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.

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.

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:  

https://github.com/jonasIWES/terrainBlockMesher

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.