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Highway Air Pressures

  1. Oct 4, 2005 #1
    Okay, sorry for that useless title, but I couldn't think up how to have it reflect my question in short.

    I went on a trip to Rome and got there by bus. Along the way, the bus passed a lot of trucks and some of them had some sort of cloth covering their sides (I'm sure you know what I mean). It was normally just pressed against the trucks, more or less. However, when the bus moved in to overtake the trucks, this 'cloth' was sucked outwards. I'm assuming that somehow the air pressure between the vehicles decreases. I was wondering what causes this.
     
  2. jcsd
  3. Oct 4, 2005 #2

    Astronuc

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    When two vehicles are traveling parallel, both are pushing air out of the way. The air flows to side, over the top, or somewhat underneath, but it flows into the path of least resistance. The air then recombines behind the vehicles, and that is where a lot of turbulence will be seen.

    Think about the momentum imparted to the air by each vehicle. The sideways momentum toward the inside of each vehicle cancels more or less, and what is left is the upward or downward flow.

    Since the airflow is diverted up (mostly), the pressure between the vehicles drops. Toward the tops of the vehicles though the pressure is more or less equal or try to equalize.
     
  4. Oct 5, 2005 #3
    But if the pressure between the vehicles decreases, isn't that exactly where the air would want to go?
     
  5. Oct 5, 2005 #4

    Astronuc

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    The pressurized air in front of the vehicles gets an upward momentum which carries it up over the vehicles, then it has to flow under a pressure gradient back to the low pressure regions mostly behind the vehicle and somewhat toward the volume between the vehicles. This is a similar effect to the air flow over a wing of an aircraft.

    You will feel a similar affect if you stand on a platform very close to the edge when a high speed train passes by, or if you stand on the edge of the traffice lane when a fast truck or bus passes by.

    If you have a smoke generator you can see the effects.
     
  6. Oct 5, 2005 #5
    Okay, I asked the wrong question. How do the flows to the inside cancel eachother out?
     
  7. Oct 5, 2005 #6

    Astronuc

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    If one looks at one vehicle traveling alone, one would see the air move sideways, roughly equal in either direction, as well as up over the top. Trucks and buses are generally more square or flat, i.e. the front is essentially vertical with repect to the air.

    Anyway, when two vehicles are side-by-side, the air that would normally move to the side where the other vechicle is now located now encounters the compressed air from the other vehicle. The side flow momentum from one vehicle opposes the other. However, both flows have upward momentum, so the flow moves up.

    The gap between the two vehicles also offers more flow resistance partly from the resistance from the sides of the vehicles, but also because that air is carried along with the vehicles now. The latter is probably the more important factor. The air between the vehicles is now pulled along to it has forward momentum with respect to the still air before the vehicles.

    One also has the much low pressure behind the vehicles which also draws the air from between the vehicles.

    The air pushed out of the way by the vehicles certainly tries to flow back between the vehicles, but not quickly enough - and the vehicles have passed before the air can move back between them.
     
  8. Oct 6, 2005 #7
    Ah so mainly the flows just bounce eachother up, keeping eachother from going inbetween the vehicles.

    And the air that is carried along with the vehicles (how does that happen, by the way?) also bounces back the flows? More or less making the two vehicles one whole for the air they pass through?
     
  9. Oct 6, 2005 #8

    Astronuc

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    Yes, that is essentially the case.

    The air will flow to the point of least resistance. Certainly some air leaks between the vehicles. One would a computational fluid dynamics model to demonstrate, or do an experiment with smoke generators (flow visualization).

    Remember, for the most part, the air is relatively still (unless there is some breeze) and the vehicles are moving. The air is 'displaced' as the vehicle moves through it.

    The air between the vehicles is carried along with the vehicles because the air is in contact with the surface of the vehicles. Friction transfers some momentum from the vehicle to the air. The gap between to adjacent vehicles is long (~10-15 m), high (~4-5 m) and relatively narrow (~1-2 m).
     
  10. Oct 6, 2005 #9
    I get it, thanks.
     
  11. Oct 6, 2005 #10
    To and fro

    Excuse me but as someone who has driven a vehicule, truck, car, and motorcycle, the airflows around heavy trucks generates a Flapping (reciprocating) wind behind the truck due to the differential of Flows rates on either side of the truck, added to that is the simple notion that the Airflow passing towards the rear on the side of the truck creates a slight vacuum effect, as evidenced to anyone when they are the first to open a Window in a Car, the initial airflow there is from the inside of the car, to the outside, as the vacuum pressure abates slightly first. The reason why the 'sidedrapes' pillow, is due to that vacuum effect in a very temporary situation wherein the Air pressures between the two trucks lowers slightly while they pass, by restiction of airflow, usually when the two are proximal enough in their passing to have that immediate an effect upon each others airflows, also a very brief effect.

    Most certainly I have driven a Motorcycle behind Large trucks and have experienced the somewhat violant, or vigorous 'tossing to and fro' attempt that the alternating side-Winds Blasts at you.

    Open the window of just about any car, at speed, slightly, and it will give you a suction effect, not a positive Pressure effect.

    Am I mucking things up?

    LD
    Stretches, **YAAAAAAAAWNS** thinks maybe he should go back to his Sumpteous Warren, to sleep, again.. .. ..soon.. .. ..←{Know what those are?}
     
  12. Oct 8, 2005 #11
    Open the window on just about any fast moving vehicule and you will immediately have a Venturi effect as the outside air is rushing past at a much greater speed, then the inside air.
     
  13. Mar 30, 2010 #12
    what will happen to the flow behind the bus or trunk?
     
  14. Mar 30, 2010 #13

    russ_watters

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    There's an awful lot of words here that don't seem to make a lot of sense, for what is really a straightforward issue: between two vehicles moving in parallel, a venturi effect is created: http://en.wikipedia.org/wiki/Venturi_effect

    The same thing happens to two ships sailing in parallel with each other. For navy ships doing underway replenishment 100 yards apart, it occasionally causes collisions.
    http://www.fas.org/man/dod-101/navy/docs/swos/stu2/STU12.html

    [edit] oops...very old thread. Sorry I missed it 5 years ago!
     
  15. Mar 30, 2010 #14

    russ_watters

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    What do you mean? What happens to it when/what?

    Are you just asking about the wake that all moving objects make? They drag an area of low pressure behind them.
     
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