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How to understand flow of air in a car

  1. Sep 5, 2011 #1
    I am in a disagreement with some co-workers about cigarette smoke contaminating the air inside a vehicle.

    With all windows up except for the driver window which is down approximately 1 inch.

    The smoke appears to be directed out the window but the smell of smoke is evident in the back seat.

    I would like to prove the concept that even tho it appears the smoke is going out the window it is still contaminating the entire volume of air inside the vehicle.

    If there is an article written about this subject please kindly point me to it.

    Thank you.
  2. jcsd
  3. Sep 5, 2011 #2
    There's diffusion, where the smoke particles diffuse from an area of higher concentration to that of lower concentration.

    There's the understanding of how heavier air particles move towards the back of the car when the car is accelerating.
  4. Sep 5, 2011 #3
    I don't think diffusion is the answer to my issue. Smoke floats therefore being lighter in it's non-defused state.

    Heavier air particles moving to the back of the vehicle because acceleration forces also implies smoke would be heavier than air.

    Great response, just not what i was looking for.

    Thank you
  5. Sep 5, 2011 #4


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    C'mon, some common sense! That's neither diffusion nor centrifugal separation. Just air in the car circulates and mixes according to some non-obvious pattern, pretty impossible to be modelled theoretically, but maybe car manufacturer tested it experimantally. Not to protect non-smokers, but to design efficient ventilation and heating/air-conditioning.
    The experimental proof that it happens is sufficient anyway: put a blindfolded non-smoker at the back seat and ask her what she smells when the driver is smoking or not.
  6. Sep 6, 2011 #5

    Ranger Mike

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    read post on this forum - Balloons In A Car Jun23-03, 04:53 AM
  7. Sep 6, 2011 #6
    I would think that it is simply a matter of the human nose being able to detect cigarette smoke at concentrations that defy visible detection with the human eye. Unfortunately, I cannot think of a citation to support this opinion.
  8. Sep 6, 2011 #7


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    Staff: Mentor

    I think the air is drawn towards the window since you have incoming air that flows to the back, but it also will spread out slightly in other directions as well thanks to diffusion and such. Since air is coming into the window, then air is also leaving the car somewhere, so I'm sure the smoke also gets drawn out there with the normal air. All in all I don't think this has any benefit, but I could be wrong.
  9. Sep 7, 2011 #8
    With the window cracked only an inch, I doubt that any significant amount of air will be coming in that window. The Bernoulli Effect will draw air (and smoke) out at a pretty good clip. This air will be replaced by air coming from sources other than the window. As you pointed out, there will be a certain amount of circulation within the vehicle. A rear-seat passenger should easily be still able to smell tobacco smoke.

    This same Bernoulli Effect is why convertible soft-tops are sucked outward while the car is moving and not pushed inward. It will also suck heavy sheets of 4x8 plywood right out of the bed of moving pickup trucks unless the load is properly secured.
  10. Sep 7, 2011 #9


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    Staff: Mentor

    So if air is being sucked out by this effect, then rolling down the window does work to clear the air better?
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