Porosity of Trees

Ruth

I am doing some wind modelling around a building which is overshadowed by a number of large trees. I plan to model the tree canopy as a porous subdomain, but the question is how porous are trees.

Having done a literature search, it appears that either the figure 50% porosity is plucked out of thin air, or detailed analysis is undertaken resulting in very specific and complex models.

Does anyone have a reference for the 50% figure, or a more accurate figure with some sort of validation? There are also seasonal, and species variation in porosity so any guidelines taking this into account would also be welcome.

Thank you, Ruth