• Question: You mentioned on your profile that you use lasers to determine how fast a flow of air is travelling. How does that work? (Especially considering it must be particularly difficult to accurately measure the time period of something travelling so fast!)

    Asked by xmali to Mike, Pip, Tianfu, Tim, Tom on 25 Jun 2012.
    • Photo: Tom Lister

      Tom Lister answered on 25 Jun 2012:

      I think they use interference patterns, rather than seeing how long it takes for the light to travel a certain distance. I’ll leave the answer to someone who knows a bit more about it though!

    • Photo: Philippa Bird

      Philippa Bird answered on 25 Jun 2012:

      I don’t know whose profile you meant but I guess this question isn’t for me…

    • Photo: Tim Stephens

      Tim Stephens answered on 25 Jun 2012:

      I think that this is meant for me…

      The way that we do it is by using a camera to take pictures of the flow which has been seeded with some sort of tracer particle. In other words, we put smoke or oil droplets into the flow and then take pictures of those as they move past the camera.

      The laser is used to light up the droplets with a very short flash of light (10 nanoseconds, or 1/100,000,000 of a second) which makes the droplets seem like they’re standing still. The camera takes two pictures with a really short amount of time between them (about 200 nanoseconds, which is 1/5,000,000 of a second), each of which is lit up by a laser flash.

      Once we have the two pictures of the particles, we use special computer software to track them from one picture to another and because we know how far they’ve moved and what the time between the pictures was, we can work out how fast they’re going.

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