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Why does wind enter a car window?

  1. May 15, 2015 #1
    When you are driving and open the window, wind blows into it, but what causes the perpendicular direction of the air's motion to your direction of travel into the car? For instance, a bus. A bus is basically a rectangular prism with parallel window sets on either side, completely parallel to the other side and the direction of motion. Why doesn't the wind just go straight past the window, and what causes it to curve into the window's opening?
     
  2. jcsd
  3. May 15, 2015 #2
    The wind and the bus are very dynamic. Neither are moving in a perfectly straight line. Even if they were, some of the wind will hit the corner of the bus window and be directed into the bus.
     
  4. May 15, 2015 #3

    A.T.

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    In general it's the viscosity or air:

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

    You cannot have still air in the car, and then suddenly very fast air just outside. Also note that the air also exits the car at some parts of the window. What exactly will happen in a certain scenario depends on the shape of the car and window.
     
  5. May 20, 2015 #4
    air moves to differential pressure.. the pressure in the car is different than the air going over the car.. especially near the roof where the pressure is very low . this is why things usually get sucked out the window.. however, sometimes the air along side the car is going slower, and creates a higher pressure zone and that becomes higher than inside the car, then the air tends to seemingly enter the car. but it usually has to have an exit to really go in the car. It will usually just be turbulence around the window or opening and it seems as if its entering the car, but its really not. open another window, and you can get some cross flow if that window opening has lower pressure around it.
     
  6. May 20, 2015 #5

    DaveC426913

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    The buses blunt nose has rapidly displaced the air in its path - both compressing it and making it highly turbulent. An open window allow the air a chance to try to equalize the pressure.
     
  7. May 20, 2015 #6

    russ_watters

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    A lot is due to the turbulence described in the posts above, but even if the flow were completely uniform, air would still flow in the front windows and out the back windows due to wake/venturi effects. The air flowing around the car creates a low pressure zone behind the windshield that pulls the airflow into the car. The back windshield blocks that flow and pushes it out the back out.
     
  8. May 20, 2015 #7
    the air compresses out front, but the turbulences is in areas of stagnation, higher pressure, the sped up areas are not as turbulent, but lower pressure.
     
  9. May 20, 2015 #8
    There are high and low pressures all over the car... middle of the hood low pressure, while base of the windshield is high pressure, top of roof is very low and rear window has a mix depending on the position. crack your sunroof or side window and hold a dollar bill on the crack.. it will get sucked out . :)
     
  10. May 20, 2015 #9

    russ_watters

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    Today on the way to work I was driving behind a pickup-truck that had an empy plastic grocery bag in and fluttering around the cargo bed. The eddy/dead spot behind the cab confined it for several minutes.
     
  11. May 20, 2015 #10

    boneh3ad

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    You should look up the Kelvin-Helmholtz instability and other shear layer instabilities. Basically, the fast stream is rubbing up against the slow stream in the car, creating a region of high shear that tends to form vortices and draw air in and out of the window. It has the added effect of dragging some of the air in the car along with it at setting up circulation cells inside the car.

    Further, if you have the window open just right without sufficient ventilation available from other windows, the frequency of the vortices going around the downstream side of the window can lead to the beating phenomenon you sometimes observe. It's the same way a flute or blowing over a beer bottle works.

    The air on the front face of a bus is very unlikely to be turbulent. It is in a region of favorable pressure gradient and not generally moving very fast along the surface and is likely to be laminar over a great portion. Of course obstructions like windshield wipers may create local turbulent regions.
     
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