Forestry
Forestry Flow Displacement Methodology
In order to account for wind flow being impeded by forestry or vegetation, it is common to apply a "flow displacement" at masts and turbines where nearby forestry results in an effective reduction in measurement or hub height that is proportional to the height of nearby forestry and its distance from the mast/turbine.
Digital forestry maps of tree heights in metres (e.g. a lidar scan as a *.grd or geotiff *.tiff file) can be loaded into WindFarmer:Analyst and a method is provided to automatically calculate and apply the effective displacements for all masts and turbines in a Workbook as detailed below.
For each mast and turbine, a single displacement height value is derived based on the nearby forestry in all directions and the wind climate at the mast/turbine.
In each direction sector (default 120 sectors), working along a line extending out from the mast/turbine, the maximum value of effective displacement of forestry back at the mast is found.
These effective displacements are then aggregated to a single overall effective displacement by applying a weighted averaging with the wind climate (so that forestry regions that are in the more common upstream wind directions have a larger impact on the overall displacement).
Note
When calculating flow field grids, displacement heights are not considered
Displacement Height Calculation Command Parameters
The following parameters can be specified for the calculation of forestry displacements:
| Parmeter | Default value | Description |
|---|---|---|
| Angle Step | 3 | The interval in directions to look out from the mast/turbine to evaluate forestry heights. A value of 3 is equivalent to looking out in 120 direction lines. |
| Distance Step | 10 | The step in metres along each line for checking the forestry height and calculating effective displacement at the mast/turbine. This setting should be considered according to the resolution of the forestry map. |
| Tree Displacement Height Ratio | 1 | A factor applied to reduce the effective displacement height of a tree (eg if the zero wind speed plane should not be assumed to be at the top height of the tree) |
| Tree Decay Slope | 50 | The reduction in effective displacement relative to distance away from the mast/turbine. The value is set as distance away per metre of reduced effective displacement. For example, with a value of 50, a 20 m tree at 50 m away will have an effective displacement of 19 m (assuming other default parameter values). |
| Max Distance | 2000 | The maximum distance to look out from the mast/turbine for forestry in each direction. With a Tree Decay Slope of 50, trees of up to 40 m in height at 2000 m from the mast/turbine will have zero effective displacement so 2000 m is considered more than sufficient in most cases. |
| Clearing Radius | 0 | Can be used to ignore forestry close to the turbines when calculating the displacement heights if forestry clearing around turbines is expected. It is specified in metres. NB the clearing radius is only applied to turbines, not masts. Where clearings around masts and turbines need to be created more precisely, it is recommended that this should be done by editing the digital map before loading it into WindFarmer and the clearing radius should be set to 0. |
Forestry Map Extents
The forestry height is defined as 0 m in all areas that are outside of any loaded foresty maps. It is therefore important to load sufficient data around the masts and turbines to ensure a correct calculation of displacement heights. Using the default value of 2000 m for Max Distance, the forestry map data should extend a minimum of 2000 m radius around all masts and turbines.