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How fast do you have to heat air to create a shock wave?

  1. Oct 24, 2011 #1
    Suppose you have a 1cm diameter heat source suspended 2m above the ground. How many watts of heat energy does it need to expend to create a shock wave?
     
  2. jcsd
  3. Oct 24, 2011 #2

    cmb

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    It'd depend on the function of thermal transfer between the heat source and the air. I suspect this becomes very non-linear if you get to the rate of heat transfer you're talking about.

    I'd also suspect that in the real world, any known material 1cm would not withstand the thermal resistance in that scenario, and, itself, ablate and explode.

    Therefore, as it is a result likely only possible by experimentation, and as there are no materials to perform that experiment, I'd hazard to say the question is moot.
     
  4. Oct 24, 2011 #3
    Ok, well since we're not imaginative enough to conceive of an object or material that can release that amount of heat without exploding then lets just consider explosions:

    Do all explosions create shockwaves?
     
  5. Oct 24, 2011 #4

    cmb

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    Yes. Why would you imagine an explosion not causing a shock wave? Are you thinking of 'super-sonic' shock waves?

    If an explosive causes a supersonic shockwave, it is called a 'high explosive'. If the shockwave is slower than local sound, it generates a wave more like a solition, though I am unclear the difference between a solition and a sub-sonic shock wave.
     
  6. Oct 24, 2011 #5
    I did not know exactly what a shock wave is so I checked here:

    http://en.wikipedia.org/wiki/Shock_waves

    I still don't know, but it's interesting reading.....

    it says: "...A shock wave ....is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium (solid, liquid, gas or plasma) or in some cases in the absence of a material medium, through a field such as the electromagnetic field.

    Shock waves are characterized by an abrupt, nearly discontinuous change in the characteristics of the medium.[1] Across a shock there is always an extremely rapid rise in pressure, temperature and density of the flow..."

    These two statements, for example, appear inconsistent.

    Can we get a shock wave in outer space...in a vacuum...sure, see the latter part of the article......how does that fit (or not) in these descriptions??
     
  7. Oct 24, 2011 #6
    Thanks, thats actually very helpful.
     
  8. Oct 24, 2011 #7
    Alright, I looked at High vs Low explosives, but does anyone know how much energy has to be released how quickly to create that supersonic shock wave? Maybe another way to ask that is:

    What is the weakest (in terms of joules per second) high explosive known, and the strongest (once again in terms of joules per second) low explosive known?
     
    Last edited: Oct 24, 2011
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