OFWind CFD

 

 

 

 

 

 

 

 

OFWind CFD is an OpenFoam CFD model specifically designed for use together with windPRO. Only a few steps are needed to run a CFD calculation with this powerful software combination:

  • Setup export of flowrequest in windPRO based on orography, roughness and WTG-area, optional define direction and heights.
  • Start OFWind and import flowrequest file. Choose Resolution – a compromise between accuracy and calculation time.
  • Modify wind directions for simulations if needed.
  • Start the fully automated simulation process, will typically run 3 to 7 days on a standard PC.
  • Automatically generated flowres-Archive import the simulation results back into windPRO.
  • Use the CFD results in windPRO for energy yield estimation and site suitability analysis.

What is OFWind CFD and how is it working?

OFWind CFD is a CFD-Tool to compute the flow field over a complex terrain. It uses the OpenFOAM libraries and applications from version 2.3.x. As input, OFWind CFD needs a flowrequest-Archive coming from windPRO with the following content:

  • Orography map of your terrain
  • Roughness map of your terrain
  • Xml-Document containing the coordinates of your WTG-Area and desired wind directions and heights to compute and the spatial resolution the Flowres file is written into.

Based on the WTG-Area, OFWind CFD will cut a circular patch from the terrain with the WTG-Area in the middle.

The user can select how fine the orography is resolved inside the WTG-Area by setting the slider position to “Fast” (coarser resolution), “Middle” or “Accurate” (finer resolution).

Subsequently, OFWind CFD automatically generates a mesh which is needed to compute the flow over the terrain. The volume of the mesh resembles a cylinder to be able to compute the flow from each direction with the flow always perpendicular at the inlet.

The terrain around the WTG-Area influences the flow inside the park area. Therefore, not only the park area is used to create the mesh volume but also the surrounding of the WTG-Area.

Inside the WTG-Area a higher terrain resolution is used to compute the flow more accurately.

In order to be able to compute the flow field over the terrain on standard PCs and in reasonable time, a couple of simplifications have to be made. These simplifications regard for example the turbulence modelling and the assumption that the flow field is in a steady state condition.

For each cell the mass and momentum conservation is computed throughout the whole domain. This is done in an iterative process and calculation time depends on the number of cells inside the domain. The iterative process of solving the equations in OFWind CFD for each cell ends when a steady state is reached.

The resolution and heights chosen when generating the Flowreq file from windPRO define a three dimensional grid above the WTG-Area. This is used when OFWind CFD sample the simulation data for the result files.

Once each direction has been simulated, OFWind CFD stores the following data at each park area grid position into files called “SiteName_Direction.flowdat” containing:

  • Horizontal wind speed
  • Turn
  • Turbulence intensity (TI)
  • Inclination

After simulating all directions, there is a flowdat-file for each direction containing information of the flow field above the WTG-Area. All these flowdat-files are stored together into an archive inside the directory specified by the user in the “Output” pane at the top of the graphical user interface. This archive is called “….flowres”. This can be imported back into windPRO to conduct predictions and analyse the flow field in the WTG-Area by the user. There is a strong graphic support in windPRO for illustration of the Flowres files content.