Goody! This is my field.
To get an idea of how vortices and eddies form around a building, watch how the snow swirls around a building when there's a breeze or wind. On the leeward side of the building you will sometimes see the "falling" snow
rise. On the leeward side of a slanted roof, there is also a counter-rotating vortex, so a westward wind might be measured as eastward at certain locations near the roof, for example. The air motion is further complicated by what are called "shed vortices." These are vortices that form and shed (migrate away from the obstruction), and then a new vortex develops and sheds, etc. This causes not only 3 dimensional motion, but
unsteady flow... flow that changes with time...very complicated.
As another, simpler example, take a look at how snow drifts and gullies form odd shapes at different spots around a building. It's because the building causes disturbances in the flow field (motion). This is made even more complicated by other nearby obstructions like trees, bushes, neighboring structures, etc.
So yeah, the best location for wind instruments is atop a pole far from
any obstructions. Placing wind instruments on a pole atop a building (as I do with mine) will produce pretty unreliable wind readings. You can even get unreliable readings if the wind instruments are atop a pole but there is an obstruction (even trees) on the leeward side (downstream) of the pole. In this case the obstruction (or trees) affect the air flow
upstream.
Here's a fairly simplistic animation of shedding vortices on Wikipedia:
http://en.wikipedia.org/wiki/Vortex_sheddingIt's simplistic because it shows only two dimensional flow around a cylinder. Real life is three dimensional and houses aren't round
. They have have complex, irregular shapes and corners and overhangs and soffits, etc., all of which produce odd eddies. Nevertheless, the animation shows how the air flow is unsteady and how vortices shed.
Regards,
Kevin...
