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Supersonic Flow Before and After the Shockwave

  1. Aug 24, 2015 #1
    After a Compression (Oblique) Shockwave during supersonic flight, the air speed (Mach number) is always reduced. I was wondering if an aerodynamic shape could be created to increase the speed of the affected stream to a Mach number greater than the freestream Mach number. I am looking for an Idea that is not an expansion Wave because it will be difficult if not impossible to create it in front of an aircraft before a compression wave.
     
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  3. Aug 24, 2015 #2

    Nidum

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    Four 593's do this spectacularly in their internals but it takes rather a lot of fuel .

    No plausible way of doing same thing over external surfaces of an aircraft . Even if it was theoretically possible there would have to be a very large input of energy into the air flow .
     
  4. Aug 24, 2015 #3

    boneh3ad

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    This is not theoretically possible, as it would require an "expansion shock" to exist, which would violate the second law of thermodynamics.
     
  5. Aug 24, 2015 #4

    Nidum

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    Barnes Wallis proposed a trans orbital aircraft with propulsion provided at higher Mach numbers by burning hydrogen fuel externally to the aircraft in the wake of controlled oblique shock waves .
     
  6. Aug 24, 2015 #5

    Nidum

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    The intakes for a jet engine for supersonic flight generally uses shock waves to decrease velocity and raise pressure to levels suitable for the engine .The engine then adds energy to the air flow and it comes out of the exhaust nozzle at higher speeds than it went in at the front end .

    Whittle made a considerable study of supersonic flow in variable area ducts with energy addition .
     
  7. Aug 24, 2015 #6

    caz

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    Release shocks exist in solids, Fe for example. They happen due to features in the hugoniot. For Fe it is the alpha epsilon phase transition.
     
  8. Aug 24, 2015 #7
    Do you know where I could find Barnes Wallis research or any of the work he did on the subject you mentioned. Google is not giving me anything of substance.
     
  9. Aug 24, 2015 #8

    Nidum

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    I doubt whether there is anything much on Google . I heard about his ideas when I was working on jet engine design . I'll see if I can find any references for you in next couple of days .
     
  10. Aug 24, 2015 #9

    boneh3ad

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    Doing this by combusting fuel on the exterior of the vehicle would be extraordinarily inefficient, though, and would not get rid of the shock. I suppose it depends on the goal.
     
  11. Aug 25, 2015 #10
    My goal is to form a sound theoretical basis to reduce the Shockwave or completely get rid of it so as to justify an experimental research being done. So at the moment I am just trying to explore ways that have not been explored and also reading up on other people's research for inspiration and to see if I can notice something that might help. Do you guys know of any particular area in this study I could pay much attention to or of any ideas that may yield answers?
     
  12. Aug 25, 2015 #11

    boneh3ad

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    This is already an area of active research. You might try a search for sonic boom reduction or mitigation. I know a company called Aeron plans on releasing a supersonic business jet in the next couple years using some of these ideas and NASA actively pursues these ideas. I believe Boeing is still looking into it as well.
     
  13. Aug 25, 2015 #12

    cjl

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    Expansion shocks can exist in certain, rather unusual circumstances. You're right that they aren't really relevant to aircraft flight though.

    (For example, http://rd.springer.com/article/10.1007/BF02511381 )
     
  14. Aug 25, 2015 #13

    boneh3ad

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    Of course atmospheric air is not a van der Waals gas. In a perfect has, expansion shocks are not possible. I am not on my work network at the moment so I can't see the references in that article, but I'd also be skeptical if it's just a computational study done without experimental validation on such an unusual phenomenon.

    I suppose I'll also point out that even if an object was flying at supersonic speed through a van der Waals gas, you'd still get compression shocks since there is no way around the fact that the flow must turn into itself to get out of the way of the body, resulting in compression. There just is no way to move a body through a gas at supersonic speeds without some compressive shock. If there was, we'd have more common supersonic transports by now.
     
  15. Aug 25, 2015 #14

    cjl

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    Yes, of course. I was presenting that as an interesting and unusual exception to the idea that expansion shocks are not possible. You're right that it has no bearing on a perfect gas (or atmospheric air, for that matter), and that even in a Van der Waals gas, you would still have compression shocks ahead of a supersonic object.
     
  16. Aug 25, 2015 #15

    K41

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  17. Aug 25, 2015 #16

    boneh3ad

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    You won't encounter such expansion waves external to a body moving my at supersonic speeds without the presence of a shock as well, particularly for any shape expected to generate lift.
     
  18. Aug 26, 2015 #17

    Nidum

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    (1) I don't think that trying to actually eliminate the shock waves is a realistic quest .
    (2)
    Need to stress that mention of lift . At supersonic speeds the shock waves are an intrinsic part of the wing lift aerodynamics .
     
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