1. Giorgio CRASTO, Francesco PARAGGIO
giorgio.crasto@gmail.com
fparaggio@gmail.com
Third Symposium on OpenFOAM® in Wind Energy. Politecnico di Milano 15-17 June 2015
RANS Simulations of Askervein
hill with OpenFOAM®

2. The Askervein Hill project
• Askervein is a well known test-site where
extensive wind measurements were carried
out in ‘82 and ‘83.
• Is considered to be an Isolated hill
– Althought some other hills are close at NE ... Map
A and Map B are available (thanks to Prof. P.Taylor)
• Neutral stratification ... Run TU03-B
• Homogenous roughness? (z0 0.03m or what?)
2

3. • Map A: 257x257 – 16km x 16km => 62.5m
• Map B: 257x257 – 6km x 6km => 23.4375m
Askervein Hill: an isolated hill?
3

4. Askervein Hill: resolution consequences
4
• Askervein, a 116m high hill: Map A Map B
115.108m 116.837m

5. Askervein Hill: measurements points
• HT Hill Top
• CP Center Point
• RS Reference Site
• Line A through HT
• Line AA through CP
• Line B main axis
• TU03-B (210°)
5

6. Askervein Hill: homogeneous roughness?
6
Negligible?

7. Simulations 2D, rectangular domain
7
• Reynolds Averaged Navier-Stokes (RANS)
• Standard k-e turbulence model
• Effects of model constant se on the wall-
functions
• Periodic BC
– Mass flow given
0,000
0,005
0,010
0,015
0,020
0,025
0,030
0,035
0,040
0 5 10 15 20 25
z0obtainedbyinterpolationh/2-100m]
ratio first cell over roughness length (h/z0)
se 1.44
se 1.30
se 1.11
~11.8
Height of first (ground adjacent)
cell ~ 11.8 * 0.03m = 0.354m

8. 3D simulations on a cubic domain
8
• The snappyHexMesh applied directly to the
orography of the site gave a poor quality grid.

9. terrainBlockMesher (TBM) + SHM
9
• snappyHexMesh only NO!
• terrainBlockMesher => boundary-fitted grid ok
• terrainBlockMesher + SNM ... More promising
• ZephyTOOLS prismatic cells over a cylindrical
domain (mesher M1)

10. TBM + snappyHexMesh
10
1. terrainBlockMesher (body-fitted)
2. 1° level snappyHexMesh
3. 2° level snappyHexMesh
PRO: reduced height first cell; increased resolution close to the ground
CON: expansion ratio 2, generally accepted <1.2

11. Further settings
• Standard k-e model
• simpleFoam (incompressible)
• First cell height 2m (= 66.67*z0) - not 0.354m!!
– Non-orthogonality alarms reducing the grid size
• Convergence criteria:
11
OF 2.3.1 default (for ABL test-case) ZephyTOOLS default
p 1e-3;
U 1e-4;
"(k|epsilon)" 1e-4;
p 5e-4;
U 1e-5;
"(k|epsilon)" 1e-5;
ZephyTOOLS
vref 15m/s
href 500m
CUBIC GRID
vref 16.016m/s
href 1008m
ut = 0.63 m/s

12. 1 and 2 cells per sHM level (Speed)
12

13. 1 and 2 cells per sHM level (TKE)
13
- 2 cells for each level
- More was not accepted by my and
the grid generated had only one
cell per level!

14. Discretization schemes (Speed)
14
Residuals:
p 5e-4
U 1e-5
"(k|epsilon)" 1e-5

15. Discretization schemes (TKE)
15
Residuals:
p 5e-4
U 1e-5
"(k|epsilon)" 1e-5

16. Discretization schemes
16

17. Discretization schemes
17/21

18. Discretization schemes and RMSE
map Grid generation Scheme (divSchemes) RMSerr. Speed RMSerr. TKE
A TBM + 2 levels of SHM bounded linearUpwind 1.239643 0.766631
B TBM + 2 levels of SHM bounded linearUpwind 1.253133 0.748267
A TBM + 2 levels of SHM b. skewCorrected linearUpwind 1.293558 0.781646
B TBM + 2 levels of SHM b. skewCorrected linearUpwind 1.309883 0.777852
A TBM + 2 levels of SHM b. linear (2nd ord. Central) 1.290942 0.899257
B TBM + 2 levels of SHM b. linear (2nd ord. Central) 1.474857 0.92132
A TBM + 2 levels of SHM bounded linearUpwindV 1.321186 0.799412
B TBM + 2 levels of SHM bounded linearUpwindV 1.350313 0.801742
TBM terrainBlockMesher
SHM snappyHexMesh
A ZephyTOOLS (OF 2.3.0) linearUpwind; upwind for k|e 0.993898 1.034235
B ZephyTOOLS (OF 2.3.0) linearUpwind; upwind for k|e 0.973765 0.995486
18/21
gradSchemes laplacianSchemes
TBM + 2l of SHM Gauss linear Gauss linear limited corrected 0.333
ZephyTOOLS cellMDLimited Gauss linear 0.5 Gauss linear limited 0.5

19. Speed [m/s]: TBM+SHM vs. ZephyTOOLS
19/21

20. TKE [m2/s2]: TBM+SHM vs. ZephyTOOLS
20/21

21. Conclusions & Future Steps
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 Highlighted the grid sensitivity (vertical and horizontal)
 SHM not satisfying; TBM + SHM promising
 Need to improve the code to automize the gridding while
satisfing the h/z0 rule; some limits due to PC;
 No substantial difference between cubic and ZT in terms of
results (speed&tke) ... Probably differences due to the SHM
 ZT slightly underestimeted the speed-up at HT and the step
raise of TKE;
 Both methods understimated significantly the peak of TKE
(due to turbulence modelling?);
 Improve (automize) the gridding;
 Test cases with roughness maps (Bolund);
 Compute the production of a Wind Farm.