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Bow waves and air Ripples

  1. Jan 17, 2007 #1
    What I would like to Know is the number of air compressions created at the front of an aircraft as well as the intensity of these compressions and if each of these ripples are of the same strength or vary. The same info would be greatly appreciated for the bow waves of watercraft.
     
  2. jcsd
  3. Jan 17, 2007 #2

    russ_watters

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    Every discontinuity produces a wave. Bigger discontinuities produce bigger waves. So with a boat, by far the biggest waves are at the bow and stern.
     
  4. Jan 18, 2007 #3

    Clausius2

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    Just to say that a wave is not a discontinuity. Even better, I have heard complaints coming from well known scientists of gasdynamics claiming that a shock wave shouldn't be called wave but front.
     
  5. Jan 18, 2007 #4
    Thanks for creating a discontinuity to the answer. I suggest you reread the question several more times and come up with a continuous answer.
     
  6. Jan 18, 2007 #5
    and by the way discontinuity does not create anything do the math!!!!
     
  7. Jan 18, 2007 #6

    Clausius2

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    Easy dude, take it easy. First of all, your question is naive, vague and not understandable. You don't refer to any fluid regime, mach number or whatever conditions the plane is involved in. What you mean with ripples or number of air compressions?. That question does not have any meaning.

    And secondly say thanks to russ for even trying to answer your question.
     
  8. Jan 18, 2007 #7

    FredGarvin

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    Judging by the way the original question was posed, you don't have the engineering chops to be making that statement. I would suggest reposting your question in a more specific and technically understandable way. What exactly do you mean by "number of air compressions at the front of an aircraft"? How can that be answered when you specify no geometry or speed? I guess I'll answer zero to all of them because both the aircraft and the boat are standing still.
     
    Last edited: Jan 18, 2007
  9. Jan 18, 2007 #8

    russ_watters

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    I said a shock wave occurs at a discontinuity, meaning a discontinuity in the object the fluid is flowing around - not that a shock wave is a discontinuity itself. I realize, though, that that isn't universally true - ie, in the transonic region.
    That sounds reasonable, but that's not the terminology I learned. For the purpose of this thread, it probably doesn't matter. This is the way the terminology was in my gas dynamics class: http://en.wikipedia.org/wiki/Shock_wave

    But anyway, none of this is that big a deal because...
    Yeah. It's pointless for us to argue over what a badly worded question is really asking. Jack, do you want to try this again...?
     
    Last edited: Jan 18, 2007
  10. Jan 19, 2007 #9
    yes,
    I will call it the front, at speed air or water is pushed ahead as expected ahead of the front pushes back, a compression of the air and water occurs. in water it resembles a ripple, in air a shock wave. Is their any way to know how many of compressions are formed at x speed and how intense these compressions might be. And sorry for the abruptness, though I did get some attention!!!
     
  11. Jan 19, 2007 #10
    I am not an engineer, My field is Biology, I need this info for a project I am working on, so be gentle.
     
  12. Jan 19, 2007 #11

    russ_watters

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    How intense is extremely difficult to calculate in all but the simplest situations, how many shock fronts/waves is a property of the object, not the speed.
     
  13. Jan 19, 2007 #12
    I don't dispute your statement because i don't know enough about the subject, but can you clarify why the number of fronts is not a property of the speed? I see why the geometry is a factor, but i would have thought speed to be a factor on first impressions. :confused:
     
  14. Jan 19, 2007 #13

    Clausius2

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    That is absolutely not true. A shock wave is a "discontinuity" caused by the supersonic flow around a body. The usual flow around an aircraft is not supersonic, so that it is not necessary to have a shock wave.

    Yes there are, but it depends on the geometry and the flow regime. And usually it is not doable by hand except for a couple of easy geometries. For even giving you an order of magnitude I would need a rough description of the geometry and the velocity of the flow.
     
  15. Jan 20, 2007 #14
    so then is their any resistance created? My initial quest was to find out if this front of resistance was a single front or multiple. Example: If my object was not aerodynamic and was traveling through the air at a supersonic speed would the resistance encountered at the front be a single front or a series of smaller fronts much like an accordion bellows. That is what I meant when I said "number of ripples". Forget the intensity
     
  16. Jan 20, 2007 #15

    AlephZero

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  17. Jan 20, 2007 #16

    Danger

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    Any time an object moves through a medium, there is resistance. If you want to include inertia, then movement even without a medium involves resistance.
    Jack, Clausius2 is the undisputed master of fluid dynamics on PF. Heed his words.
     
  18. Jan 20, 2007 #17

    Danger

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    That's a bit nostalgic. Even before I started flying, I always heard of bow waves, sonic booms, etc. referred to as 'fronts'. Until now, I thought that it was universal.
     
  19. Jan 21, 2007 #18
    Hello Jack,
    I am going to try and help Clausius help you out. You need to help us out by understanding the things we are describing to you, verifying to us you understand, and then giving us more information about your question with regards to what we have described to you. I would first suggest that we clarify the 3 domains of fluid flow around a body, because the shape of pressure waves on or around a body are different with respect to these 3 domains. The domains are:

    1) Subsonic
    (when Mach number is well below the Critical Mach number where sonic flow first appears. Usually this is at Mach 0.8 and below.)

    Predicting the structure and interactions of "bow waves" in this regieme is not impossible, but not terribly easy as well. As others have said, the structure and interaction of such bow waves is a function of the shape of the body itself. And this is true in ALL THREE of these domains. You can either perform tests in a wind tunnel, or use Computational Fluid Dynamic (CFD) software to predict what the "bow wave" flowfield looks like.

    2) Transonic
    (Generally Mach 0.8 up to and including Mach 1.2)

    This is the most difficult regieme to predict flowfield effects. Things are highly non-linear, and the shape of the body has a great impact on the structure and interaction of the flowfield such that even small changes in the geometry can have drastic effects on the aero/hydrodynamic situation. Typically, you have "sonic bubbles" forming at specific geometric points on the body depending on how the body accelerates the flow even before you have shock waves forming. Drag forces (viscous resistance inherent to the fluid itself) on the body rise in a non-linear fashion in this regieme.

    3) Supersonic
    (Generally from Mach 1.2 up to Mach 2 or 3)

    Of the three regiemes of flow, this regieme is probably the "easiest" to predict. Mostly because the geometries you tend to use in supersonic flows are simple and are more intended to reduce drag than to increase lift. In your question of "bow waves" and how they disturb the air in front of the body, there are no such "bow waves". There are shock waves and expansion waves, but the body is moving so fast in the fluid (by definition, faster than the speed of sound) there is no way for the fluid in front of the body to have any sort of "advance notice" that the body is there.

    Specialists in aerodynamics and fluid dynamics spend a LOT of time trying to quantify fluid flowfield effects around various body shapes in these three domains. So in order to help you we will need you to be more specific about what you are trying to understand. Perhaps by describing the specifics of the problem or scenario you are trying to analyze.

    Rainman
     
  20. Jan 21, 2007 #19

    Clausius2

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    I'm with Rainman on everything except on this:

    That statement is not totally true, and you already know a lot about bow shocks as an aerodynamicist. When the nose of the aircraft is a sufficiently blunt nose we do know there is an eliptic (subsonic) flow around the nose, and the shock gets detached forming a bow. Actually it is easy to estimate the shock strength near the nose, since the bow shock can be approximated to leading order as a planar shock.

    Danger, the way as I see it is that a front means a discontinuity whereas a wave does not mean a discontinuity. An inviscid shock is essentially a discontinuity. Moreover, I think that in french they say shock front instead of shock wave. In spanish we say Onda de Choque, which still is a wave (onda).
     
  21. Jan 21, 2007 #20

    russ_watters

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    Did you look at the picture in the link I provided...?
     
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