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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'How does the wind blow behind wind turbines and in wind farms?' taking place on Tuesday, 11 March 2014 at 16:30-18:00. The meet-the-authors will take place in the poster area.

Carlos Diaz-Asensio Mancebo Uppasala Universtiy - Gotland Campus, Sweden
Co-authors:
Carlos Diaz-Asensio Mancebo (1) F P Bahri Uzunoglu (1)
(1) Uppasala Universtiy - Gotland Campus, Visby, Sweden

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Abstract

A COMPARISON STUDY FOR TWO COMMERCIAL WIND RESOURCE ANALYSIS CFD SOFTWARE FOR AN EMBANKMENT

Introduction

For wind resource assessment, Computational Fluid Dynamic (CFD) packages can be adjusted to model wind flow in the boundary layer [1]. The boundary layer over complex terrain comprises phenomena like acceleration of flow over embankments, hills, mountain tops or ridges [2]. Two commercial CFD wind resource assessment tools namely Meteodyn WT and Windsim have been compared for an embankment site named Hjardemål. A controlled experiment is carried out for a fixed set-up with same domain size and same number of cells in the X, Y and Z directions for both software tools. Vertical profiles observed at site validate software output.

Approach

The methodology of the Hjardemål controlled experiment comprises five steps. The approach includes an intial site definition, a parameterization on input data, results, a discussion and conclusions section, this methodology is shown schematically in figure 1 below:



Site definition: In this section the relevance of site selection for the experiment is highlighted, and the following question is answered: Why has this site been selected to test the wind resource assessment tools?

Parameterization: In order to be able to compare both wind resource assessment tools, all parameters that can be controlled by the software user have been set equal; where software limitations did not allow setting the exact same parameters, convergence has been seek. The parameters adjusted to carry out the comparison are the following:

• Domain dimensions,
• Orography and roughness,
• Mesh generation and
• Wind input

Results: After setting all parameters, simulations are run in both wind resource assessment tools. Vertical wind profiles at four different locations distributed (-397.69 m) before, (0 m) at, (30.83 m) just after and (199.16 m) after the embankment are shown graphically and contrasted with the available averaged wind speed observations on site.

Discussion: The discussion of results is based on the limitations of both software tools which have conditioned the comparison. More specifically discussion addresses: domain size, orography and roughness, mesh generation, wind input, convergence and turbulence models.

Conclusions: After carrying out the experiment where a clear methodology has been used, input parameters are explained step by step, results are analysed and software limitations are considered, conclusions are finally drawn.

Main body of abstract

Site definition: The Hjardemål site has been selected for this study for its topographical attributes: its orography and roughness make Hjardemål site very appropriate to test the software. The Hjardemål site contains an embankment with high gradient. Elevation figures range from 0.3 m the lowest to 27 m the highest, two almost flat terrains at different heights are joined by the embankment. Roughness is uniform over the whole terrain and is defined by 0,03 mm roughness lines. Figures 2 and 3 below show the site elevation displayed in Meteodyn WT and Windsim software correspondingly.





Available averaged wind speed measurements at four different locations distributed (-397.69 m) before, (0 m) at, (30.83 m) just after and (199.16 m) after the embankment generate vertical wind profiles which are plotted in figure 4 below which also shows terrain orography.



Parameterization:

• Domain size: Same domain size has been used for both software simulations. A squared domain ranging from -893 (m) to 893 (m).
• Orography and roughness: Same topographic files have been inserted in both software tools, matching the site definition section.
• Mesh generation: Same number of cells in the X, Y and Z directions same for both software figures 5 and 6 show mesh generation in Meteodyn WT and Windsim software correspondingly.





• Wind input: In Meteodyn WT a tab file has been generated to produce a 4.87 m/s average wind speed, this climatological input corresponds to the first observation location at 10 m height in the first tower, before the embankment. In Windsim, a logarithmic law was required to modify the settings of the Wind Fields module.
Results: Results depicted in figures 7 to 10 show that Meteodyn WT predicts closer vertical wind profiles before, at and after whilst wind Windsim predicts a closer wind profile just after the embankment.






• Discussion: Domain size and site orography can be set totally equal in both wind resource assessment tools. Roughness lengths in Meteodyn WT are computed using a roughness ratio whilst in Windsim the roughness height is defined by a logarithmic-law. Although mesh generation is composed by the same number of cells in both experiments, horizontal coefficients of expansion in Meteodyn WT cannot be set for a perfect uniform grid. Similarly, in Windsim, the first vertical cell could not be modified in the bws terrain file. With regards to wind input Meteodyn WT does not accept an average wind speed value, it requires to insert a wind file, to cope with this impediment, a tab file has been generated, the tab file contained the desired wind speed but with a different distribution than in reality. This tab file could be inserted in Windsim but since in Windsim, vertical profiles are not scaled against measurements [3] a logarithmic law has been applied to find convergence in between both wind inputs. This leads to a source of error in the comparison, especially visible in tower one results since Meteodyn’s tab file is associated to this exact location. The change in settings of Windsim do not allow to associate the wind speed to an exact location, wind comes only from the 270ª direction and not from tower one, this explains the closer Meteodyn WT vertical profile at tower one. In Meteodyn WT turbulence intensity can be inserted in a turbulence file which considers standard deviation of velocity fluctuations, in this case, a turbulence file from measurement period is not available and therefore, automatically turbulence is modelled from the turbulence ambient results obtained from the computational directions module run earlier [4]. In Windsim the turbulence model selected is the standard K-epsilon model. Concerning convergence Meteodyn WT generates a different mesh for each sector independently: Iteration number to reach convergence is lower than 25. Windsim generates a unique mesh that will is run in all sectors, although sectors can be run independently, this system requires higher number of iterations to reach convergence.

Conclusion

Both CFD wind resource software tools have been considered, the following conclusions are drawn:

- Observed vertical wind profiles at site match with the defined theory at [2]; acceleration of wind flow over the embankment can be clearly identified in both software tools at site in tower 8, located 30.83 meters just after the embankment.

- Domain size, grid generation, orography and roughness parameters are very adaptable in both wind resource software tools. These parameters can be and were adjusted equal or almost equally for the comparison carried out in this experiment. However software user constraints addressed in the text have limited some of the comparison, differences in wind input parameter are especially visible at tower 1, located 397.69 m before the embankment, where although extracted profiles before the embankment irregularity are very close, they are not identical.

- Meteodyn WT has given a closer result to reality at tower 1 location, at embankment location and at tower 10, 199 meters after the embankment. In contrast, Meteodyn WT gives a too conservative prediction at tower 8, 30.83 m just after the embankment. On the other hand Windsim gave a higher prediction, at embankment, 30.83 meters after the embankment and 199 meters after the embankment. These general higher results made Windsim over predict at embankment and specially 199 m after embankment. Contrariwise, this general higher prediction made Windsim vertical wind profiles closer to reality at tower 8, 30.83 meters just after the embankment where a wind acceleration phenomenon occurs due to orographic causes.

Finally the two software comparison is presented while addressing some constraints to users on control of parameters.





Learning objectives
Getting to know the software performance in an embankment site, which is a simplification of a irregular/complex terrain, as well as identifying software limitations of both wind resource assessment tools.


References
[1] M.Strack, V. Riedel, 2004, State of Art in Application of Flow Models for Micrositing, German Wind Energy Institute (DEWI)
[2] S. Emeis 2013 Wind Energy Meteorology – Atmospheric Physics for Wind Power Generation - Winds in Complex Terrain p75 to 92.
[3] Meteodyn WT user guide, 2013
[4] Windsim Support Team, 2013 Vertical Profile, email sent May 29th 2013