According to the World Bank, currently 56% of the world’s population – 4.4 billion inhabitants – live in cities. This trend is expected to continue, with urban population more than doubling in size by 2050, at which point nearly 7 of 10 people will live in cities. As city growth and regeneration continues there is a corresponding aspiration for space. However, this can meet planning resistance as, rightly so, surrounding greenbelt is a protected resource. With our understanding of climate change and the importance of vegetation, there has been a subsequent push to vertical city growth through multi storey buildings instead.
With high-rise urban growth comes a greater demand for green space where people can congregate, relax and enjoy their surroundings. However, the presence of these green spaces within a myriad of high-rise structures makes it tricky to ensure pedestrian comfort and usability of the space throughout the year. It becomes necessary to understand the suitability of external pedestrian wind comfort to ensure green spaces are optimally designed and located to maximise their function throughout the year, which includes alleviating the effects of flow around nearby high-rise buildings.
When buildings and sites are being planned, early stage conceptual modelling can measure the risk. Pedestrian comfort analysis, using Computational Fluid Dynamic (CFD) simulation, is performed to study the conditions surrounding the site under a range of wind speeds and directions. Consider it as a virtual wind tunnel, where a CFD simulation model of the site is massed and subjected to the test considerations. Air speed is measured at points of interest, and this data along with local weather data is used to obtain an annual perspective of the local air speed across the area of interest. Statistical analysis can then be performed to check compliance against various wind comfort criterion. The brilliance of the model is that the speed feedback can be introduced and the tests rerun to optimise the design.
CFD simulation is imperative to understanding the challenges of airflow surrounding buildings in built-up areas.
Take the case when the wind impinges on the face of a building. Bernoulli’s equation states the air will slow down and the building face pressure will increase significantly. The natural tendency then is the high-pressure air will flow towards a lower pressure area which typically exists at the building’s base; an effect called downwash. These flows are regularly observed at the base of tall buildings, especially when surrounded by shorter buildings. Once the downwash reaches the ground level it spreads horizontally with pedestrians experiencing this as sudden gusts. These gusts occur even in conditions which are termed as a ‘light breeze’ by recorded weather data and can lead to micro climates.
A second example of air accelerating is when street layouts are funnelled from a wide to narrow path. This is a regular situation in existing city centre layouts, however this can be considered in new and regeneration schemes. It is also observed in cases where there are large openings at the base of the buildings used to create spaces like a reception concourse. Although visually impressive and imposing, its weakness is the potential for accelerating air.
Too often both situations come together with the effect multiplying, and at times generating hazardous conditions. There have been many cases where pedestrians have complained of difficulty opening doors and comfortably crossing open spaces due to pressure imbalance. By incorporating CFD simulation into the planning process, these issues can all be addressed to ensure optimum pedestrian comfort levels.
Now, turning to the popular metric currently in use, the Lawson criterion, which is divided into two parts. Firstly, it covers pedestrian comfort for regular activities like walking, standing and seating. This helps in determining the usability of a location or site against that particular activity. The criterion sets out threshold local air speeds based on the activity, which cannot be exceeded for more than 5% of the year. The other aspect is the safety criterion which stipulates the air speeds which cannot be exceeded even once a year. This ensures that pedestrians and cyclists are not in danger of physical harm from high air speeds.
The impact of urban developments is felt globally through carbon emissions, their internal conditions (as we live and work within them), but also in how we commute and enjoy the space between them. We can make the best of all three through CFD simulation modelling.
IES Consulting’s CFD experts use a combination of IESVE MicroFlo and the 3rd party CFD software package Engys® HELYX®. MicroFlo was developed to provide our customers with a relatively simple and accessible CFD simulation package which can be used by engineers in conjunction with our ApacheSim tool. The Engys® HELYX® CFD package, derived from the OpenFOAM® libraries, allows us to use bespoke tools for performing complex analyses like data centres, external pollutant studies, pedestrian comfort and external flows with heat transfer, etc.