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General questions about modelling with WAsP


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Hi,

i am a new user to WAsP and after having studied the theory behind the model and done some work on it, i have a few rule-of-thumb type of questions when applying these principles to real problems:

  1. Maps: what would be an adequate horizontal resolution [m] for elevation maps?
  2. Maps: what would be an adequate vertical resolution [m] for elevation maps?
  3. Maps: what would be an adequate horizontal resolution [m] for roughness maps?
  4. RIX: one of the limitation of the model is when we have complex terrain (RIX>30%). Does this threshold refers to the average RIX of a site or to the RIX of each sector (ie if the average RIX<30% but for specific sectors RIX>30% would you consider the site complex?)
  5. DeltaRIX: what would be an adequate range of deltaRIX values to consider two sites similar?
  6. Power density: could you provide the formula used to calculate the power density for a grid resource map or a turbine site?
  7. Site with multiple wind farms: for a site with multiple wind farms, to model the wake effects of all turbines on each others, do i need to put all turbine sites in the same wind farm on WAsP, or can i create different wind farms and still account for all wake effects?

thank you!

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Hi Filiberto,

The terrain elevation in WAsP is specified as a digital height contour or vector map. The resolution of these maps depends to some extent on the terrain steepness (RIX). For example, in a relatively flat country like Denmark, you can use maps with 2.5-meter contour intervals, while in complex terrain, 5 to 10-meter intervals might be more appropriate. Roughness maps should contain the essential roughness transitions, e.g. from water to land and from open areas/farmland to forests/urban distracts. The maps should extend at least 15 km in all direction from your sites of interest.

The ruggedness index (RIX) is a relatively simple indicator of flow separation. RIX is defined as the percentage fraction of the terrain with terrain slopes higher than a critical value of 0.3. If the slopes are below 0.3, the RIX value is 0. WAsP will give you the RIX value for every wind direction bin and an average value. I mainly use the wind direction averaged value. 

A critical terrain slope of 0.3 for all hills is a conservative indicator of flow separation. I would therefore not be concerned if I see a RIX value of, say, 5% and similar dRIX values. In WAsP 12.6, there is a method to evaluate the similarity between two sites, including the combined effects of extrapolation distance and RIX. If you are interested, you can find the paper that describes the method here: https://doi.org/10.5194/wes-5-1679-2020

Kind regards
Andreas

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