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Two different physical phenomena can be used to optimize natural ventilation : overpressures/depressions caused by wind or turbulence outside the building and thermal draft.
The neutral pressure plane is a decisive factor for the dimensioning of natural ventilation openings subject to thermal draft effects. The neutral plane is an imaginary horizontal plane, where the internal pressure is equal to the external atmospheric pressure.
At the neutral pressure point, the air inlets are very inefficient. Above the neutral plane, the internal pressure is higher than the external atmospheric pressure. That is why the air outlets should always be located here. The air inlet openings are always located below the neutral plane, as a negative pressure zone is produced there.
When designing a natural ventilation system, it is necessary to predetermine the height of the neutral plane and determine the proportion of pressure difference that is available to overcome flow losses in the air inlets and the proportion that will be used by the air outlets. The precise position of the neutral plane and thus the arrangement of the air inlets on the facades can only be verified by an iterative method
When a roof is exposed to the wind, a phenomenon of pressure is created on the side subjected to the wind and depression on the opposite side.
By nature, a difference in air pressure between two environments tends to balance : the depression on the opposite exterior face will cause air to be sucked in from the atmosphere inside the building. An overpressure 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 the entry of air.
Beyond the thermal effects of the processes, the depressions at the level of the outlets are the engine of the natural ventilation of the building. To this, it must be understood that the depression in the roof is strong enough to convert all the openings in the lower part into an air inlet, even on the facades downwind (in the standard configuration).
The sizing of natural ventilation is based on precise physical concepts.
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