UPWIND, Aerodynamics and aeroelasticity

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• UPWIND, Aerodynamics and aero-elasticity
• Rotor aerodynamics in atmospheric shear flow
• N. N. Sørensen12 and J. Johansen2
• 1Department of Wind Energy, Risø National
  Laboratory, DTU, Denmark
• 2 Department of Civil Engineering, Aalborg
  University, Denmark

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Introduction

• The basic idea is to provide new insight about
  rotor operation in shear, with the aim to improve
  engineering models.
• The problem of modeling the atmosphere and the
  rotor using CFD with RANS type turbulence
  modeling
• How to account for the shear
• How to avoid excessive eddy viscosity
• What will we look for
• Azimuth variation of the rotor loads at different
  radial positions
• Azimuth variation of the inflow velocity
• The wake behavior downstream of the rotor
• Disturbance of the upstream flow due to the rotor
  loading

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Extreme wind gradient over flat terrain

• During day time the velocity gradient over flat
  terrain will typically be lt 2 m/s over a 100
  meter rotor
• RANS equation are not well suited to model the
  atmosphere and the rotor in a single computation
  for day time conditions
• During night time the velocity may vary as much
  as 5-6 m/s over a rotor diameter of 100 meters.
• During night time the stable stratification
  suppress the mechanical turbulence and makes the
  flow low turbulent

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Modeling the wind shear

• We will take advantage of the fact that the flow
  is nearly laminar during stable stratification at
  night
• The computational domain is spherical
• The velocity profile is approximated by a power
  law

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Rotor computations

• EllipSys3D Navier-Stokes solver
• Moving mesh technique
• Nacelle and tower are neglected
• The grid consists of 14 million cells
• Steady state/transient computations
• Time step 2.d-3

The Upwind turbine Hub height 90 meter Rotor
diameter 126 meter
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Power and Thrust
Deviation due to airfoil data
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Azimuth variation of low speed shaft torque

• The shaft torque is not constant, and the load
  even though periodic possesses some phase lag

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Spanwise variation of the axial force

• The hysteresis of the spanwise forces between the
  90 and 270 degree position is clearly visible
• The blade remember the load history

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Phase lag of axial force and velocity
R
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Phase lag of axial force and velocity
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Phase lag of axial force and velocity
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Vertical profile of axial-velocity
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Wake patterns

• The wake is tilted due to the vertical velocity
  gradient
• The non-uniform azimuth loading is reflected in
  the wake pattern
• The expansion of the wake is non-uniform

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Conclusion

• 3D CFD rotor computations of a wind turbine in
  severe atmospheric shear have been performed
• The time history of the mechanical power exhibits
  a clear 3P variation, even though the amplitude
  is very low
• The computation show a large phase lag in the
  axial and tangential forces especially at central
  part of the rotor blades and decreasing towards
  the blade tips
• The phase lag observed in the forces is also
  observed for the upstream axial velocities
  especially at central part of the rotor blades
  and decreasing towards the blade tips
• The azimuth variation of the blade loads are
  clearly reflected in the rotor wake

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Future work

• We plan to compute and compare to measurements
  form Høvsøre tests showing the extreme wind shear
  for a real turbine
• Using LIDAR the up- and downstream effect of the
  rotor can be measured
• Knowledge about the velocity field around the
  turbine at high shear may be important for power
  curve measurements
• We hope to provide detailed data that can be used
  to improve the BEM type methods