Schlieren system

The Schlieren technique allows us to see any change in the refractive index of transparent fluids such as air and water. Changes in refractive index are typically caused by a change in temperature or pressure or simply a change of fluid (e.g. two different gases mixing). We use this technique at 91快活林 to study the combustion process inside the cylinder of an engine as it lets us visualise the fuel evaporation, its mixing with the air and the subsequent flame development.

The technique is very simple, yet visually stunning as it lets you see for example the heat rising from a hand. It is based on the principle that light rays deviate slightly when travelling through different refractive indices. Using one or more mirror to focus a point light source back to a point, we can block some of the rays that have bent with a knife-edge causing bright and dark areas in the image corresponding to the different refractive indices.

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Schlieren of the heat from a hand

Using the equipment for an experiment with the BBC

Research Fellow Guillaume de Sercey used the Schlieren technique to capture the sonic boom of a whipcrack for a feature on BBC1's The One Show, broadcast on 8 June 2016. To achieve this feat, the setup used for the BBC was extremely simple yet very effective.

It started with a point light source (a blue LED in this case) that was placed at the focal point of a telescope mirror resulting in a cone of light directed towards the mirror. The mirror reflected that light back to a point. A knife-edge obstructing part of the light was placed at that point and past that point there was a high-speed video camera focussed on the mirror. In the absence of disturbance, the knife edge only effect was to dim the image.

However, any change in density in the air in front of the mirror caused the light rays to bend slightly (the shimmering effect you see on a hot road) and some of them were be blocked by the knife-edge causing dark areas in the image, while others that were previously blocked were visible causing brighter areas.

See to demonstrate what a sonic boom looks like (starts 24.44).

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Sonic boom from a whipcrack