / DES

� Moving / Overlapping meshes (CHIMERA)

� Added features:

� Time-resolved atmospheric

turbulent inflow

� Higher order WENO scheme

for convective fluxes

� Time-accurate strong fluid

structure coupling

Process chain for wind turbine simulationsFlow solver FLOWer

(IAG)

(SWE) IAG & SWE



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� Simulation of blade aerodynamics

� Turbine boundary layer fully resolved (Y+ ≈ 1)

� 3D-viscous and unsteady effects are considered

� Simulation of tower and nacelle

� Calculation of near wake

� Highly resolved by using grid refinement and higher order schemes

� Simulation of interactions with atmospheric boundary layer

� Unsteady inflow

� Large scale turbulence is resolved

Process chain for wind turbine simulationsResolution



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Atmospheric boundary layer simulation (DES)Computational setup

Iso surface of x-velocity at 14m/s of LES in a yz-plane over time (extract from ForWind data)

Setup of simulation:

• 16 million cells

• LES inflow data from ForWind (based on FINO

1 site specific maritime boundary layer)

• Linear interpolation of LES inflow data to fit IAG

time step

• No-slip walls on ground

• Periodic boundaries used on both sides of

background mesh

• DES with Spalart Allmaras turbulence model

Computational domain



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Atmospheric boundary layer simulation (DES)Turbulence propagation

Turbulence intensity at 95m height at several downstream positions and for higher order WENO scheme (simulations without turbine)

turbine position

Previous questions:

• How is turbulence transport affected by dissipation?

• Where should the turbine be placed?

∆y

∆y ≈ 46m



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Setup of simulation:

• Full model (35.5 million cells) with extract of LES inflow data (ForWind)

• Fixed time step 0.020833 sec (1.5°azimuth) over 38 full rotations

• No-slip walls on every surface

• Periodic boundaries used on both sides of background mesh

• DES with Spalart Allmaras turbulence model

• 11.2°constant pitch

LES inflow data by ForWind

Simulation of the REpower 5MComputational setup



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Simulation of the REpower 5MOut of plane normal blade force over one revolution (5 seconds)

Uniform inflow(periodic)

Atmospheric inflow(not periodic)

shift

titi

shift

turb

turb

turb

Both normalized with mean Fax of uniform inflow



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Simulation of the REpower 5MOut of plane normal blade force & spectrum

Out of plane normal blade force over 60 seconds (one inflow period)

Out of plane normal blade force spectrum of uniform and atmospheric inflow (evaluated over 60 seconds; rotor freq. 0.2 Hz)

• Distinct load fluctuations at blade passing frequency and higher harmonics

• Broadband excitation of load fluctuations for atmospheric inflow

• Amplitudes of 0.5% up to 0.5Hz (atm. Inflow)



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Simulation of the REpower 5MBlade load distribution

1. Blade tower interaction is higher for inner section

2. Higher amplitudes at low frequencies for the inner sections

3. Frequency range of linear time interpolation of inflow data

: normalized fluctuation of chord normal force coefficient



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� Method for simulation of wind turbines in atmospheric boundary layer was presented

� Turbulence transport with CFD till wind turbine position is possible with less dissipation

� Turbulence transport can be improved by using higher order methods

� DES simulation of REpower 5M

� Turbulence of the ABL has significant influence on loads

� Detailed load analysis by using CFD is possible

� Comparison of results to simple/faster models (BEM)

� Consideration of time accurate fluid structure interaction in an ABL of the REpower 5M

(Together with Endowed Chair of Wind Energy (SWE) – Stuttgart)

� Coupling of the CFD simulation to a pitch controller

Conclusion & Outlook



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Acknowledgements

OWEA:All Project Partners

The IAG is member of the WindForS Alliance



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Thank you for your attention!

simulation of a 5mw wind turbine in an atmospheric boundary layer

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