Originally posted by TWILITE22
I just read one of the other posters and I stand corrected,when I roll down just one back window in my car it sounds like helicopters,(really
annoying)so the air pressure is quite possible,does this happen to anyone else and why does this happen?
Yes that's exactly what I referred to
in my earlier post though I didn't mention helicopters, that's not a bad description of the effect. I'm sure it's got something to do with fluid
dynamics, standing waves, and interference patterns, though I never tried to model it precisely. But what I suspect is happening is when air comes in
the windows from both sides, there's some turbulence and maybe a wave-like pattern to the turbulence and where the two different wave sources meet,
you get an interference pattern, sort of like in the double slit experiment with light:
In that diagram the equivalent would be air coming in each side of the car causing turbulent waveforms that collide and interfere something like that,
except the interference isn't stable, so you hear the exaggerated peaks and valleys of the interference wave pattern as it moves around, giving the
effect of the helicopter blades.
I don't hear it as badly with just one side window open, but I can see how it could still happen, perhaps when it hits the back window there may be
some kind of reflection of air bouncing off the back window and you might get an interference pattern that way.
I'm doing a fair amount of educated speculation here but I've determined this pattern is quite repeatable and I haven't been able to think of a better
explanation which fits the data. There's a lot more speculation at this link:
townhall.edmunds.com...
What they say there is NOT consistent with my experiments, since I can make the vibrations go away by only opening the windows on one side. The
"helicopter" effect is most pronounced for me with windows open on both sides which makes me think interference.
edit on 15-7-2011 by
Arbitrageur because: clarification