Hello , I was wondering how big the studied plot/urban area should be so the DragonFly component can give accurate results ?
I was testing the effect of different buildings morphology (the ones in grey)
The results are very different if I remove or keep the surrounding area …
I’m testing two options , 25x25 buildings and 15x15 buildings with the same densities and same DF propreties.
If I keep the surrounding area the second option has worse effect on the UHI then the first option and if I remove the surrounding by taking the plot alone (110x50m) the results are the contrary…
You should aim to model a neighborhood with a diameter of around 100m. The building energy calculation of Dragonfly’s UWG component is based on the Building Energy Model Town Energy Balance (BEM-TEB) scheme, which can model the building energy consumption of a city (approx. 1000m to 10,000m diameter) from the resolution of a neighborhood (approx. 100m). From this neighborhood model, the UWG component will derive a representative urban canyon with geometric dimensions reflecting the weighted average of your neighborhood typologies.
However, that neighborhood has to represent a reasonable city typology. The UWG uses a lot of microclimate, and mesoscale parameters based on this assumption. If you model a neighborhood that is more suburban, or rural in nature (i.e very low site coverage, density, and high amounts of vegetation) the UHI results are going to start being more and more incorrect.
I’m not sure how you are representing that extra site area (vegetation? roads?) but I think you need to rethink your assumptions. If you want to test the UHI impact of the 15 x 15 versus 25 x 25 typology, then you should just build an entire neighborhood of just those typologies, that is then extrapolated to a larger scale to derive the potential UHI impact. If there is a large non-built site area in your neighborhood, you can incorporate that into your model, but just be aware the probability of error is higher, and the larger site area is going to dominate your microclimate interactions and wash out differences between your two different typologies.
If you want to test the UHI impact of the 15 x 15 versus 25 x 25 typology, then you should just build an entire neighborhood of just those typologies, that is then extrapolated to a larger scale to derive the potential UHI impact.
So it’s not precise to use dragonfly to test the effect of one building or few buildings on the neighborhood?
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I’m not sure how you are representing that extra site area (vegetation? roads?)
I’m using the extra site area and the roads in the surrounding as an input in the terrain of the DF city component… As far as I know it’s the only option other than the grass input , right?
So it’s not precise to use dragonfly to test the effect of one building or few buildings on the neighborhood?
Yes, as a rule of thumb, I would suggest thinking about using the tool to test “neighborhoods” and not “buildings”. Adding or subtracting one or two buildings isn’t enough to change the UHI of a site. You can look up various papers and thesis done on the Urban Weather Generator (UWG) to see examples of valid site area precedents. In my earlier post I said to aim for 100m diameter (which is based on the original UWG author’s thesis suggestion), but having just done some more precedent investigation, I think the more traditional scope is a range between 200 - 450m diameters. And, as I mentioned, these neighborhoods are assumed to be within a larger urban area.
I’m using the extra site area and the roads in the surrounding as an input in the terrain of the DF city component… As far as I know it’s the only option other than the grass input , right?
I don’t have the GH components in front of me right now, but I assume then that the terrain/site area is subtracted from the building footprints to determine road area. Then the grass fraction you specify is used to determine road versus grass coverage. You can confirm this by checking the GH component hints. You’ll want to make sure your various vegetation fractions reflect your actual site area.
