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Thread: Confused about Bernoulli effect

  1. #1 Confused about Bernoulli effect 
    nuk
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    Howdy, all - first post here.
    I'm a HS physics teacher, and I taught fluid mechanics for only the second time this year. It never was my strong suit.

    I thought I more or less knew what I was doing, but when I was having dinner with my parents the other day, they brought up some comment that Romney made about opening the windows on an airplane. So I started talking about how that in addition to reduced air pressure at high altitudes, you'd also have reduced air pressure due to the Bernoulli effect, since air outside the plane is whipping by at 800 kph, which would contribute to sucking more air out of the plane.

    But later, I started wondering. From the point of view of the air, the plane is moving at 800kph. From that point of view, the air in the plane is moving fast, would have lower pressure, and air should get sucked into the plane.

    What am I missing?


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  3. #2  
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    The air right at the surface (boundary layer) is moving at the same speed as the plane. Open a window and air goes out due to pressure difference.


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  4. #3  
    nuk
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    But why out? Why can't you say that the boundary layer is moving with the plane from the atmosphere's point of view, and switch directions for the air flow?
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  5. #4  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by nuk View Post
    But later, I started wondering. From the point of view of the air, the plane is moving at 800kph. From that point of view, the air in the plane is moving fast, would have lower pressure, and air should get sucked into the plane.

    What am I missing?
    Surely, the air outside the plane is moving past the opening (or, equivalently, the opening is moving past the air outside) therefore there will be a reduction in pressure sucking air out.

    The air inside the plane is not moving with respect to the opening and so will not suck air in.
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    This remind me of an invention called "air curtain". Its a device often put at (the door of) shopping mall or supermarket to seal in the cool air by shoting a jet of air (similar to what it would felt like if you have air rushing across your window). I'm not sure what's going on but you can look at this links and video (see [1]).

    Here's how I think it work (jets flowing freely in open air):
    The jet create a low pressure boundary (in which all static air will rush toward the jet), but since both side of the jet also have a static air: both side put an equal pressure into the jet; therefore the low pressure area of the jet cancelled off and the static air will just be blown sideways and recirculate.

    Jets over a small opening (like windows in moving cars):
    I guess the opening itself can be thought of as skiming thru the bulk of the air (instead of thinking that air is moving over across the opening). The static air relative to the opening will have higher chance of escaping since it is motionless relative to the opening, while the bulk of the air (which skim over the opening) will have only a tiny fraction of time to encounter the opening thus lower chance of getting into the opening. -As result: more static air going out of the opening while less moving air going into the opening [2].

    [1]:
    air curtain
    discription: Air Curtain Manufacturer India | Supplier Air Curtain Delhi | Industrial Air Curtain Exporter Noida | Invisible Air Curtain Manufacturer
    Air curtain create barrier for hot smoke: Air-Curtain Fume Hood (Downblowing Jet) Hot Smoke Test - YouTube
    Air thermal profile for the air curtain use in supermarket: HCR Thermal Video (Before and After a HCR Air Curtain is Opened) - YouTube

    [2]:
    using probability and chances to explain behaviour for gasses: http://en.wikipedia.org/wiki/Statistical_mechanics
    Last edited by msafwan; October 23rd, 2012 at 11:41 AM.
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  7. #6  
    nuk
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    Interesting ideas, thanks.
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  8. #7  
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    Bernoulli force dynamic pressure will make the cross window pressure difference even worse by about 1/2*density*v^2. At airplane height and speed (say 200 m/s and 0.3 kg m^-3), it's a dynamic pressure reduction of about 100 mb or so. The static pressure differential is ~700mb between outside and inside. As you can see from the back and envelope calculation, the dynamic pressure is small by comparison.

    Good to have a teacher here--I'm about six months out from a cert myself.
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    Also, well to keep airplanes from staying well breathable they reduce the air pressure to about a third of normal. Which is still much higher then the eirpressure at high altitude. Once the aircraft is de-pressurised there will be hardly any outflow and equilibrium will set in.
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