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The notions of overpressure and depression are relative and depend on the observer’s point of view. By convention, we consider here that we observe the building from the outside. Unless otherwise stated, it must then be understood by overpressure that the pressure outside the building is higher than inside.
The wind corresponds to a balancing current between zones of different pressures. The part of the air flow that meets a building will be dammed up in front of it, creating a state of overpressure.
The value of this overpressure is called “stagnation pressure” expressed in pascal. The level of pressures and their distribution over a building are the result of the interaction between the incident wind (average velocity gradient, turbulence, etc.), the building (through the interplay of its shape, dimensions, wall roughness) and the juxtaposition of immediate obstacles (or the nature of the immediate environment).
These pressure differences cause air movements through the openings of the building. This phenomenon causes air infiltration on the windward side, and air extraction from the low-pressure areas.
The objective of sizing natural ventilation is to best support the air flows naturally generated by the prevailing winds:
In this context, the objective is to position the air inlets and outlets according to the pressure distributions; for that, it is a question of finding the zones having the same behavior (depression or overpressures) according to the prevailing winds. This helps to promote the intended operation of the opening.
When a roof is exposed to the wind, a phenomenon of pressure is created on the face subjected to the wind and depression on the opposite slope.
By nature, a difference in air pressure between two environments tends to balance each other: the negative pressure on the opposite exterior face will cause air to be sucked from the atmosphere inside the building. An excess pressure is then created in the volume, proportional to the force of the wind, causing the natural circulation of the interior air.
The overpressure zones will then tend to favor air inlets.
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