The Smart Cube: a national calibration standard for urban canopy flows

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Air quality in cities has been recognised to be a matter of urgent concern. One of the major factors driving urban air quality is the flow of air between buildings: this can affect how vehicle exhaust and other pollutants are carried away from street level (and building fresh air intakes), how hot, stagnant air within buildings can be replaced with cool, fresh air from outside with minimal energy input, ultimately with repercussions for public health. The presence of cities also affects wind, temperatures and local weather patterns. A major problem in studying the wind around buildings is the wide range of spatial scales involved: to get accurate results, you typically need to be able to track pockets of air the size of melons. Over an entire city block, then, there are just too many melons: our fastest and most powerful computers are still not able to manage this for a large enough city area (like a neighborhood). The best way to study the wind between buildings, then, is in a wind tunnel. This poses a different problem: the wind that flows over a city is far from uniform. It has already been affected by the terrain upwind, with a complicated and nuanced impact on temperature and speed. We can simulate these upstream conditions in an atmospheric wind tunnel (such as EnFlo at Surrey), but it can never be perfect. Each wind tunnel will, therefore, be slightly different - and the effects of these differences on the measurements can be large. The aim here is to produce a generic model building which can be mounted into any wind tunnel, carrying a sophisticated set of pressure, airspeed and temperature sensors. The same instrument could then be used in different wind tunnels to make sure that their inherent imperfections are not affecting the results of the research. Because the same instrument can be used in different facilities, we will know for certain that that any differences in the results are due to the wind tunnel, and not to the way in which the measurements were done.Also, once the wind tunnel is known to be producing reasonable results, the instrument can be used to return extremely valuable data for scientists. Pressure measurements are critical, not only because these tell us how well ventilation systems will work, but also because these inform us about how wind is circulating around the buildings without the need of any other measurement tools which may interfere with the flow. By comparison, most common current practices need probes which can interfere with the measurements by their very presence. One of the most important quantities - the drag exerted by the wind on the Earth, used to approximate the effect of the city on the atmosphere - can be obtained from the pressure as well. Currently, we cannot acquire enough pressure measurements at a time for this to be accurate: the pressure sensors have usually been just too expensive. Since temperature also can have a strong effect on how wind moves around buildings (i.e warm air rises), we will install heaters and monitor temperature with multiple sensors as well. Once complete, we will obtain a detailed set of measurements of pressure and air flow around the model building. This will be used as a benchmark during tests, to make sure that everyone using the model is getting the same results: if not, this provides clear evidence that something is wrong. This instrument will become a valuable NCAS-AMF resource; it will be made available for use at research laboratories around the country, either to ensure that their wind tunnels are producing a good approximation of the wind approaching a city or to be able to collect data during city-flow experiments that would otherwise not have been available. The measurements may also be resolved down to the size of typical windows or ventilation intakes, allowing the effects of exterior wind on interior air quality to be modelled (an emerging area of great importance to public health).