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Vortices, Resonators, and Foam sleeving

  1. Nov 26, 2013 #1
    Hi,

    I recently became acquainted with foam sleeving as a material for forming a resonant cavity via this link: http://www.phon.ucl.ac.uk/home/mark/vowels/ . I'd actually read about it before used in an infrasound detector for detecting approaching tornado. Researchers used something like this to amplify the infrasound.

    I have also read of these things: http://en.wikipedia.org/wiki/Air_vortex_cannon .

    My question is, 1) will these air vortices propagate down foam tubing?

    2) What class of solutions of the vorticity equation can be used to model these types of vorticity effects (eg. long-distance-propagating vorticity solutions, self-"contained" solutions where the end points don't require physical boundary conditions to be set up.)

    Is it possible to do other experiments with vortices (perhaps create an unusual vortex) using some combination of resonators, maybe some of this foam tubing, and some ingenuity?

    3) Can information signals be sent with this kind of air vortex cannon tubing by sending rapid vortices in sucession?


    As far as I can tell, foam tubing is a perfect device for conducting sound perhaps due to sound's high speed in it due to its low density (and low attenuation), also the foam may function to dampen oscillations inside of its walls and strengthen oscillations within its walls due to its flexibility.

    Please tell me more!

    I was looking at a tornado video and you can hear a broadband resonance in it similar to a duck call, here: https://www.youtube.com/watch?v=HFrgSVoJi1U

    That's what started getting me interested in this broadband resonance phenomenon.

    Sorry if I'm hard to follow due to my intermingling of related concepts, acoustical in nature.

    DoYouKnow (Michael)
     
  2. jcsd
  3. Nov 27, 2013 #2

    Bobbywhy

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    Gold Member

    DoYouKnow (Michael), welcome to Physics Forums.

    An air vortex (or vortex ring) travels quite well through open air, as you’ve seen in the videos. If you’ve not yet, you can read about them here at Wikipedia: “A vortex ring usually tends to move in a direction that is perpendicular to the plane of the ring and such that the inner edge of the ring moves faster forward than the outer edge.”
    http://en.wikipedia.org/wiki/Vortex_ring

    Here was an attempt to design a weapon to send irritant chemicals like pepper spray carried by the vortex rings for crowd control:
    Battelle Vortex Ring Gun
    http://www.battelle.org/media/press-releases/battelle-develops-vortex-ring-gun-for-firefighters-pesticide-delivery [Broken]
    and for a video: http://www.sciencespacerobots.com/battelles-vortex-gun-shoots-rings-of-gas-42720128

    A high power military weapon: “The vortex ring gun is an experimental non-lethal weapon for crowd control that uses high-energy vortex rings of gas to knock down people or spray them with marking ink or other chemicals. The 40mm grenade machine gun was selected because firing at 4-10 shots per second resonates with many body parts and causes a stronger impact effect.”
    http://en.wikipedia.org/wiki/Vortex_ring_gun

    You ask,

    1) “will these air vortices propagate down foam tubing?”

    A vortex ring probably will not travel very well inside a tube because its outer regions of axial flow would interact with the inside walls of the tube because of the viscous friction between its flow and the foam. Therefore, it would become perturbed and its internal energy would dissipate. Why would you want to send them through tubes, since they seem to travel so well in free air?

    2) “What class of solutions of the vorticity equation...?”

    I always recommend going to the “main man” who first figured out these structures; his name is Herman Von Helmholtz and here is his landmark paper. Of course, there are lots of other, newer and probably clearer descriptions you could also learn from. Google is your friend to help find them:
    “On Integrals of the Hydrodynamic Equations That Correspond to Vortex Motions” by Hermann Von Helmholtz
    http://www.21stcenturysciencetech.com/Articles_2009/Helmholtz.pdf

    Here is a doctoral thesis Analytical Vortex Solutions to the Navier-Stokes Equation:
    Thesis for the degree of Doctor of Philosophy, Växjö University, Sweden 2007.
    Coherent structures in a pipe flow is modeled. These vortex structures in the pipe are of interest since they appear for Re in the range where transition to turbulence
    is expected and considers the motion in a viscous vortex ring. The model, with diffusive properties, describes the experimentally measured velocity field as well as the turbulent energy spectrum:
    http://www.diva-portal.org/smash/get/diva2:205082/FULLTEXT01.pdf

    The vortex rings of interest here are exact solutions of the Euler equations of inviscid, incompressible flow. The flow is axisymmetric and independent of time except for translation along the axis:
    http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.50.6896&rep=rep1&type=pdf

    2a) Is it possible to do other experiments with vortices (perhaps create an unusual vortex) using some combination of resonators, maybe some of this foam tubing, and some ingenuity?

    My suggestion is to try to create Hill’s Spherical Vortex using your impulse generator and foam shapes. That would be great fun, and novel.

    3) Can information signals be sent with this kind of air vortex cannon tubing by sending rapid vortices in succession?

    See the “Vortex ring gun” Wiki site above for “rapid fire” of 4-10 rings per second. Will you please elaborate on your question? What kind of information would you want to send?

    Good experimenting!
    Bobbywhy
     
    Last edited by a moderator: May 6, 2017
  4. Nov 27, 2013 #3

    AlephZero

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    No. The sound in those resonators is all conducted through the air, not the foam.

    For that type of demo, it doesn't really matter what material you use to make the resonators, so long as they have the right internal shape. Actually foam is probably a poor material from an acoustic point of view because it absorbs a lot of the sound energy, but its big advantages are that it is cheap, and easy to cut to any shape you want.

    A resonator of the same shape made from wood or metal would work better, but it would be much harder to make than cutting a piece of foam.

    As Bobbywhy said, vortex rings in air don't need "guiding", and they persist for quite a long time before the energy is dissipated by the viscosity of the air. The vortices shed from the wing tips of a large plane like a 747 can persist for many kilometers behind the plane and still be strong enough to cause big problems if a light aircraft flies into one by accident.
     
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