Does the City Affect the Ventilation of my Home?

The boundary layer wind tunnel of Göttingen design fills the laboratory in Inffeldgasse in Graz. The air circulates in a closed circuit. At the bottom is the measuring section with the test set-up, which can be viewed through glass panes, and above it is the air flow recirculation system.

More than half of the 8.6-metre-long test track is covered with blue Lego bricks in a regular pattern and at different heights. Behind it are model buildings made of Plexiglas on a black rotating plate that can be orientated at different angles to the flow. “The Lego bricks influence the near-surface flow in a similar way to buildings, vegetation and topography in urban areas,” explains Christoph Irrenfried, researcher at the Institute of Fluid Mechanics and Heat Transfer. The wind conditions in and around Graz, for example, are simulated in this way.

Ventilation tests with tracer gas

Different building development situations are simulated on the turntable – for example, a building with a window on only one side of the façade, as is often the case in a city, and several buildings in the immediate surroundings, which affects ventilation. “We use the tracer gas method, in other words, we introduce CO₂ into the closed model building,” explains Irrenfried. “Then we open the window for exactly five seconds while the wind tunnel is running. From the change in the concentration of CO₂ in the building, we can calculate how much air has been exchanged.” The buildings can then be rotated at different angles to the wind.

An experiment with a model building of this type yielded exciting findings. If the air flows into the window from the front, the air exchange rate is the lowest, as there is a build-up in front of the window and hardly any fresh air flows into the building. If you turn the house by 90 degrees so that the wind blows directly past the window surface, air transport is much better. Ideally, the house should be turned away from the wind by 85 degrees, as the airflow then detaches from the edge of the house and reattaches itself directly to the window, thereby directing air into the room, which then leaves the room together with the stale air.

Tests were carried out with rotations from 0 to 180 degrees – i.e. also in cases where the window is on the side facing away from the wind. The researchers use particle image velocimetry to measure exactly how the air moves in space. To do this, an oil is atomised in the model building and illuminated by laser. High-frequency photographs show the movement of the oil droplets and allow conclusions to be drawn about local flow velocities.

This is just one of many basic studies being carried out at the institute. Other projects have looked at the influence of buildings of different heights and positions on ventilation – or how the length of an airport hall and the number of windows affect ventilation. The surprising finding is that three windows in a long hall lead to a poorer air exchange rate than two windows. “We hope to make a positive contribution to urban development,” says Irrenfried. “This won’t be so easy in an existing city, but it can be taken into account in new buildings. This saves electricity for air conditioning, and it’s great when it happens automatically just by opening a window.”

Scientific challenge and meaningful contribution

Christoph Irrenfried himself is fascinated by the diversity and scientific challenges in his field. “Building aerodynamics is incredibly exciting and there are still many unanswered questions. I like the fact that I am simultaneously contributing something meaningful to society and facing top level scientific challenges.” And he realises that students are becoming enthusiastic: “I’ve been teaching a course on building aerodynamics for several years now, and more and more students are interested in it every year.”