Air flow patterns around buildings in Built up areas can be quite complex, especially when there are multiple buildings in close proximity to one another. The effects of air flow on pedestrians at ground level are of particular concern, and should be evaluated carefully to ensure pedestrian safety.
As wind strikes the surface of a building, the flow typically splits as it flows over and around the building. This results in jetting off of the windward corners and strong vortex formation downstream of the leeward face as the flow streams reattach. Depending on the wind velocity, these flow phenomena can cause pedestrian discomfort, and in extreme cases, be dangerous. The following describes the behavior of the wind flow at several positions relative to the building:
At the windward surface, where the wind first impacts the building, the wind stream splits above and around the sides of the building.
The air flow accelerates around the corners of the building. This causes a high speed separated region off the corner and a calmer stagnation region on the sides of the structure:
The wind flowing over the top of the building descends rapidly on the leeward side:
As the flow streams over the top and sides of the structure reconnect on the leeward side, vorticies form in the wake region. A stagnation region can occur against the leeward surface, followed by a highly energetic region of vortex flow just downstream of the structure:
A channeled flow effect occurs when wind gusts pass between adjacent buildings, resulting in a "canyoning" effect. These can be particularly dangerous for pedestrians, especially when there are no landscaped or structural elements to disrupt the stream:
These effects can have a significant effect on pedestrians, and should be reviewed carefully to ensure safety.
The following table from (Shane 2011) lists a range of comfort classes based on wind speeds and limiting criteria. These classes attempt to quantify pedestrian comfort assuming suitable attire. They only consider wind speed and do not account for other weather conditions such as temperature and humidity:
| Comfort Class | Wind Conditions | Activity Description |
|---|---|---|
|
Sitting |
Wind speed < 3.9 m/s Occurs > 70 % of the time |
Acceptable for sedentary activities. |
|
Standing |
Wind speed < 6.1 m/s Occurs > 80 % of the time |
Acceptable for standing, strolling, and other slow movements. |
|
Walking |
Wind speed < 8.3 m/s Occurs > 80 % of the time |
Acceptable for walking, running, and other rigorous activities. |
|
Uncomfortable |
Wind speed > 8.3 m/s Occurs > 20 % of the time |
Unacceptable for walking. |
|
Dangerous |
Wind speed > 25 m/s Occurs > 0.01 % of the time |
Dangerous for walking. |
References
Fadl, Mohamed Sakr and Karadelis, John (2013). CFD Simulation for Wind Comfort and Safety in Urban Area: A Case Study of Coventry University Central Campus. Coventry CV1 5FB, United Kingdom.
Shane, F. (2011). Pedestrian Level Wind Study. Toronto, Ontario, Canada.