useless conundrum
useless conundrum
(OP)
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
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





RE: useless conundrum
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.
RE: useless conundrum
RE: useless conundrum
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.
RE: useless conundrum
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>>
RE: useless conundrum
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.
http://virtualpipeline.spaces.msn.com
"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain
RE: useless conundrum
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
RE: useless conundrum
RE: useless conundrum
RE: useless conundrum
<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying " Damn that was fun!" - Unknown>>
RE: useless conundrum
http://virtualpipeline.spaces.msn.com
"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain
RE: useless conundrum
RE: useless conundrum
http://virtualpipeline.spaces.msn.com
"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain
RE: useless conundrum
<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying " Damn that was fun!" - Unknown>>
RE: useless conundrum
Let's not go there !
RE: useless conundrum
RE: useless conundrum
http://virtualpipeline.spaces.msn.com
"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain
RE: useless conundrum
<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying " Damn that was fun!" - Unknown>>