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Simulating the Bolund hill flow by CFD approaches

S. Sanquer, C. Bezault, T. Clarenc and J.C. Houbart

www.meteodyn.comNZWEA 2010 PALMERSTON NORTH

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Context and purposes of the Bolund Round robin test

Technical background

Round robin test results

Further works

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Purposes of blind comparison (From Ris DTU)

Make the Bolund data visible

Evaluate flow modelling accuracy

1st European Wind Energy Technology Platform.

Challenge : uncertainty less than 3% on wind speed

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Method and budget : Computation or experiment

Complexity of terrains : Linear and non-linear resolution

How to choose the good approach devoted to the wind

energy assessment with our own users criteria ?

o e : ne-equat on, two equat ons,

Results expected : Speed-up or turbulence fitting

Computing time and/or precision (Academic or industrial using)

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Interest of the Bolund Round Robin Test

Sharp terrain

Representative topography

Transition of roughnessRoughness well defined

10 masts and two axisBoundary conditions well definedLots of results

Lots of attendees49 with 49 results sets !

Blind test for competitors

Improvements for everyone

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RANS equations (Reynolds Average Navier Stokes)

Stationary and incompressible flow

Mass conservation

Solving the fluid dynamics inmeteodynWTandUrbaWind

0=

i

i

x

u

Momentum conservation

Reynolds stress tensor evaluated from a one-equation closure scheme.

( ) 0'' =+

+

+

ijii

j

j

i

jij

ijFuu

x

u

x

u

xx

P

x

uu

+

=

i

j

j

itji

x

u

x

uuu '' TT Lk

2/1=where

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meteodyn WT

Mesh : Cartesian structured

horizontale resolution : 3 m

verticale resolution : 0.5 m

Mesh points :1.5 M

Mesh generation duration : 15 minutes

Simulation duration : 24 minutes with1 CPU by direction

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UrbaWindMesh : Cartesian unstructured

horizontale resolution around result points : 0.5 m

verticale resolution around result points : 0.1 m

Mesh points :2.2 M

Mesh generation duration : 18 minutes

mu a on ura on : m nu es w y rec on

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meteodynWTand UrbaWind: Speed up factor at h=2 and 5 m (270)

Speeding up and slowing down are stronger for UW than for WT.

Grids close to the ground are differents

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meteodynWT: Speed up factor (239)

The speed-up profil at the mast n1 is well predicted

The speed-up profil at the mast n2 is over predicted f or h

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meteodynWT: Turbulence (239)

RANS models have some difficulties to well predict the turbulencepeak at the mast 2

Correct at the three others locations

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UrbaWind: Speed up factor (239)

The speed-up profil at the masts n1 and n2 are well predicted

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UrbaWind: Turbulence (239)

UW predicts better the turbulence peak at the mast 2

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UrbaWind: Error at h=5 m => 7% best of the one equation model

MeteodynWT: Error at h=5 m => 11 % Not so bad for a structured mesh

Few % between the best k- and the best 1 equation model

Lack of comparison on turbulence energy errors

From Ris DTU

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Further comparisons :

How long to carry out the computations

How to make the perfect mesh according the convergence and the

accuracy?

Meshing and computing time (order of magnitude)

Further works :

Explanation of the gap between meteodynWTand UrbaWind

Mesh type grid points Meshing timeComputing time

per direction Error on speed-up

k-l (WT) Structured 1.5 M 15 min 24 min 11%

k-l (UW) Unstructured 2.2 M 18 min 90 min 7%

k- Unstructured up to 10 M fews hours 5 h 5-10%

LES Unstructured 1 M fews hours 24 h Unknown

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The first point of the grid closest to theground

=0.5 m, =0.2 m, and =0.1 m

Modification of the wind shear (mast 2)

meteodynWTand UrbaWindgive

15

20

25

30

Z(m)

DZ= 0.5 m

DZ=0.2 m

DZ=0.1 m

Discrepancies with RANS should be more a question of grid than a

question of models when speed-up is evaluated according theEuropean plate form challenge

V/Vref (5m) V/Vref (2m) (I-Iref)/Vref (2m)

Riso 1.25 0.92 0.16

WT - =0.5 m 1.15 1.10 0.10

WT - =0.2 m 1.18 1.05 0.13

WT - =0.1 m 1.20 1.06 0.13 0

5

10

0.80 0.90 1.00 1.10 1.20 1.30

V/Vref

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MERCI BEAUCOUP