1. C. Albrecht, D. Delaunay, A. Grözner, M. Kohiert et L. Pauscher, «Atmospheric

stability: general classification and simulation in Meteodyn WT,» in EWEA, 2014.

2. A. Grachev, E. Andreas, C. Fairall, P. Guest et P. Persson, «The critical richardson

number and limits of applicability of local similarity theory in the stable boundary

layer,» Boundary Layer Meteorology, vol. 147, pp. 51-82, 2013.

3. P. J. Hurley, «An evaluation of several turbulence shemes for the prediction of mean

and turbulent fields in complex terrain,» Boundary Layer Meteorology , vol. 83, n° 1,

pp. 43-73, 1997.

4. S. S. Zilitinkevich, T. Eleperin, N. Kleeorin, I. Rogachevskii, V. L'vov, I. Esau et R.

Kouznetsov, «Turbulence closure for stably stratified flows: local and non-local

formulations,» in 8th European conference on applied climatology, Zürich,

Switzerland, 2010.

Up to now CFD computations in wind resource assessment mainly focused on wind

statistics treatment, and then considered average thermal structure of the atmosphere.

With an increasing demand for a more accurate description of these statistics, including

time series, there is a need for considering more specific situations, and particularly

stable thermal stratifications (ref 1). We present here a new turbulence model allowing

to consider the strongly stable cases in CFD computations. This model is implemented

in Meteodyn WT 5.2

Applying a k-L multi-layer model in CFD computations is a promising way to reproduce

behavior of the Stable Boundary Layer, especially when the height of the SBL becomes

inferior to the wind turbine hub height. This is an important step forward for a better

consideration of the atmospheric stability in wind resource assessment.

Abstract

Objectives

Conclusions

References

The objective of this work is to develop a new turbulence model for the Stable Boundary

Layer, by considering measurements at two sites with well documented measurements

up to 200 m height above ground.

A new approach for CFD modelling in stable situations

Figure 2: Meteodyn WT simulations of wind profile at Cabauw using the new turbulent multi-layer viscosity model

The main effect of stable stratification in the atmospheric boundary layer is to reduce the

vertical turbulent exchanges. In CFD computations these exchanges are modelled via

the concept of turbulent viscosity, which is the product of a wind speed scale and a

turbulent length scale. The model is focusing on the determination of the length scale

vertical profile, allowing a multi-layer approach, adapted to stable situations.

The model considers 3 layers: The Monin-Obuknov similarity theory (MOST) layer, the

local z-less model, and the layer above the SBL (ref 1, 2,3).

A first calibration has been made with 3 years data at Cabauw (NL) (flat open terrain),

with data sorted according to a Stability Index (z1= 10 m and z2= 80 m), given by:

A verification has been conducted on a more complex site at Rödeser Berg (hilly and

forested) with 1 year data sorted according to the measured Monin-Obukhov length L at

40 m height. A dense forest lies in the South-West and South sectors. In the East sector,

the roughness is more moderate.

2

1212

1212

1 )/())()((

)/())()((

zzzVzV

zzzzgSI

S03 S04 S05 S06 S07 S08 S09 S10 S11 S12 S13 S14SI range 0.00 - 0.01 0.01-0.05 0.05-0.10 0.10-0.15 0.15-0.20 0.20-0.25 0.25-0.32 0.32-0.40 0.40-0.50 0.50-0.70 0.70-1.00 > 1.00

frequency 7.3% 9.4% 14.4% 11.2% 9.0% 6.9% 6.7% 4.5% 3.4% 3.1% 1.6% 1.9%

power contribution 9.1% 24.1% 21.4% 9.6% 6.0% 4.0% 3.5% 2.1% 1.4% 1.0% 0.4% 0.2%

(V80+V140)/2 (m/s) 8.1 11.3 9.5 8.0 7.4 7.1 6.8 6.5 6.2 5.8 5.2 4.3

Table 1: SI classes and characteristics at Cabauw (wind direction: 255-285 deg)

Considering thermal stratification in

CFD modelling for wind resource assessment Bin FU1, Didier DELAUNAY2, Ru LI2

1 METEODYN, 48 avenue Dong Zhi Men Wai Jie, 100 027 Beijing, China 2 METEODYN, 14 bd Winston Churchill, 44100 Nantes, France

Poster

24

Comparison with measurements at Rödeser Berg

Comparison with measurements at Cabauw

Figure 1: Views of Rödeser Berg and Cabauw sites

Figure 6: Meteodyn WT simulations of wind profile and TKE in Rödeser Berg (East Direction)

Figure 5: Meteodyn WT simulations of wind profile and TKE in Rödeser Berg (South Direction)

Acknowledgements

In the East direction, the wind speed gradients increase according to the thermal stability

(L decrease) as computed in Meteodyn WT, but the measured TKE are less dependant

on stability than expected in the computations. Calibrations on the dissipation terms in

the TKE equation will have to reduce this discrepancy.

In the S direction, the surface friction generated by the tree weakens the thermal effect

for the wind speed gradient, but the TKE is affected by stability as expected. Globally, the

results of the Meteodyn WT computations are quite satisfactory.

We acknowledge Anselm Grötzner (Cube Engineering) for his help for Cabauw data

analysis and Tobias Klaas (IWES Fraunhöfer) for the data at Röderser Berg.

Rödeser Berg Cabauw

Figure 3: Meteodyn WT simulations of TKE at Cabauw using the new turbulent multi-layer viscosity model

Presenters

The results show the good agreements between Meteodyn WT and measurements for the

different stability classes. The wind speed gradients increase and the turbulence kinetic

energies (TKE) decrease according to the thermal stability (L decrease).

FU BIN

Director of Meteodyn Beijing

Engineer Univ. of Xi’an Jiaotong

Email: fu.bin@meteodyn.com

Julien BERTHAUT-GERENTES

Innovation&Research

Project manager

PhD Ecole Centrale Lyon

Email: julien.berthaut-gerentes

@meteodyn.com