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

andreas.bechmann@risoe.dk

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!