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Question about sonic boom...

  1. Jan 11, 2016 #1
    if a airfoil profile has velocity less than 340.29m/s (sea level) but close to this #, the air velocity above the airfoil compresses some and thus has a greater velocity than the airfoil itself (thus an upward force on the airfoil due to pressure diff). if the air above the airfoil has velocity of 350m/s does that air itself make a sonic crack? is this phase the transonic phase where you have super sonic and sub sonic things happening in tandem (one causing the other)?
     
    Last edited: Jan 11, 2016
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  3. Jan 11, 2016 #2

    Simon Bridge

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    I think the short answer is "no" ... the "crack" occurs as the pressure wave from the front of the aircraft passes over the observer.
    You appear to be using a common but incorrect idea of how airfoils make lift.
    https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html
     
  4. Jan 11, 2016 #3
    ok, airfoil lift is more complex than i suggest, but the P diff is due to diff in airflow velocities between upper & lower surfaces. the site you reference talks about turning of the flow.

    what about a bullet which has a symmetric bullnose profile? if the bullet has V=330m/s (sea level) does the airflow over the bullet speed up as it traverses the bullet front to back?

    and then i read about transonic in context of aircraft props where tips get into that 340+m/s zone while the other parts of the prop are less than 340m/s, and this creates stability issues in the prop itself. another article references transonic in context of a bullet as some parts are 340+m/s while other parts of the bullet are not. for this bullet example i did not understand, but the prop example is easy because every piece of the prop is at same rpm's, but the bigger radius means more distance in same time ("rpm"), thus larger the radius larger the V.
     
    Last edited: Jan 11, 2016
  5. Jan 11, 2016 #4

    russ_watters

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    No, this discussion has very little to do with any particular misunderstanding of how airfoils make lift.
     
    Last edited: Jan 11, 2016
  6. Jan 11, 2016 #5

    russ_watters

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    You have it more or less correct. The shock wave occurs where velocity transitions between supersonic and subsonic. As the plane approaches Mach 1, the airflow over the top (and often bottom) surface of the wing is accelerated to Mach 1 first near the top of the wing (the thickest part). At this point, a shock wave forms. As the plane continues to accelerate, the area of supersonic flow moves forward until it reaches the leading edge when the plane reaches Mach 1. See:

    181CFFig7.gif

    And: [sorry if big...]

    shock-wave.jpg
     
  7. Jan 11, 2016 #6

    boneh3ad

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    @russ_watters gave a good explanation of the transonic region. To answer the original question, I would be willing to bet that the shocks there would be unlikely to produce too much of a boom on account of the very low Mach number and the relatively small amount of air involved when only a small part of the flow is supersonic. Any sound from that is likely to dissipate and be too small to notice over the engines anyway.
     
  8. Jan 12, 2016 #7
    ah, makes sense.
    so for a pointy bullet that is 0.9mach, w/o digging into the math, the airflow around the nose is partially supersonic, but so small it doesnt produce sonic noise.
     
  9. Jan 12, 2016 #8

    boneh3ad

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    It would be hard to say with any certainty, but if you had a transonic bullet where it was moving at something like M = 0.9 and had some region that was locally supersonic, I would think it would be a very minute noise. Even a supersonic bullet is just a crack kind of sound, so as you make the Mach number ever closer to 1 and decreases the supersonic region's size, it will tend to get more quiet. I'd say test it to be sure but that would be difficult and (obviously) dangerous to test.
     
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