Atmospheric Flow over Terrain using Hybrid RANS/LES European Wind Energy Conference & Exhibition...
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![Page 1: Atmospheric Flow over Terrain using Hybrid RANS/LES European Wind Energy Conference & Exhibition 2007 A. Bechmann, N.N. Sørensen and J. Johansen Wind Energy.](https://reader035.fdocuments.in/reader035/viewer/2022062808/56649dd15503460f94ac74f2/html5/thumbnails/1.jpg)
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
![Page 2: Atmospheric Flow over Terrain using Hybrid RANS/LES European Wind Energy Conference & Exhibition 2007 A. Bechmann, N.N. Sørensen and J. Johansen Wind Energy.](https://reader035.fdocuments.in/reader035/viewer/2022062808/56649dd15503460f94ac74f2/html5/thumbnails/2.jpg)
Appetizer:
Simulation of Wind over Complex Terrain
![Page 3: Atmospheric Flow over Terrain using Hybrid RANS/LES European Wind Energy Conference & Exhibition 2007 A. Bechmann, N.N. Sørensen and J. Johansen Wind Energy.](https://reader035.fdocuments.in/reader035/viewer/2022062808/56649dd15503460f94ac74f2/html5/thumbnails/3.jpg)
European Wind Energy Conference & Exhibition 20073
Outline1. Introduction: Terrain flow using CFD
2. New approach: Hybrid RANS/LES model
3. Simulation results: Askervein Hill
4. Conclusions
![Page 4: Atmospheric Flow over Terrain using Hybrid RANS/LES European Wind Energy Conference & Exhibition 2007 A. Bechmann, N.N. Sørensen and J. Johansen Wind Energy.](https://reader035.fdocuments.in/reader035/viewer/2022062808/56649dd15503460f94ac74f2/html5/thumbnails/4.jpg)
European Wind Energy Conference & Exhibition 20074
Introduction: Complex terrain?
![Page 5: Atmospheric Flow over Terrain using Hybrid RANS/LES European Wind Energy Conference & Exhibition 2007 A. Bechmann, N.N. Sørensen and J. Johansen Wind Energy.](https://reader035.fdocuments.in/reader035/viewer/2022062808/56649dd15503460f94ac74f2/html5/thumbnails/5.jpg)
European Wind Energy Conference & Exhibition 20075
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
![Page 6: Atmospheric Flow over Terrain using Hybrid RANS/LES European Wind Energy Conference & Exhibition 2007 A. Bechmann, N.N. Sørensen and J. Johansen Wind Energy.](https://reader035.fdocuments.in/reader035/viewer/2022062808/56649dd15503460f94ac74f2/html5/thumbnails/6.jpg)
European Wind Energy Conference & Exhibition 20076
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
![Page 7: Atmospheric Flow over Terrain using Hybrid RANS/LES European Wind Energy Conference & Exhibition 2007 A. Bechmann, N.N. Sørensen and J. Johansen Wind Energy.](https://reader035.fdocuments.in/reader035/viewer/2022062808/56649dd15503460f94ac74f2/html5/thumbnails/7.jpg)
European Wind Energy Conference & Exhibition 20077
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
![Page 8: Atmospheric Flow over Terrain using Hybrid RANS/LES European Wind Energy Conference & Exhibition 2007 A. Bechmann, N.N. Sørensen and J. Johansen Wind Energy.](https://reader035.fdocuments.in/reader035/viewer/2022062808/56649dd15503460f94ac74f2/html5/thumbnails/8.jpg)
European Wind Energy Conference & Exhibition 20078
Outline1. Introduction: Terrain flow using CFD
2. New approach: Hybrid RANS/LES model
3. Simulation results: Askervein Hill
4. Conclusions
![Page 9: Atmospheric Flow over Terrain using Hybrid RANS/LES European Wind Energy Conference & Exhibition 2007 A. Bechmann, N.N. Sørensen and J. Johansen Wind Energy.](https://reader035.fdocuments.in/reader035/viewer/2022062808/56649dd15503460f94ac74f2/html5/thumbnails/9.jpg)
European Wind Energy Conference & Exhibition 20079
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
![Page 10: Atmospheric Flow over Terrain using Hybrid RANS/LES European Wind Energy Conference & Exhibition 2007 A. Bechmann, N.N. Sørensen and J. Johansen Wind Energy.](https://reader035.fdocuments.in/reader035/viewer/2022062808/56649dd15503460f94ac74f2/html5/thumbnails/10.jpg)
European Wind Energy Conference & Exhibition 200710
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)
![Page 11: Atmospheric Flow over Terrain using Hybrid RANS/LES European Wind Energy Conference & Exhibition 2007 A. Bechmann, N.N. Sørensen and J. Johansen Wind Energy.](https://reader035.fdocuments.in/reader035/viewer/2022062808/56649dd15503460f94ac74f2/html5/thumbnails/11.jpg)
European Wind Energy Conference & Exhibition 200711
New approach: turbulent lengthscale
![Page 12: Atmospheric Flow over Terrain using Hybrid RANS/LES European Wind Energy Conference & Exhibition 2007 A. Bechmann, N.N. Sørensen and J. Johansen Wind Energy.](https://reader035.fdocuments.in/reader035/viewer/2022062808/56649dd15503460f94ac74f2/html5/thumbnails/12.jpg)
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New approach: turbulent lengthscale
![Page 13: Atmospheric Flow over Terrain using Hybrid RANS/LES European Wind Energy Conference & Exhibition 2007 A. Bechmann, N.N. Sørensen and J. Johansen Wind Energy.](https://reader035.fdocuments.in/reader035/viewer/2022062808/56649dd15503460f94ac74f2/html5/thumbnails/13.jpg)
European Wind Energy Conference & Exhibition 200713
Outline1. Introduction: Terrain flow using CFD
2. New approach: Hybrid RANS/LES model
3. Simulation results: Askervein Hill
4. Conclusions
![Page 14: Atmospheric Flow over Terrain using Hybrid RANS/LES European Wind Energy Conference & Exhibition 2007 A. Bechmann, N.N. Sørensen and J. Johansen Wind Energy.](https://reader035.fdocuments.in/reader035/viewer/2022062808/56649dd15503460f94ac74f2/html5/thumbnails/14.jpg)
European Wind Energy Conference & Exhibition 200714
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
![Page 15: Atmospheric Flow over Terrain using Hybrid RANS/LES European Wind Energy Conference & Exhibition 2007 A. Bechmann, N.N. Sørensen and J. Johansen Wind Energy.](https://reader035.fdocuments.in/reader035/viewer/2022062808/56649dd15503460f94ac74f2/html5/thumbnails/15.jpg)
European Wind Energy Conference & Exhibition 200715
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
![Page 16: Atmospheric Flow over Terrain using Hybrid RANS/LES European Wind Energy Conference & Exhibition 2007 A. Bechmann, N.N. Sørensen and J. Johansen Wind Energy.](https://reader035.fdocuments.in/reader035/viewer/2022062808/56649dd15503460f94ac74f2/html5/thumbnails/16.jpg)
European Wind Energy Conference & Exhibition 200716
Askervein hill: flow field
Hybrid LES
RANS
![Page 17: Atmospheric Flow over Terrain using Hybrid RANS/LES European Wind Energy Conference & Exhibition 2007 A. Bechmann, N.N. Sørensen and J. Johansen Wind Energy.](https://reader035.fdocuments.in/reader035/viewer/2022062808/56649dd15503460f94ac74f2/html5/thumbnails/17.jpg)
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Askervein hill: speed-up
![Page 18: Atmospheric Flow over Terrain using Hybrid RANS/LES European Wind Energy Conference & Exhibition 2007 A. Bechmann, N.N. Sørensen and J. Johansen Wind Energy.](https://reader035.fdocuments.in/reader035/viewer/2022062808/56649dd15503460f94ac74f2/html5/thumbnails/18.jpg)
European Wind Energy Conference & Exhibition 200718
Askervein hill: turbulence
![Page 19: Atmospheric Flow over Terrain using Hybrid RANS/LES European Wind Energy Conference & Exhibition 2007 A. Bechmann, N.N. Sørensen and J. Johansen Wind Energy.](https://reader035.fdocuments.in/reader035/viewer/2022062808/56649dd15503460f94ac74f2/html5/thumbnails/19.jpg)
European Wind Energy Conference & Exhibition 200719
Outline1. Introduction: Terrain flow using CFD
2. New approach: Hybrid RANS/LES model
3. Simulation results: Askervein Hill
4. Conclusions
![Page 20: Atmospheric Flow over Terrain using Hybrid RANS/LES European Wind Energy Conference & Exhibition 2007 A. Bechmann, N.N. Sørensen and J. Johansen Wind Energy.](https://reader035.fdocuments.in/reader035/viewer/2022062808/56649dd15503460f94ac74f2/html5/thumbnails/20.jpg)
European Wind Energy Conference & Exhibition 200720
Conclusions
![Page 21: Atmospheric Flow over Terrain using Hybrid RANS/LES European Wind Energy Conference & Exhibition 2007 A. Bechmann, N.N. Sørensen and J. Johansen Wind Energy.](https://reader035.fdocuments.in/reader035/viewer/2022062808/56649dd15503460f94ac74f2/html5/thumbnails/21.jpg)
European Wind Energy Conference & Exhibition 200721
Thank you for listening!