1. Windy Cities http://www.youtube.com/watch?v=8NSW9R1IVAk http://www.youtube.com/watch?v=SEBLt6Kd9EY http://www.youtube.com/watch?v=porZ2jOaYew

2. London Smog Then … … and now

3. What is wind?

4. Turbulence http://www.youtube.com/watch?v=1jge03Qls6s http://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

5. How does air flow around a building? Flow around a single building

6. Strong surface winds Flow around a single building

7. Flow down Parallel Streets Streets far apart Streets close together

9. http://www.youtube.com/watch?v=5lSvDKv7sTg http://www.youtube.com/watch?v=7HUUENCQFto How does air Flow around Buildings?

10. Source of Dye Wind direction Flow down Parallel Streets Source www.dapple.org.ukwww.dapple.org.uk

11. Wind Tunnel Experiments flat roof H/W=0.6 flat roof H/W=1.0 high pitch H/W=0.6high pitch H/W=1.0 Model scale ~ 1:400  ~ 400 times faster than in nature !!!

13. BT Tower NE wind direction

14. B. Blocken et al. / Environmental Modelling & Software 30 (2012) 15e34 Probability of Wind Nuisance at Eindhoven University Hall to be replaced

15. Computer simulation of whole city air flow http://www.youtube.com/watch?v=x3w6w0 B6pf8http://www.youtube.com/watch?v=x3w6w0 B6pf8

16. 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.