Microclimate and urban planning

Modélisation de ventilation naturelle sur un bâtiment

Why do we need to assess the pressure coefficient field on the building envelope?

It is well known that the natural ventilation of a building is driven by the combined forces of wind and thermal buoyancy. However, when the opening areas are large enough, and except for large volumes, natural ventilation is mainly driven by wind forces.

Therefore, for the evaluation of natural ventilation in warm climates, the coupling between thermal forces and wind driven forces is generally assumed to be negligible.

The pressure field on the building envelope generates flows through openings that allow the interior space to be refreshed.

Assessment of natural ventilation in urban areas with thermal dynamic software

The cross ventilation efficiency is estimated by considering the flow rates through the openings. All these characteristics are fundamentally dependent on the external wind pressure at the openings. The macroscopic approach gives the air exchange rate from the pressure field characteristics.

Many numerical thermal software used for the design of naturally ventilated buildings consider the values given in the "AIVC applications guide" for low-rise buildings (Liddament, 1987) as universal values.

Unfortunately for designers, the pressure value on the outside walls of buildings could not be assessed with this method in urban places for three reasons:

  • The pressure coefficient depends strongly on the shape of the building.
  • The pressure coefficient on buildings depends strongly on the influence of neighboring buildings.
  • The reference wind velocity needed to convert the pressure coefficient into pressure cannot be easily defined due to the inhomogeneity of wind in urban places.
Natural ventilation in urban area

When buildings have non-standard shapes or when the flow is disturbed by nearby obstacles, these external wind pressures must be evaluated either by wind tunnel tests or by computational fluid dynamics (CFD) methods such as UrbaWind.

Typical CFD computational domains cover an area of nearly 500 m x 500 m, allowing the effects of neighboring buildings to be considered and thereby providing an efficient pressure coefficient.

Poor IT resources? Discover how to break the computational problems for large, complex urban areas: read the article.



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