Urban Planning: Real Zone Urban Heat Island (UHI) and Urban Air Quality

Urban Heat Island (UHI) CFD Simulation on a Real Urban Zone, ANSYS Fluent Training

Description

This project simulates airflow and heat transfer over a real urban area — the Auckland University of Technology (AUT) campus in Auckland, New Zealand — to study the Urban Heat Island (UHI) effect using ANSYS Fluent.

Urban heat island and pedestrian comfort are central concerns in urban planning. As cities grow denser and taller, buildings reshape local wind patterns and trap heat, creating uncomfortable or even unsafe conditions at street level. CFD lets planners predict wind speed and temperature around a real building layout while the design is still on the drawing board, so problem areas can be identified and fixed before anything is built.

The study has two goals: to map pedestrian wind comfort and flag locations where wind speed exceeds 3.8 m/s, and to check outdoor thermal comfort, where the aim is to keep the campus area below 295 K.

The real building footprints were extracted from Google Earth Pro, and the corresponding geometry and building volumes were reconstructed in ANSYS Design Modeler as a main domain (the campus itself) surrounded by a larger subdomain that captures the incoming wind.

Simulation Methodology

The analysis is carried out in two parts: wind comfort and thermal comfort.

For the wind-comfort study, note that Auckland's airflow is predominantly from the southwest, shifting toward the northeast in summer as the high-pressure belt moves south, and that coastal areas are consistently windier than sheltered inland ones. Using representative wind conditions for the site, the model resolves the wind field around the buildings and evaluates it against standard pedestrian wind-comfort criteria.

For the thermal-comfort study, solar loading is applied with the Discrete Ordinates (DO) radiation model, set from the site's geographic coordinates for mid-February at 1 p.m. The building surfaces are assigned a representative heat flux, the ground surface is fixed at 286.15 K, and the incoming free-stream air is set to 288.15 K, based on local meteorological data. Together these drive the temperature field that forms the urban heat island.

Results & Conclusion

The velocity contours show that buildings directly exposed to the wind experience speeds up to about 30 km/h (≈ 8.3 m/s) at some points, well above the pedestrian-comfort limit. As the air moves into the passages between buildings it slows to around 10 km/h (≈ 2.8 m/s), which is comfortable, and it is damped further in the rear rows of buildings.

On the thermal side, the incident solar radiation over the domain ranges from about 1560 to 1700 W/m²; taller buildings absorb more, while the passages between them receive less because of shading. Temperature contours were extracted at several heights above the ground: at 0.5 m the average is about 287.46 K with a local peak of 295.77 K, at 1 m the average rises to 287.96 K (peak 292.58 K), at 1.5 m the field changes little, and at 2 m the peak reaches 294.87 K. The warm zones between buildings come from heat rejected by the surrounding surfaces — the signature of the urban heat island — and one low but wide building (the meeting hall) cools more slowly and holds the highest roof temperatures.

Overall, the wind-comfort criterion is exceeded in several exposed areas, particularly toward the suburbs, so the study points to mitigations such as windbreaks or added vegetation and greater spacing between closely packed buildings to avoid narrow, high-speed street canyons. The thermal-comfort target of 295 K is met across almost the entire campus at pedestrian height, with only a small, localized area reaching it, so heat is not expected to cause meaningful hardship for people using the campus.