Wind Speed Terrain Type (Ladybug) VS Landscape (Butterfly)

Hello everyone,

I am a bit confused about two different strategies to reduce the wind speed of the epw file for a urban context for example. In Ladybug you can choose for the WindspeedCalculator a terrainType between 0 and 3. In Butterfly there´s a option for landscape, where you can choose a roughness level from 0 to 7.

When I´m doing a site analysis and get my wind directions with the average wind speeds, I first thought I can then use theses average wind speeds for my CFD in Butterfly. But isn´t then the wind speed value reduced two times, once in WindspeedCalculator and once in create case from Wind Tunnel?

Am I completely wrong with this?

Any ideas? Maybe @TheodorosGalanos, you are the best expert to ask this?

Thank you so much.

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I’m not entirely sure if the same landscape classification is used in both of the tools. In Butterfly we follow the international standard of the updated version of the so-called Davenport classification. The roughness value is used for the calculation of the physical parameters of the flow (mainly k, epsilon, omega, etc.), which BF does for the user in the background.

Perhaps @chris can give us some more information on what the Windspeed Calculator is using under the hood (unless that info is already there in the tooltip, make sure to check!). In any case, these two tools are quite independent as you don’t really need the WindspeedCalculator for your BF studies. Instead you can simply plug in the EPW wind speed and let the reference height be the same with EPW values (almost universally @10m).

Kind regards,

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@davidbriels and @TheodorosGalanos ,

The ladybug Windspeed calculator is intended to be versatile enough that it can characterize both the wind speeds of butterfly (which uses a log law based on roughness length) and the the wind speeds that E+ uses (which is a power law and is based on an exponent and a coefficient). You can switch between the two with the powerOrLog_ input to the component.

This section of Ladybug_ladybug shows you what assumptions are being made for the coefficent and exponent (d, a) and the roughness length (rl) for each of the terrain types:

I know we are drawing a parallel between the Davenport classification and the E+ classification when we do this.

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