Simulations of Wind Turbine Wakes in Sheared and Turbulent Inflow

1
Simulations of Wind Turbine Wakes in Sheared and
Turbulent Inflow

• Niels Troldborg1, Jens N. Sørensen2 Robert
  Mikkelsen2
• 1 Wind Energy Department, Aeroelastic Design
  Program, Risø DTU
• 2 Department of Mechanical Engineering, Fluid
  Mechanics Section, Technical University of
  Denmark

2
Outline

• Objectives Motivation
• Numerical models
• Actuator Line model
• Atmospheric boundary layer modelling
• Validation
• Results
• Conclusions

3
Objectives Motivation

• Document the wake development in a standard shear
• What happens when the wake impacts the ground?
• Study influence of turbulence on wake stability.
• Importance for downstream turbines.
• Investigate fundamental features of the wake
  including vortex breakdown, interference factors,
  turbulence properties and wake meandering.

4
The Actuator Line Technique

• Flow field governed by full 3D Navier Stokes
  Computations (EllipSys3D) using LES.
• Blades represented only as lines.
• Unsteady aerodynamic blade forces governed by
  velocities obtained along lines using tabulated
  2D airfoil data.
• Body forces subsequently distributed along and
  away from lines.

5
Atmospheric Boundary Layer Modelling

• Atmospheric boundary layer modelling consists of
  two parts
• Steady wind shear
• Ambient turbulence
• Wind shear model based on immersed boundary
  technique, i.e. body forces used to
  prescribe a given velocity profile
• Forces determined from the Navier-Stokes
  equations
• Present work assumes a power law shear profile

6
Atmospheric Boundary Layer Modelling

• Atmospheric turbulence modelled by introducing
  artificial turbulent fluctuations in a plane
  upstream of the turbine.
• Imposed turbulence field generated using the
  method of Mann
• Velocity fluctuations generated by introducing
  unsteady concentrated body forces in the plane.

Rotor
Computation domain (8.4106 cells)
Turbulence plane
7
Results

• Computations

• CP-curve

• Wind turbine
• 3MW NM80 turbine, D80m
• Inflow velocity
• V8 5, 10, 15 m/s
• Rotor thrust
• CT 0.87, 0.71, 0.43
• Ambient turbulence
• s 8/ V8 0, 1 and 9
• Shear coefficient
• a 0.2

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Comparison with Measurements

• Streamwise velocity 2.5D downstream (V810 m/s,
  s8/V89)

• Turbulence intensity 2.5D downstream (V810 m/s,
  s8/V89)

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Results Qualitative Description

• Contours of instantaneous vorticity at V8 5 m/s
• s8/V80
• s8/V81
• s8/V89

10
Results Qualitative Description

• Contours of instantaneous vorticity at V8 15
  m/s
• s8/V80
• s8/V81
• s8/V89

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Results Mean Properties

• Streamwise velocity at V8 5 m/s

• Streamwise velocity at V8 15 m/s

s8/V8 0 s8/V8 1
s8/V8 9
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Results Mean properties

• Center line induction for various CT at s8/V8
  9.

• Center line induction at CT 0.87 and different
  turbulence levels.

?Vzz-0.7 to z-0.85
13
Conclusions

• Comprehensive study of wake of turbine operating
  in a sheared inflow with various degrees of
  ambient turbulence.
• Ambient turbulence significantly alters the flow
  properties of the wake.
• The inflow velocity shear and presence of wall
  causes an asymmetric wake development where the
  wake expands more upwards and to the sides than
  downwards.
• There seems to be a speed up under the rotor and
  wake.
• Other INTERESTING topics covered in the
  conference paper are Added turbulence and wake
  meandering.