Base Roughness Calculation

Base roughness is an important input to the large wind farm model. For WasP users the average base roughness at turbine locations may be computed via a user script. The script calculation performs a frequency-weighted average of the directional base roughness exposure at each turbine location followed by a simple average of the base roughness found on each turbine so as to get to a final base roughness representative of the wind farm. More detailed directional roughness exposure data may be exported as a text file report with an additional script call.

Requirements

• A valid WasP installation must be present
• Currently, base roughness calculation only supports the pointwise "WasP from Frequency Distribution" flow model as shown below, it does not support an area flow model nor any other flow model type like "External Flow Model", "WasP from Wind Atlas" or "Simple Flow Model". If your project has one of these unsupported flow models see below how to proceed to be able to calculate base roughness.

Workflow if flow model is pointwise "WasP from Frequency Distribution"

• You're all set, just run the script below:

public void Execute()
{
    //Calculate base roughness
    double baseRoughness = Toolbox.CalculateBaseRoughness();

    //Set the calculated base roughness in WFA
    Workbook.ModelSettings.EnergySettings.LargeWindFarmCorrectionSettings.BaseRoughness = baseRoughness;

    Toolbox.Log(string.Format("LWF base roughness set to: {0}", baseRoughness));

    //Set the increased roughness in WFA (change INCREASED_ROUGHNESS_INCREMENT as needed)
    const double INCREASED_ROUGHNESS_INCREMENT = 0.05;
    double increasedRoughness = baseRoughness + INCREASED_ROUGHNESS_INCREMENT;
    
    Workbook.ModelSettings.EnergySettings.LargeWindFarmCorrectionSettings.IncreasedRoughness = increasedRoughness;

    Toolbox.Log(string.Format("LWF increased roughness set to: {0}", increasedRoughness));

    //(Optional) If you wish to analyze in detail roughness values per sector and turbine,
    //uncomment below to output a tab-separated table of roughness calculation results
    //Toolbox.ExportBaseRoughnessReport(string.Format(@"C:\MyProject\base_roughness_export.tsv"));
}

Workflow for any other flow model

• You'll have to change the flow model type to WasP from Frequency Distribution.
• Then delete any Flow Field Boundaries that may be present.
• Finally set flow target heights and run the calculation.
• Now you can run the script above.
• After running you now have to reload / restore your previous flow model calculation.

Increased roughness

Increased roughness represents the effect of the wind turbines. An increase from the Base roughness between 0.02 and 0.03 m has been found to provide a good match. A higher value of 0.05 m can be used, when looking at specific wind speed cases with deep wakes. The default setting for the increased roughness is 0.06 m.| You may set a project-specific increased roughness value by editing the variable INCREASED_ROUGHNESS_INCREMENT in the script as necessary.