Atmospheric Flow over Terrain using Hybrid RANSLES

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• Atmospheric Flow over Terrain using Hybrid
  RANS/LES
• European Wind Energy Conference Exhibition 2007
  
• A. Bechmann, N.N. Sørensen and J. Johansen
• Wind Energy Department
• Risoe National Laboratory
• Technical University of Denmark
• andreas.bechmann_at_risoe.dk

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• Appetizer
• Simulation of Wind over Complex Terrain

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Outline

• Introduction Terrain flow using CFD
• New approach Hybrid RANS/LES model
• Simulation results Askervein Hill
• Conclusions

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Introduction Complex terrain?
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Introduction Why CFD?

• Structural loads in complex terrain - most
  important issue

• The critiquel wind happens locally - difficult to
  measure

• Computational fluid dynamics (CFD) -provides
  supplemant

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Introduction EllipSys3D code

• Risø/DTU code, Applications
• Terrain computation
• Airfoil computations
• Rotor computations

• EllipSys3D code
• Incompressible Navier-Stokes
• Finite-volume (non-staggered)
• Pressure/Velocity formulation
• Patched multi-block grids
• Parallellized using MPI for distributed computers

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Introduction problem formulation

• RANS-method
• Provides the mean wind speed and turbulence
  intensity
• Average amount of modelling
• Wall-func. easily implemented
• Computational affordable

• Complex terrain
• Wall-functions are needed due to surface
  roughness (buildings, grass, trees etc.)
• Simple flow models for complex terrain are
  unreliable.
• The unsteady wind is important when predicting
  wind loads

• LES-method
• Provides detailed and transient wind information
• Minimum of modelling needed
• Wall-functions are problematic
• Near-surface flow is computational very expensive

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Outline

• Introduction Terrain flow using CFD
• New approach Hybrid RANS/LES model
• Simulation results Askervein Hill
• Conclusions

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New approach basic equations

• Stress term replaces viscosity term (high Re)
• RANS- and LES-equations written in same form
• different only by how stress term is modelled

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New approach turbulence model
Similar to Detached-Eddy Simulation Spalart et
al. (1997)
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New approach turbulent lengthscale
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New approach turbulent lengthscale
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Outline

• Introduction Terrain flow using CFD
• New approach Hybrid RANS/LES model
• Simulation results Askervein Hill
• Conclusions

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Askervein hill background

• Askervein hill
• Hebrides, Scotland. 1982-1983
• H116m, planform 2x1km

• Computational grid
• N288x240x96 (7mill cells)
• Domain size 8.8x5.5x1.5 km
• Resolution ?23m, z10.03m

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Askervein hill precursor

• Precursor
• Wind over flat-terrain with homogeneous roughness
• Simulation is run until statistics are converged
• Velocity slice are stored at each timestep

• Successor
• Wind over terrain with in- homogeneous roughness
• Variables are read at each timestep
• At least one flow through time before results are
  sampled

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Askervein hill flow field
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Askervein hill speed-up
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Askervein hill turbulence
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Outline

• Introduction Terrain flow using CFD
• New approach Hybrid RANS/LES model
• Simulation results Askervein Hill
• Conclusions

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Conclusions
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• Thank you for listening!