atmospheric flow over terrain using hybrid rans/les european wind energy conference & exhibition...
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Atmospheric Flow over Terrain using Hybrid RANS/LESEuropean Wind Energy Conference & Exhibition 2007
A. Bechmann, N.N. Sørensen and J. Johansen
Wind Energy Department
Risoe National Laboratory
Technical University of Denmark
Appetizer:
Simulation of Wind over Complex Terrain
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Outline1. Introduction: Terrain flow using CFD
2. New approach: Hybrid RANS/LES model
3. Simulation results: Askervein Hill
4. Conclusions
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Introduction: Complex terrain?
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Introduction: Why CFD?
• The critiquel wind happens locally - difficult to measure
• Computational fluid dynamics (CFD) -provides supplemant
• Structural loads in complex terrain - most important issue
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Introduction: EllipSys3D codeRisø/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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RANS-method
1. Provides the mean wind speed and turbulence intensity
2. Average amount of modelling
3. Wall-func. easily implemented
4. Computational affordable
Introduction: problem formulation 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
RANSRANSLESLES
LES-method
1. Provides detailed and transient wind information
2. Minimum of modelling needed
3. Wall-functions are problematic
4. Near-surface flow is computational very expensive
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Outline1. Introduction: Terrain flow using CFD
2. New approach: Hybrid RANS/LES model
3. Simulation results: Askervein Hill
4. Conclusions
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Momentum equations for resolved motions:
New approach: basic equationsMomentum equations for an incompressible Newtonian fluid:
Components are decomposed into resolved and unresolved parts:
• 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 modelTurbulent stresses: product of fluid strain and eddy-viscosity:
Equation for TKE modified for LES and RANS length scales:
‘Standard‘ model:
Similar to Detached-Eddy SimulationSpalart et al. (1997)
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New approach: turbulent lengthscale
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New approach: turbulent lengthscale
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Outline1. Introduction: Terrain flow using CFD
2. New approach: Hybrid RANS/LES model
3. Simulation results: Askervein Hill
4. Conclusions
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Askervein hill: backgroundAskervein hill:• Hebrides, Scotland. 1982-1983• H=116m, planform: 2x1km
Computational grid:• N=288x240x96 (~7mill cells)• Domain size: 8.8x5.5x1.5 km• Resolution: Δ=23m, z1=0.03m
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Askervein hill: precursorPrecursor:
• 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
Wind database
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Askervein hill: flow field
Hybrid LES
RANS
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Askervein hill: speed-up
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Askervein hill: turbulence
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Outline1. Introduction: Terrain flow using CFD
2. New approach: Hybrid RANS/LES model
3. Simulation results: Askervein Hill
4. Conclusions
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Conclusions
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Thank you for listening!