Mountain unicycling and fluid dynamics

Posted by William M. Connolley on March 3, 2008
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At the “lookout” in Bracknell. Bracknell gets a bad press, and the center is indeed horrible, but it has nice paths for walkers and quite a decent bit of woodland. Where I saw my first ever mountain-bike unicycle. I didn’t see anyone riding it, but they did have a lot of pads.

Inside, they had this rather nice fluid-dynamics toy: a perspex cylinder about 1/2m wide and 2m high, with a circulation about the vertical axis imposed by the water flowing in at the top. And a valve you could turn, that did something slightly unspecified, but which we’re fairly sure was to change the speed of outflow from an approx 1″ dia plughole in the center at the bottom. Which lead to the effect you see in the pic, of a very long thin tube of air reaching down to the bottom, much like a tornado. Note the interesting corkscrew patterns imposed on the sidewalls of the tube; those were quite stable, and didn’t turn. A coriolis effect thing; but of course they didn’t explain it.

Here’s a small random girl admiring it. Or is she admiring her reflection?

It takes more than good intentions / And a big bloke on the door / And though it’s never the same after the first time / That doesn’t stop them coming back for more

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Comments

  1. #1 Siamang
    2008/03/03

    Water spiraling down the drain isn’t long-lasting enough for the coriolis effect to work on it.

    See http://www.ems.psu.edu/~fraser/Bad/BadCoriolis.html

    [Of course, not for the one that comes from the Earths rotation. But this is from the bulk rotation of the water column. Unless you have an alternative explanation? -W]

  2. #2 Hank Roberts
    2008/03/04

    >corkscrew patterns

    “Spiral waves form in the water surface of the vortex. These waves appear to move in slow motion as they travel upward through the downward flowing water.”
    http://www.exploratorium.edu/snacks/vortex.html
    (not explained but they invite questions there)

    I know this has been written up but don’t recall where, it’s been years. When I was a kid I went to an ‘Engineering Show’ every year at the local university where they had lightning bolts, and quicksand, and lots and lots of fluid demonstrations; I think you’re seeing random waves on the surface getting wrapped and stretched and interacting making the patterns. But that’s a vague recollection. If you have a good graphics program you might take a flat picture of ripples (shadow tank, photograph the flat white bottom of a water tank) and stretch and rotate the center of the image.

    You get the same patterns in vapor vortexes.

    I do recall, years back, something (Scientific American’s Amateur Scientist?) about how it was possible to drip pigment into a rotating can of paint, producing a long spiral, then reverse the rotation of the can and ‘unwind’ the shape.

  3. #3 Hank Roberts
    2008/03/04

    duh …. spiral wave vortex in Google brings up plenty; Scholar does better; Image Search has lots of pictures. It’s a field of mathematics.

    Well, isn’t everything, really?

    Nice image here:
    http://www.exo.net/~pauld/lectures/patternscostarica/vortex600.jpeg

    From http://www.exo.net/~pauld/ — it’s a wonderful physics lectures/demonstrations/travels page by a good teacher. I didn’t find explanations but it’s a huge site.

  4. #4 stef
    2012/05/12

    hi there

    i need to purchase a cylinder like this one. where did you get this?

    stef

    [Its in Bracknell, UK. I've no idea where they got it from, though -W]