Why is the car less windy with two windows open than one?

June 7, 2007 9:15 AM

When I'm driving, why is the wind less strong when two windows are open than when I just have the driver's window down?

I've noticed this in several vehicles. If I roll down the driver's side window, the wind tends to buffet me in the side of the head pretty good. If I open one of the passenger-side windows as well, the wind isn't nearly as strong. I'd expect more openings to equal more wind, so can one of you physics-empowered folks explain why this happens?

posted by L. Fitzgerald Sjoberg to science & nature (8 comments total)

Because the window has a place to blow out of the car.
posted by dobbs at 9:24 AM on June 7, 2007

Assuming you drive at a steady speed in the same direction, the air flow is going to settle into some kind of pattern, which will be different in the two window configurations you mention. The air is flowing in the direction of higher pressure areas to lower pressure areas. Your car is displacing a chunk of air as it moves, which creates an envelope of high pressure air around the car. With one window open, it's an assymetric setup. Probably the flow settles into a circular pattern, from the high pressures flowing around the car's body into the lower pressure inside. It swirls around in the car, and then somewhere in that window opening there has to be a compensating airflow out, which you could locate with a wet hand or a piece of tissue.

With two windows open, you have a symmetric arrangement. Air will now flow into both windows from the high pressure envelope around the car. But since the "pull" of the inside volume of lower-pressure air is the same as when one window is open because the cabin space is the same, the flow will be half from the left window and half from the right (and back out again equally on both sides).
posted by beagle at 9:37 AM on June 7, 2007

somewhere in that window opening there has to be a compensating airflow out, which you could locate with a wet hand or a piece of tissue.

Although we do not recommend that you attempt this maneuver while driving at a high rate of speed.
posted by grateful at 10:59 AM on June 7, 2007

I'm curious about the way that the sound changes. With only the driver's side window open, I get a pain in my left ear that is similar to the kind of pain that I experience if I'm standing too close to the speakers at a concert. I think it might have to do with a low frequency sound of some sort. It doesn't happen when both windows are open.

Maybe, though, the pain is just caused by the wind itself, and not the noise it makes?
posted by solipsophistocracy at 11:16 AM on June 7, 2007

[Some comments deleted. More helping, less faux-etymology, please.]
posted by cortex at 11:36 AM on June 7, 2007

From here:

The bending airstream creates a low pressure just outside the front window. When you crack that window open, you don't change the pattern of air much, but you do allow car air to accelerate toward that window and the low pressure outside it. As air leaves the car, the air pressure inside the car drops. Opening the rear window allows atmospheric pressure air outside that window to flow into the car and replace the missing air molecules. The car's air pressure rises back toward normal atmospheric pressure.

And from here:

This resonant effect is common in cars when one rear passenger window is opened slightly. In that case, air blowing across the opening in the window is easily deflected into or out of the opening and drives the air in the passenger compartment into vigorous vibration. In short, the car is acting like a giant whistle and because of its enormous size, its pitch is too low for you to hear. Instead, you feel the vibration as a sickening pulsation in the air pressure.

posted by Johnny Assay at 11:40 AM on June 7, 2007 [1 favorite has favorites]

I have recently discovered that the best airflow, not too buffeting and comfortably cooling is to have the diagonal windows open. I.e. if I am driving having my window open (front left [USA]) and the back right window open. I suppose that this only applies if I am alone in the car.
posted by iurodivii at 5:11 AM on June 8, 2007

iurodivii - I've done this for years, passengers be damned.
posted by grateful at 4:12 PM on June 8, 2007