Wind Noise in Car

[Plifzig]

When I drive with my sunroof down and all the windows up and slow down to around 45 mph I hear that annoying pulsating sound of the air in my car. Know what I'm talking about?

I was wondering if there is a way to mathematically explain what is happening?

I know it must be a function of wind shear, sunroof opening area normal to the wind, and air space in the car.

Sounds like a MechE/Physicist could figure it out, but I guess you might need to run experiments to obtain a mathematical explanation. Just throwing this out there. Catch!

 

[Plifzig]

Well, even a layman's explanation would suffice. I'm just wondering where the periodic noise comes from. What's the source of the wave?

 

[Cattlegod]

Its called "body boom" in the engineering world. I'm pretty sure it is the resonant frequency of the vehicle body you are hitting. It is caused by the wind coming off of the windshield and changing the pressure above the car at a certain rate ( the resonant frequency of your car which happens to be at 45mph for you ).

 

[Plifzig]

Sounds good. Body boom, eh? You learn something every day. Thanks!

 

[Pulsar]

Well, if you want the layman's explanation it is this:

The air moving over the outside of the sunroof or window causes the air pressure inside the car to be less than outside the car.

A common middle school experiment to demonstrate this is to blow across the top of a straw and watch the water come up the straw.

Now, the problem this causes is that the air is turbulent. The air is sucked out, sucked out, sucked out, until the turbulence causes the air to suddenly flow back INTO the car a bit. It essence, it is a standing wave that involves the size of the opening, the interior volume of the vehicle, and the air (car) speed.

Another experiment that will demonstrate this is flowing air past a suspended cylinder. The cylinder will start oscillating up and down as the turbulence occurs first on one side and then on the other.

Try opening the vent on your car, or if you car doesn't have a vent, crack the other windows somewhat. That will help. You can also turn on your AC a bit.

 

[George Powell]

I find on my car if I just have the sunroof open the buffet is worst at about 35 to 45 mph.

The buffet decreases then as I go faster, and is hardly noticeable at about 80mph and above.