With snow blanketing much of Western Europe at the moment, here's a chance to do some aerodynamic investigation in the field. 
Somewhat counter-intuitvely, it turns out that under certain conditions snow fails to accumulate on the windward side of a tree. The reason for this is that the snow at the base of the tree is scooped out by a horseshoe vortex. A high-pressure stagnation point forms on the windward side of the tree, and this creates an adverse pressure gradient for the flow approaching the tree. The adverse pressure gradient causes the boundary layer of the flow across the ground to detach and roll-up into a vortex. (The photo and first diagram here are taken from Internal flow: concepts and applications, p117, Greitzer, Tan, and Graf, 2004). The vortex is then bent down the sides of the tree, where it also scoops out some snow.
Similar vortices can be found on cars or aircraft at the junctions between a wing-section and the endplate or fuselage.
6 comments:
I believe some notable bridge failures are due to the same effect, where the foundations have been eroded in The same way with water...
The other thing I like to see is the 'wire' boundaries put at the edge of fields, which create a vortex, slowing blowing snow and reducing the chance of a whiteout on roads... There were lots of these in the mountains of Colorado, and I am sure elsewhere!
Indeed, it's fascinating stuff isn't it?
I saw this today and thought of you... (that sounds weird...) It's like a massive wind tunnel experiment with high rise buildings...
http://photoblog.msnbc.msn.com/_news/2012/02/10/10375026-spectacular-cloud-tsunami-rolls-over-florida-high-rise-condos
Amazing photo. A tower-block is certainly a very bluff body!
There is a particularly bluff body in Leeds: see this scary news story:
http://www.bbc.co.uk/news/uk-england-leeds-16968325
Sam
Interesting that people have been complaining of a 'wind-tunnel effect'. They should at the very least insert a very large fan-blade, and use the electricity it generates to power the lighting and air-conditioning in the adjacent buildings.
Might be a bit turbulent downstream of the fan though.
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