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Banging doors

Discussion in 'General Discussion' started by Handyman, 17 Jul 2004.

  1. Handyman

    Handyman

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    Why is it that if a door is open, it blows shut, and if it's shut it blows open? This process will go on until the door is restrained either open or closed. I know that wind is effectively air movement between areas of differing pressures, but why do these pressure areas reverse every minute or two and cause my back door to crash backwards and forwards if I leave it unlatched?. Bloody door hit me thre times the other day when I was working on the frame.
     
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  3. kendor

    kendor

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    Probably when the wind blows past at different rates of speed and direction it blows the door shut and when the speed picks up it follows the shape of the wall causing a vacuum to form in front of the door and the pressure inside the house closes it again.

    Talking of wind ( no, not the smelly stuff ), when they measure wind speed is this measured in open space? as that may bear no relation to the speed around buildings, skyscrapers where the speed could be many Magnitudes higher!
     
  4. Handyman

    Handyman

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    I understand your reasoning, Kendor, but if you watch an unlatched door, it will blow shut and bang a few times, and then blow open and bang against its hinges a few times. If the wind wants to blow it shut, why does it have to drive the point home by banging the door several times? Either it wants to blow it shut or blow it open. It's not right that it should prevaricate about its intentions, and then throw a tantrum of banging if it gets ignored.
     
  5. oilman

    oilman

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    Wind, by definition is moving air. It would be rare to have still air, and it would be rare to have constant pressure. So it's going to move. If you have your door positioned mid way between shut and fully open, the wind could shift it in either direction. So the fact that it blows open if it's shut, and shut if it's open is only that it can react to the pressure change present at the time. If it's on its way to shutting and the pressure isn't high enough, it won't shut. When the pressure gets the door moving fast enough to shut then it can't open again, but the pressure is still changing.

    Now if you are a handyman, why do you let the door bash you around , and why don't you put a wedge under it to keep it still?
     
  6. masona

    masona

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    The door will only shut in a greater force of wind and won't in light wind because of a vacuum create from the edge of the door and frame.
    Have you considered a door closer which are adjustable for your need ?
     
  7. masona

    masona

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    Ahhh, Oilman beat me to it !
     
  8. AdamW

    AdamW

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    Perhaps a much more dense door is in order?

    I am pretty sure bank vault doors don't blow in the wind, and are quite effective against burglarious attempts.

    TO explain the bouncing in the direction where the door opens, assuming that the house has infinite mass:

    1) the door is blown/sucked by the wind towards it's opening direction
    2) the door hits the wall or the "end-stop" of the hinges.
    3) the door still has considerable momentum
    4) this momentum is transferred to the wall (or in the case of the hinge, stored as elastic potential energy)
    5) due to a degree of elasticity between the wall and door (even though it isn't a rubber door!), or the energy stored in the hinge, momentum is transferred back to the door in the direction from whence it came.
    6) because there isn't a 100% transfer of momentum, the door swings back with less force than it swung open
    7) the wind catchs it again, blows it open.
    8) because the wind applies a force to the door over a shorter distance now, the door absorbs less momentum from the wind than before
    9) the door hits the end stops with less momentum than before, so bounces back even less
    10) when the momentum required for the door to swing back against the wind becomes less than the momentum applied back to the door by the hinge or wall, the door remains open.

    I hope that makes sense. The door blowing open probably has it's "bounce" in the internal air pressure of the house and the hinges not being perfectly level (as nothing ever is)
     
  9. securespark

    securespark

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    It's very easy to explain why this happens. The controller of wind is a cat, and cats change their blo*dy minds every nanosecond about which side of the ef*ing door they want to be on.........so wind is the same.
     
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  11. breezer

    breezer

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    wind has been known to open doors in a "shut building" setting the burglar alarm off.

    It also sets off PIR detectors becuse of the venturi effect.

    a small amount of wind goes through a small gap, (constriction) to get the same volume of air through the gap the wind must increase speed, this high speed wind if adjacent to a PIR will set it off. but if the window had been left open it wouldnt have gone off
     
  12. AdamW

    AdamW

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    What, does it make the PIR move about in the breeze?

    I know of an office development where the designers hadn't taken into account the venturi effect. There were two buildings running parallel to each other with tall, flat sides and were separated by a narrow street. The wind blew directly down this street and they actually had cases of glazing units being sucked out of the windows!!!

    I wouldn't want to be underwriting the new building warranty on THAT development!
     
  13. breezer

    breezer

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    no, when it stops blowing the temerature suddnely rises, so it thiks some one is present
    why do you think Chicago is called the windy city? it has the same problems
     
  14. Handyman

    Handyman

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    Securespark has identified the answer. It definitely is controlled by cats. And small yappy dogs.

    It's not that I want to prop the door open, simply adjust the hinges, clean the rebate, and fix a draught excluder. Now I've done it, the door stays where I leave it. Probably becuase the cats have stopped the wind.
     
  15. mildmanneredjanitor

    mildmanneredjanitor

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    On a similar vein, why is it that my children (and our best friends!) are incapable of closing the front door behind them???

    Shouting 'Do you live in a barn?' over the years hasn't helped.

    Is there an scientific explanation to this? :rolleyes:
     
  16. securespark

    securespark

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    You are either born a door-closer or a person whose thought processes do not cover "should I shut the door I have just opened?"......
     
  17. TexMex

    TexMex

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    The thing is air is compressable and acts like a spring therefore. If there is higher pressure outside the house, the air will blow in through the door. This does not, however stop once the pressure is equallised as the air has momentum and will continue to flow past the point of equilibrium.

    Finally once the airflow finally stops, there is a higher pressure inside the house than outside. So the whole thing starts again in reverse. ie Air rushes out, pressure drops, goes past equilibrium. stops Low pressure inside etc..

    If you time the interval between the door slams you should be able to calculate the resonant frequency of the air in your home. That's why the door on the outside Karsi will bash backwards and forwards faster than the back door of the house.

    Try shutting the kitchen door and you will find that the oscillations, although smaller in amplitude (less air involved) will be seperated by a shorter interval.
     
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