Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

Register Log in
  • Congratulations cowski on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

useless conundrum 1

Status
Not open for further replies.
charland

charland

Mechanical
Apr 12, 2006
26
Hi guys,

This has been rattling around in my brain for some time. It's strictly anecdotal but bothering me just the same.

Why does my wife's cigarette smoke get pulled out of the car window when I'm driving?

Bernoulli's law states that a fluid at a higher velocity will have a lower pressure but the reality is that its the air inside the car that is moving, not the air outside. Also the air is constantly flowing out of the window but my car never completely decompresses.

My best guess is that the car forces the air to move out of the way as we drive by and this very local movement of the air is what causes the decrease in pressure. In addition to this, there has to be some region of the car that is in contact with a slightly higher pressure of air (perhaps at the car's nose) and this air is leaking in to replace what is sucked out.

If anyone is having a slow day, I'd like to hear your comments,

Thanks
 
Replies continue below

Recommended for you

Sort by date Sort by votes
typically cars will let air into the vehicle through the defrost vents on the dash or through the foot vents when not in operation. You can choose the "interior circulation" on most cars today which will close the outside air inlet and prevent air from getting in. But most do have "air locks" that allow the pressure in the car to be somewhat stabilized. These are typically in the doors, they are also used to remove moisture and humidity from the shell of the car to prevent rusting and mold growth.

As for the pressures, I do think that the local displacement caused by the vehicle causes the lower pressure, but its a pressure thats relative to the cab of the car. As the air leaks into the vehicle from outside (typically from the nose) you actually build pressure up and once you crack the window you allow the pressure to begin to bleed off, but it will eventually find a equilibrium and the pressure in teh car will be slightly higher than the outside.

Now here is a good question for you, if you are driving down the road and crack the drivers window, the air begins to get sucked out, but crack the rear passenger window after the drivers and you create a draft through the car going from the drivers window the the rear passenger window.
 
Upvote 0 Downvote
Isnt the fact that the smoke is being drawn out the same as the principle of lift on an aircraft wing. The velocity of air outside is more than in the car (Over the top of the wing), simulating 'lift'.
 
Upvote 0 Downvote


limey1 -echoing. That's also my thougt - and you can also compare this 'cigarette-smoke' effect to the vacuumeffect crated by a jetstream of water passing an opening to an air-filled - sucking the air out.

The effect will at one point have larger effect than the internal/external fan-created ventilation in any car, given that the speed and window opening is large enough.



 
Upvote 0 Downvote
From what I have observed, being a smoker with a non-smoking higlhy complaining wife, there is a vacuumm effect created by the jet stream (as gerhardl correctly pointed out). Whether the air in the car is moving and the air outside can be considered stagnant, is a mute point for this experiment. All relates to relativity. The car is moving for someone standing in the road, and viceversa for someone in the car.
Air does come from the vents, but when the speed is high enough, check the backside of the window. The air is actually coming back in, where in the front is going out (might change a bit depending on geometry of the window, but typically this is what happens). This also explains why the wife still complains about the smell.

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>
 
Upvote 0 Downvote
Bernoulli

E = Elevation head + velocity head + pressure head

If elevation remains the same, an increase in velocity must produce a reduction in pressure. Assuming the air outside the car is moving faster than the air inside, the inside air is at higher pressure and exits the vehicle to the low pressure area outside.

Also a real good reason to make sure your fuel caps are on tight, if your tanks happen to be placed in the wings.


"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain
 
Upvote 0 Downvote
One point that hasn't been made clear above--the velocity in question is the speed of the air relative to the no-flow boundary at the surface of the car body. The air that an airplane flies through is not necessarily moving counter to the plane's direction at some close approximation of the plane's ground speed. The speed of the air relative to the plane's control surfaces is what provides lift. Same thing happens in your car.

At a given speed, the amount of vacuum that can be created in the center of the open window is fixed by the shape of the control surfaces. The vacuum created at the edge of the windows will tend to be less through leakage from outside (which is what keeps an open window from creating an oxygen deficient atmosphere inside the car).

David
 
Upvote 0 Downvote
What gets me is I live in the U.S, but im from England. Periodically, I get to go home, pick up a rental car, and drive with no problem. But sometimes, if I have a passenger, they may get the "fag-end" flicked at them. (Me habitually thinking the open window is to my left). That may be an opportune time to speak of the theories above.
 
Upvote 0 Downvote
The case for cigarette smoke is removal is clear. Why then, if one is unfortunate enough to break wind in a car, does it hang around for so long ?
 
Upvote 0 Downvote
Well, you'd have to put the gass generation closer to the front end of the window. I find that with leather sits, it'll linger less time. I think cloth holds and slow releases it

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>
 
Upvote 0 Downvote
Who says those instances are rare ? This the Piping Forum after all ....
 
Upvote 0 Downvote
Hahahaha, thanks biginch, you just made my day!

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>
 
Upvote 0 Downvote
A potential solution to the energy crisis ?

Let's not go there !
 
Upvote 0 Downvote
If only I could get 4$ a gallon for my gas, I would be thrilled (as would my girlfriend)!
 
Upvote 0 Downvote
Well, there's more than one source. This might work in a bus, though. However, there's also a lot of water associated.

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>
 
Upvote 0 Downvote
Status
Not open for further replies.

Part and Inventory Search

Sponsor

Back
Top Bottom