Prairie dogs are rodents who live in the soil of North American plains. The climate in which they live can be quite harsh, with temperatures ranging from 38°C in the summer to -37°C in winter. To survive they live in burrows, a sort of tunnel dwelling 2-3m below the Earth’s surface. These tunnels can reach up to 10m in length however, and without some sort of ventilation system these cute little creatures would most likely die in their sleep from asphyxiation. So how do they solve the problem of fresh air?
What they do is take advantage of the physical law described in Bernoulli’s principle, the same law that enables planes to fly, hair sprays to work and wind towers and solar chimneys to cool and ventilate people’s houses.
It´s about achieving a change in air pressure to create a movement of air in a desired direction. When the prairie dogs construct their burrows, or “towns” as they are also called, they design the entrances in different ways.
If I understood it right the mound shape of the entrance to the left creates an increase in air movement/velocity. This in turn decreases the air pressure in the left part of the tunnel. Which in turn draws air from the entrance to the right towards the entrance to the left and allows for air circulation. High pressure air always moves towards low pressure air, this is true for climate as well as for ventilation systems or spray cans.
And actually all air movement preconditions a difference in pressure between the affected areas, either in the form of wind on an atmospheric scale or in the form of stack effects created by temperature differences. When it comes to architecture, it seems creating a naturally ventilated building requires us to also understand how pressure is created and how it can be used. Architectural space has to be composed as a flowing, interconnected space as opposed to a strongly subdivided space, and air pressure has to be controlled by means of directing natural wind or manipulating air temperature.
Hassan Fathy, New Gourna
Cold air is high pressured and warm air is low pressured. This was not known before the 18th century but long before that houses were still designed following that principle. For example, Palladio’s Villa da Schio was erected in the mid 16th century and utilizes a cave to cool the interior spaces.
The low pressured cool air is drawn through the villa towards the high pressured warm air on the outside. The thermal mass of the mountain constantly cools the air in the cave and as long as the outside air is warmer the system will be active. Also, the greater the temperature difference between the cave and the outside, the greater the flow of air will be.
Building shape and size is fundamental to these principles. Different shapes will handle air movement differently and thereby create different pressure zones. This is however pretty difficult to calculate with accuracy, as air outside laboratories move in somewhat unpredictable and complex ways. Also the height of a building is important when it comes to pressure build-up from stack effect. The taller the building the stronger the effect.
Anyone who is interested in pressure and air movement could look at some of the links below. Some deal with aerodynamics, others with pressure and convection. The issuu publication is really good.
Reference and image source “figure 5.7” and “figure 5.8”:
Pressure related videos: