Numerical site assessment

Measurement is costly and usually limited to a small local area, whereas numerical methods for the analysis of wind conditions fully map a site. In simple terrain, the wind atlas method can be used, but in complex terrain ­– wooded, hilly, or mountainous – this method displays local weaknesses. This is where computational fluid dynamics comes in.

A key factor in the success of a wind farm is the correct assessment of the site. If there are insufficient measurements or if the terrain is too complex, numerical simulations are ideal for predicting the potential yield. Long-term data with temporal resolution can be obtained from mesoscale simulations based on analyses of global weather models.

OpenFOAM and flapFOAM

Wake effect of a wind farm in front of a forested hill

To determine local effects, however, computational fluid dynamics is used. This allows for the details of the terrain and the local conditions to be taken into account. For these simulations, Fraunhofer IWES uses the open code OpenFOAM. It has the advantage of being parallelizable, allowing even large areas to be simulated with ease; even transient simulations are possible.

The open code enables simulation conditions to be precisely adapted to the requirements of the atmospheric simulation. For example, in addition to a mesh generator, Fraunhofer IWES has developed tools to generate wake models for wind farms (generation of wooded areas from image files, consideration of stable and unstable stratifications) and implemented them in the code. Linking the mesoscale simulations with the local (microscale) simulations is currently under development.


TerrainMesher-generated structure for the simulation in complex terrain

A number of add-ons have been developed for site assessment: for example, a mesh generator for complex terrain and the simulation of entire wind farms. The meshes, using which numerical simulations discretize the space, are vastly important to the result of the simulations. Fraunhofer IWES offers a structured mesh generator for site simulations, which is freely available to download. In the meantime, many functionalities have been added to the tool, one important one being that any 2D meshes can be used as the base mesh. They do not necessarily have to be structured, as the mesh generator is no longer based on blockMesh – hence its name “terrainMesher”. One part of the base mesh is first projected onto the terrain in STL format. Fraunhofer IWES offers a terrainMesher for site simulation:

Then the rest of the mesh is dynamically relaxed, which prevents excessive discontinuities and thus improves the resulting cell quality. The resulting 2D mesh is expanded to a 3D mesh of a given height. The user can set the height of the first cell via either the base or the desired grading. This is outwardly adjusted so that the maximum aspect ratio is not exceeded. The vertical splines are also dynamically optimized so that they stand orthogonally on the terrain and maintain maximum distance from one another.

Based on the wind farm optimization code from Oldenburg University “FLaP” and the open field equation solver “OpenFOAM”, Fraunhofer IWES has developed the wind farm optimization software “flapFoam”. It projects the wakes of individual turbines into a simulated or modelled wind field. The wind farm is then laid out according to the best possible result on the basis of various optimization criteria and methods.

In addition to various wake models, the program also has the ability to compute the wakes of the individual wind turbines using CFD. The code is at an advanced stage of development.

OpenFoam Training Courses

Fraunhofer IWES uses primarily the open source code OpenFOAM or the derivative FOAM extended for CFD simulations. Even though the code is open, it is quite challenging to understand the program and apply it efficiently. Fraunhofer IWES offers OpenFOAM coruses for users with different levels of knowledge:

•         Introductions to OpenFOAM

•         Courses for the use of OpenFOAM in site assessment or aerodynamics

•         Courses on programing in OpenFOAM



Development and application of a grid generation tool for aerodynamic simulations of wind turbines
Rahimi, H.; Daniele, E.; Stoevesandt, B.; Peinke, J.
(Journal article)

Wind engineering 40 (2016), Nr.2, S.148-172

Link Fraunhofer Publica


Combined structural optimization and aeroelastic analysis of a Vertical Axis Wind Turbine (Conference contribution)
Roscher, B.; Ferreira, C.S.; Bernhammer, L.O.; Madsen, H.A.; Griffith, D.T.; Stoevesandt, B.

American Institute of Aeronautics and Astronautics -AIAA-, Washington/D.C.:
33rd Wind Energy Symposium 2015. Vol.1 : Kissimmee, Florida, USA, 5 - 9 January 2015; held at the AIAA SciTech Forum 2015
Red Hook, NY: Curran, 2015

Link Fraunhofer Publica

The impact of wake models on wind farm layout optimization (Conference contribution, journal article)
Schmidt, Jonas; Stoevesandt, Bernhard

Journal of physics. Conference series 625 (2015), Art.012040, 10 S.

Link Fraunhofer Publica

Numerical investigation on tower effects for downwind turbines (Abstr.); Stoevesandt, B.; Habib, F.; Mehra, B.; Rahimi, H.; Peinke, J.

UL International GmbH, Wilhelmshaven:
DEWEK 2015. Book of abstracts : 12th German Wind Energy Conference, 19/20 May 2015, Bremen, Germany
Bremen, 2015

Link Fraunhofer Publica

Roof region dependent wind potential assessment with different RANS turbulence models (Journal article)
Toja-Silva, F.; Peralta, C.; Lopez-Garcia, O.; Navarro, J.; Cruz, I.

Journal of wind engineering and industrial aerodynamics 142 (2015), S.258-271

Link Fraunhofer Publica

Wind farm layout optimization in complex terrain with CFD wakes (Conference contribution); Schmidt, J.; Stoevesandt, B.

Paper presented at EWEA 2015, Europe's Premier Wind Energy Event, 17 - 20 November 2015, Paris, France

Link Fraunhofer Publica

Wind power energy in Southern Brazil: Evaluation using a mesoscale meteorological model (Journal article, conference contribution)
Krusche, Nisia; Peralta, Carlos; Chang, Chi-Yao; Stoevesandt, Bernhard

Energy Procedia 76 (2015), S.164-168

Link Fraunhofer Publica