Engineering at the University of Brighton

Cracking the whip on TV

A scientist from our division is due to appear on BBC’s The One Show to show what a sonic boom from a whip crack looks like.

Dr Guillaume de Sercey, Research Fellow with the university’s Advanced Engineering Centre, will be using the same imaging techniques – Schlieren imaging – that he and his team use in their research into the combustion process occurring inside the cylinder of an engine.

He said: “Schlieren imaging lets us visualise the fuel evaporation, its mixing with the air and the subsequent flame.”

Guillaume used the technique to help the programme’s science presenter Marty Jopson ‘see’ the shockwave that emanates from the tip of a whip when it is cracked and the item is scheduled to be aired at 7pm on Tuesday, 31 May.

He said: “The Schlieren technique allows us to see any change in the refractive index in 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).

“The setup I’ve used for the BBC is extremely simple, yet works wonderfully. It start with a point light source (a blue LED in this case) that is placed at the focal point of a telescope mirror. You therefore have a cone of light directed towards the mirror. The mirror reflects that light back to a point.

“At that point, we place a knife edge that obstruct part of the light. Past that point we place a camera or a screen. In the absence of disturbance, the knife edge only effect is to dim the image. However, any change in density in the air in front of the mirror will cause the light rays to bend slightly (the shimmering effect you see on a hot road) and some of them will be blocked by the knife edge causing dark areas in the image, while others that were previously blocked will be visible causing brighter areas.”

Stephanie Thomson • May 27, 2016

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