Height of wind in the "probability distribution for speeds"

[agaffajoli]

Hi,

I was wondering, at what height is the wind for the definition of the probability distribution of speeds. Is it at 1.75m like in the results? 

I want to analyse a specific wind but I only know the wind speed at 10m, how can I enter it into Spacemaker ?

Thank you !

[knut.sverdrup · Accepted solution]

Hello @agaffajoli , thanks for getting in touch! 

Yes, the probability distribution graph corresponds to wind speeds at ground measurement height. Most weather data stations are defined at 10m or 100m, so we use a logarithmic Atmospheric Boundary Layer (ABL) profile to scale the data to the ground height. This is in order to give an intuitive connection between the ground results and the input speed. 

When computing the results for each direction, we use ABL conditions along the entire inlet surface, as you would expect. You can translate a given wind speed at height 10m to height 1.75m as long as you know the roughness length - this can be specified in project settings .

The formula is:

 

So if you have v_ref = 12 m/s at z_ref =10 m and the roughness length is 0.2 m, the speed at z = 1.75 m is 6.95 m/s. 

Hope that helps! 

[agaffajoli]

Thank you @knut.sverdrup ! 

That was exactly what I was looking for. Just an other question, when the roughness length is in "automatic" is it the value from Global wind atlas for the chosen location ? 

[knut.sverdrup]

We fetch the data from ERA5, as under "Boundary conditions" towards the end of this help article: https://help.spacemaker.ai/en/articles/4532016-wind-analysis-details

[knut.sverdrup]

FYI: we have now added functionality for choosing the incoming speed height in the software. Hopefully that will remove the need for users to compute this themselves 🙂 

Illustration attached.