# Wind Tunnel construction logic

Hi,
I am struggling to get the logic behind the wind tunnel construction in the attached file @TheodorosGalanos posted (No errors but also no vectors).

If I well understand, the longest dimension was first extracted, then the height of the bounding box “wrapping” the buildings; then a proportion (the longest dim/height) seems to be made, to get the multiplier for both windward and side of the tunnel, rather than just multiply the height by the factor you want? Does this links the wind tunnel dimensions to the blocakge ratio? But in this case it’d be not necessarily the longest dimension to be considered perhaps…

Other sample cases I found around, simply input sliders into the “Wind Tunnel Parameters” component. e.g. windward, leeward, etc…; in this case, I don’t get how the component “knows” what it needs to multiply by the number shown on the slider (which is the height of the bounding box, but how does BF know this, as I can’t see where the height dimension of the bounding box is specified).

Thank you for your help, V.

Butterfly calculate the bounding box of input geometries and uses the height of bounding box to determine wind tunnel size.

Ok thank you Mostapha, this greatly answers my question 2.

Regarding question 1, any chance to explain the logic, as I am trying to adapt it to my case but there’s something I must be missing here…

Thanks. V.

Hi @Voila,

I would recommend checking (Hall, 1997; Cowan et al.,1997; Scaperdas & Gilham, 2004) that recommends most of the following factors.

1. Height of the tunnel is 6Hmax
2. Span of the tunnel is governed by the blockage ratio. More can be read in (Blocken et al., 2004) and VDI (2005)
3. In front of the buildings 5Hmax
4. Back of the buildings 15Hmax
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Hi Devang,
thank you, I’ve done this type of research already, which however gives fixed parameters; therefore I was interested in understanding if the sequence that I was sent in the file I referred to in my first post, is the logic behind the construction of these parameters as well. Any idea if this is the case please? E.g. does this logic have links with the blockage ratio? Thanks. V.

Hello everyone,

I apologize for such a late reply @viola.maffessanti, I was unavailable these past few days.

I will try and explain the logic behind what I was doing in the component that calculates extension of the bounding box.

The main idea is that we want to extend the bounding box by some standard, meaning similar across different studies and projects, process. The process I have chosen there is to extend the bounding box for every case by the maximum length of the input geometry. So the first step there is to simply take a bounding box of all buildings and then select the maximum (either x max or y max depending on the geometry).

Since BF takes multiples of maximum height (z max), the next step is to divide that length by the maximum height. That gives us a number which represents the ratio between z max (the BF input of choice) and maximum bbox (bbox_max) extension.

The last step is to simply select a kind of best practice. My choice there is to extend windward (before the geometry) and sides by 1 x bbox_max, while extend the leeward by 5 x bbox_max. Those numbers give us the final multipliers for the relevant BF inputs.

That is the process more or less. I have a feeling the reason you might have been confused is that it’s not a very clear way of doing it. I made that cluster for one of my cases at some point and then never really went back to make it clear or efficient. I apologize. I hope it now makes sense to you, and anyone else that was curious.

Please let me know if there’s something I did not explain clearly.

Kind regards,
Theodore.

Hi all! I’m new in the forum.
I try to relate my question to this post, even if old.

I’m doing an urban-scale wind analysis on butterfly; and I’m reflecting on the shape of the wind tunnel. I understand that wind tunnel is a way of assigning boundary conditions to geometry block. However, I wonder if this tunnel should/can take different forms depending on the surrounding orographic/building context.

For example, if my geometry is located at the foot of a valley, the tunnel could have a trapeizoid shape, in order to recreate the channeling effect of the wind along the mountain. This is not my case, but I would like to understand better the logic to build a good and appropriate wind tunnel.

Is the parallelepiped shape of the wind tunnel always the best option?

Thanks!

Martina