What's that weird flutter in air when only one window is down in a car?

when there are 2 windows open, the air can move in and out of the 2, but with one it circulates around the car, back out tge same window. It’s called airflow.

And welcome to fluther!

If it happens as you’re passing things (trees, signs, other cars, etc.), it’s the sound waves bouncing off of that object and coming back at you.

The air in the car, moving with the car, and at rest with the car , of course, has an interface with the air outside the car’s ONE , slightly-open window moving at the car’s velocity.

If there is no wind and the car is relatively aerodynamically ‘smooth’ that interface is like a smooth drum face which is not vibrating , i.e., not being ‘struck’ by anything…. and their shall be no [or very little] ‘sound’. If there is some wind , but it is moving in line with the car(towards or away from the moving car)... there also would be minimal ‘sound’ created by ‘vibration’ of that interface-drum-membrane.

If there is some wind moving NOT in line with the motion of the car AND especially if the wind is , as one would expect , slightly varying in its speed, THEN one gets turbulent interference at that ‘membrane’ of air… and ‘vibration’ of that interface and SOUND… varying in pitch with the slight change in wind speed AND change in direction of the car with respect to that wind.

With BOTH windows open…. you have turbulent flow inside the car causing the sound, especially at the edges of the open windows….....

The pulsations begin as a feedback phenomenon: as air passes the leading edge of the open window, vortices (swirls) of air are created. These then hit the trailing side of the window frame creating a sound wave. Part of that sound wave is projected forward to the leading edge again, where it triggers another vortex. (here’s a schematic representation). At certain speeds, this feedback process will settle into a pattern where the vortices are created in ever stronger rythmic pulses.

This phenomenon is the source of the original vibration, but this alone doesn’t account for the strong pulsing you fell in the car. This happens because the interior of the car is a (nearly) closed cavity of air having a fixed volume. This cavity of air, like the air inside a pop bottle, has a particular resonant frequency. The bigger the cavity, the lower the resonant frequency. If you supply a source of vibration that matches this resonant frequency, then you’ll get that whole volume of air pulsing to that beat.

In the case of the car, at some speed the vibration set up by the vortex feedback mechanism will match the resonant frequency of the air inside the car and that whole volume of air will start pulsing right along. Opening another window creates a situation where you no longer have a fixed volume of air in the car, because air can move freely in an out that other window. So the inside of the car no longer has a resonant frequency. The open windows are still creating rythmic vibrations, but they won’t set the air in the car pulsing.

I think Harp did a pretty good job of explaining it. Although it is really vortex shedding that causes the rhythmic pressure changes. The shear forces in the air close to the open window cause alternating vortices to emerge. You can tell this is the case because vortex shedding resonates at the objects natural frequency and when one window is open the car almost always makes the sound (provided it is traveling fast enough to create vortices). However, a car with both windows open does indeed have a resonant frequency. The difference is that it is harder to get a standing wave to propagate in the interior of a car because of chaotic flows within the cabin and the fact that you have two windows vying to hit the natural frequency at the same time. Most of the time the vortices will not be in sync and will not cause a resonant frequency to be excited, or simply cancel each other out entirely. A car with one window open will have a resonant frequency twice as high as a car with one window open as well. Higher frequency states require more energy to excite which also helps to explain why both windows open often does not cause the harmonic frequency to be excited.

For anybody who wants to read about this effect this article covers it well. Harp and shuttle explained well though! I just want to make clear that the resonant frequency isnt required for you to hear the pulsing sound though, you would definitely notice it at the res freq because of the increased volume. Just the (cyclical) pressure variations caused by the vortices travelling past the window (and effects of any feedback mechanism) would allow you to hear it. (sound being caused by and in fact being pressure variations in the air.