The wind field around a building is not simply calmer than it would be without the building — in some regions it is markedly stronger.
Acceleration mechanisms
Downwash. On a tall façade the faster wind at upper levels strikes the face and part of it is directed downwards, bringing the energy of the upper levels down to ground level.
Corner acceleration. As the descending flow sweeps around the building corners it accelerates. The most uncomfortable zones at pedestrian level are typically there.
Channelling. A narrow passage between two buildings compresses and accelerates the flow.
Passages under columns. These create a short circuit between the high-pressure front face and the low-pressure rear face; the flow through them is very fast.
The criterion is not mean velocity
Comfort and safety criteria are defined in terms of exceedance frequency: what fraction of the year a given velocity is exceeded. That means a single wind direction is not enough — several directions must be assessed and weighted by the local wind rose.
The criterion also varies with activity: the acceptable velocity for a sitting area is lower than for a walking route.
Modelling requirements
Atmospheric boundary layer. The inlet profile cannot be uniform; velocity increases with height and the turbulence profile must be defined too. A flat inlet profile gets the ground-level velocities systematically wrong.
Domain size. Enough distance must be left around the building and the blockage ratio kept low. See the boundary condition note.
Separation dominates. Separation from sharp corners is fixed by the geometry, so model sensitivity is lower — that is the good part of this problem. See the separation note.
Surrounding buildings belong in the model. Neighbours steer the flow; an isolated building model is not realistic.
References
- Menter, F. R. (1994). Two-equation eddy-viscosity turbulence models for engineering applications. AIAA Journal, 32(8), 1598–1605. DOI: 10.2514/3.12149
- Ferziger, J. H., Perić, M. & Street, R. L. (2020). Computational Methods for Fluid Dynamics, 4th ed. Springer. DOI: 10.1007/978-3-319-99693-6