Windy Citieshttp://www.youtube.com/watch?v=8NSW9R1IVAk

http://www.youtube.com/watch?v=SEBLt6Kd9EY

http://www.youtube.com/watch?v=porZ2jOaYew

London Smog

Then …

… and now

What is wind?

Turbulence

http://www.youtube.com/watch?v=1jge03Qls6shttp://www.youtube.com/watch?v=HTOl0lTet9A

Turbulence is rotational, three dimensional air flow

Turbulence quickly turns the energy from moving air into heat

Unstable eddies in the wind which disrupt the flow of air

How does air flow around a building?

Flow around a single building

Strong surface winds

Flow around a single building

Flow down Parallel Streets

Streets far apart

Streets close together

http://www.youtube.com/watch?v=5lSvDKv7sTg

http://www.youtube.com/watch?v=7HUUENCQFto

How does air Flow around Buildings?

Source of Dye Wind direction

Flow down Parallel StreetsSource www.dapple.org.uk

Wind Tunnel Experiments

flat roof H/W=0.6 flat roof H/W=1.0

high pitch H/W=0.6 high pitch H/W=1.0

Model scale ~ 1:400 ~ 400 times faster than in nature !!!

BT Tower NE wind direction

B. Blocken et al. / Environmental Modelling & Software 30 (2012) 15e34

Probability of Wind Nuisance at Eindhoven University

Hall to be replaced

Computer simulation of whole city air flow• http://www.youtube.com/watch?

v=x3w6w0B6pf8

Summary • Urban air flow is complicated!• Swirling flow is generated by buildings• At street level, the flow can even be in the

opposite direction to the wind higher up• There can be large gusts at street level which

can be annoying or even dangerous• Pollution can be concentrated in areas of low

wind speed.• Architects and urban planners try to design

buildings to maximise their natural ventilation, and to prevent dangerous air flow around them.