Hi @Mo_Elsebaei - very nice project idea! 
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Sounds good but it depends if you want to see transient structural behavior on the buildings or is steady-steate fine for you? Did you already decide? For the turbulence model maybe @1318980 or @ananthu_ajit can comment as I have not tested k-omega SST myself, all I know is that k-epsilon underpredicts in the strong wind and wake regions, maybe they can comment on that.
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Maybe this approach here helps you: Passive Scalar Sources - for the calculation of the concentration of dust you could use the approach from @tcakir here: Pollution exhaust in a garage - #4 by tcakir
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You can define the porosity ( P ) of a (coniferous) tree by
where
- LAI = Leaf Area Index (LAI)
- Dc = Needle diameter
- h = Height of the canopy
- Not if you want to use the approach mentioned in 2) I would say as you need the calculator filter which is not yet implemented (but on the roadmap)
Hope I could help you a bit!
Jousef
Relevant articles: Pekin, B.; Macfarlane, C. Measurement of Crown Cover and Leaf Area Index Using Digital Cover Photography and Its Application to Remote Sensing. Remote Sens. 2009 , 1 , 1298-1320.doi:10.3390/rs1041298
Source for the porosity calculation: Porosity of Trees -- CFD Online Discussion Forums