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

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).

The impact of this innovative instrument, the "Smart Cube", will happen over a range of time scales and will be relevant to many different groups across the UK. In the short term, the shared use of a standard, calibrated, instrumented model for wind tunnel work will allow us not only to build a reliable experimental database but also to ensure confidence and repeatability in the high-quality measurements taken in different laboratories. The use of a national standard will also simplify and encourage data comparisons.

In the medium term, the Smart Cube will help the interpretation and understanding of the physics of wind flow and micro-climate around urban buildings using non-intrusive techniques, thus producing higher quality data when compared to the current state-of-the-art. The data acquired with this asset will also form a reliable and high-quality experimental database, that can be used to validate high-resolution numerical models (CFD). This will support the large urban CFD community, who are currently running a number of high-fidelity numerical simulations without any experimental validation of comparable resolution to be able have confidence in their results. Integrating the experimental and numerical techniques will also have the effect of providing a better knowledge of the physical flow phenomena, as these are complementary techniques and their combined power is more than the sum of the two methods in isolation (e.g. wind tunnel data can explore a different parameter space compared to CFD and vice-versa).

In the long term, building on a trusted and shared interpretation of the physics will lead to better fast-response mathematical models that can predict the flow, the air quality, and the micro-climate around buildings as well as support natural ventilation studies. Having better forecasting tools will have a beneficial impact on reducing air pollution in urban areas and city heat islands, with consequent improvements in public health and resilience to the impact of climate change. This should lead to significant reductions in mortality rates and improved productivity through decreases in morbidity, with large positive economic benefits both nationally and internationally.

The data produced using the Smart Cube will be shared through an open repository and actively advertised through existing networks (e.g. MAGIC, Refresh, Urban Fluid Mechanics SIG, Low-energy Ventilation SIG, the Met Office Urban Working Group); new avenues will also be encouraged. As already mentioned, the data will have a great appeal for numerical modellers who need validation data and can assist them in amplifying the reach of their studies. Interested parties start with existing collaborators (Universities of Southampton, Reading, Leeds, London South Bank, and Imperial College London) and extend to the numerous other UK groups working on urban simulations. The shared use of the instrumented Smart Cube will also be encouraged through existing networks (EnFlo is part of both NERC-NCAS and EPSRC-NWTF, and there is a large pool of potential users who are already able to access the lab and its suite of facilities).

As for a wider impact, non-academic beneficiaries will be sought, both for the data and improved knowledge produced (and some companies might also be interested in using the instrument itself). Collaborations are already in place through existing networks (MAGIC partners, Air Pollution Research In London - APRIL, SIGs, UK Wind Engineering Society) with potential beneficiaries such as local authorities (e.g. GLA), governmental institutions (e.g. Met Office), and industrial partners (e.g. ARUP, CERC, Foster+Partners, Dyson, BRE, RWDI). Finally, as discussed in the Academic Beneficiaries section, we are planning to extend the network of potential users through a workshop even at the end of the project, where all potential beneficiaries will be invited, as well as providing technical training to potential industrial partners.