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 Department Risoe National Laboratory Technical University of Denmark andreas.bechmann@risoe.dk

2. Appetizer: Simulation of Wind over Complex Terrain

3. European Wind Energy Conference & Exhibition 2007 3 Outline 1. Introduction: Terrain flow using CFD 2. New approach: Hybrid RANS/LES model 3. Simulation results: Askervein Hill 4. Conclusions

4. European Wind Energy Conference & Exhibition 2007 4 Introduction: Complex terrain?

5. European Wind Energy Conference & Exhibition 2007 5 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

6. European Wind Energy Conference & Exhibition 2007 6 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

7. European Wind Energy Conference & Exhibition 2007 7 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 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

8. European Wind Energy Conference & Exhibition 2007 8 Outline 1. Introduction: Terrain flow using CFD 2. New approach: Hybrid RANS/LES model 3. Simulation results: Askervein Hill 4. Conclusions

9. European Wind Energy Conference & Exhibition 2007 9 Momentum equations for resolved motions: New approach: basic equations Momentum 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

10. European Wind Energy Conference & Exhibition 2007 10 New approach: turbulence model Turbulent stresses: product of fluid strain and eddy-viscosity: Equation for TKE modified for LES and RANS length scales: ‘Standard‘ model: Similar to Detached-Eddy Simulation Spalart et al. (1997)

11. European Wind Energy Conference & Exhibition 2007 11 New approach: turbulent lengthscale

12. European Wind Energy Conference & Exhibition 2007 12 New approach: turbulent lengthscale

13. European Wind Energy Conference & Exhibition 2007 13 Outline 1. Introduction: Terrain flow using CFD 2. New approach: Hybrid RANS/LES model 3. Simulation results: Askervein Hill 4. Conclusions

14. European Wind Energy Conference & Exhibition 2007 14 Askervein hill: background Askervein 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, z 1 =0.03m

15. European Wind Energy Conference & Exhibition 2007 15 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 Wind database

16. European Wind Energy Conference & Exhibition 2007 16 Askervein hill: flow field Hybrid LES RANS

17. European Wind Energy Conference & Exhibition 2007 17 Askervein hill: speed-up

18. European Wind Energy Conference & Exhibition 2007 18 Askervein hill: turbulence

19. European Wind Energy Conference & Exhibition 2007 19 Outline 1. Introduction: Terrain flow using CFD 2. New approach: Hybrid RANS/LES model 3. Simulation results: Askervein Hill 4. Conclusions

20. European Wind Energy Conference & Exhibition 2007 20 Conclusions

21. European Wind Energy Conference & Exhibition 2007 21 Thank you for listening!