Why does wind enter a car window?

• loobylooby
In summary, when driving and opening a car window, the wind rushes into the car due to the difference in pressure between the outside and inside of the car. This is caused by the car's shape and the movement of air around it, creating areas of high and low pressure. Additionally, turbulence and shear layer instabilities play a role in directing the air into and out of the window. This phenomenon can also be observed with objects, such as plastic bags, getting caught in the eddy and dead spots behind a vehicle.
loobylooby
When you are driving and open the window, wind blows into it, but what causes the perpendicular direction of the air's motion to your direction of travel into the car? For instance, a bus. A bus is basically a rectangular prism with parallel window sets on either side, completely parallel to the other side and the direction of motion. Why doesn't the wind just go straight past the window, and what causes it to curve into the window's opening?

loobylooby said:
Why doesn't the wind just go straight past the window, and what causes it to curve into the window's opening?
The wind and the bus are very dynamic. Neither are moving in a perfectly straight line. Even if they were, some of the wind will hit the corner of the bus window and be directed into the bus.

loobylooby said:
When you are driving and open the window, wind blows into it, but what causes the perpendicular direction of the air's motion to your direction of travel into the car? For instance, a bus. A bus is basically a rectangular prism with parallel window sets on either side, completely parallel to the other side and the direction of motion. Why doesn't the wind just go straight past the window, and what causes it to curve into the window's opening?

In general it's the viscosity or air:

http://en.wikipedia.org/wiki/Viscosity

You cannot have still air in the car, and then suddenly very fast air just outside. Also note that the air also exits the car at some parts of the window. What exactly will happen in a certain scenario depends on the shape of the car and window.

loobylooby said:
When you are driving and open the window, wind blows into it, but what causes the perpendicular direction of the air's motion to your direction of travel into the car? For instance, a bus. A bus is basically a rectangular prism with parallel window sets on either side, completely parallel to the other side and the direction of motion. Why doesn't the wind just go straight past the window, and what causes it to curve into the window's opening?
air moves to differential pressure.. the pressure in the car is different than the air going over the car.. especially near the roof where the pressure is very low . this is why things usually get sucked out the window.. however, sometimes the air along side the car is going slower, and creates a higher pressure zone and that becomes higher than inside the car, then the air tends to seemingly enter the car. but it usually has to have an exit to really go in the car. It will usually just be turbulence around the window or opening and it seems as if its entering the car, but its really not. open another window, and you can get some cross flow if that window opening has lower pressure around it.

loobylooby said:
A bus is basically a rectangular prism with parallel window sets on either side, completely parallel to the other side and the direction of motion. Why doesn't the wind just go straight past the window, and what causes it to curve into the window's opening?
The buses blunt nose has rapidly displaced the air in its path - both compressing it and making it highly turbulent. An open window allow the air a chance to try to equalize the pressure.

A lot is due to the turbulence described in the posts above, but even if the flow were completely uniform, air would still flow in the front windows and out the back windows due to wake/venturi effects. The air flowing around the car creates a low pressure zone behind the windshield that pulls the airflow into the car. The back windshield blocks that flow and pushes it out the back out.

DaveC426913 said:
The buses blunt nose has rapidly displaced the air in its path - both compressing it and making it highly turbulent. An open window allow the air a chance to try to equalize the pressure.
the air compresses out front, but the turbulences is in areas of stagnation, higher pressure, the sped up areas are not as turbulent, but lower pressure.

russ_watters said:
A lot is due to the turbulence described in the posts above, but even if the flow were completely uniform, air would still flow in the front windows and out the back windows due to wake/venturi effects. The air flowing around the car creates a low pressure zone behind the windshield that pulls the airflow into the car. The back windshield blocks that flow and pushes it out the back out.
There are high and low pressures all over the car... middle of the hood low pressure, while base of the windshield is high pressure, top of roof is very low and rear window has a mix depending on the position. crack your sunroof or side window and hold a dollar bill on the crack.. it will get sucked out . :)

russ_watters
zanick said:
There are high and low pressures all over the car... middle of the hood low pressure, while base of the windshield is high pressure, top of roof is very low and rear window has a mix depending on the position. crack your sunroof or side window and hold a dollar bill on the crack.. it will get sucked out . :)
Today on the way to work I was driving behind a pickup-truck that had an empy plastic grocery bag in and fluttering around the cargo bed. The eddy/dead spot behind the cab confined it for several minutes.

You should look up the Kelvin-Helmholtz instability and other shear layer instabilities. Basically, the fast stream is rubbing up against the slow stream in the car, creating a region of high shear that tends to form vortices and draw air in and out of the window. It has the added effect of dragging some of the air in the car along with it at setting up circulation cells inside the car.

Further, if you have the window open just right without sufficient ventilation available from other windows, the frequency of the vortices going around the downstream side of the window can lead to the beating phenomenon you sometimes observe. It's the same way a flute or blowing over a beer bottle works.

DaveC426913 said:
The buses blunt nose has rapidly displaced the air in its path - both compressing it and making it highly turbulent. An open window allow the air a chance to try to equalize the pressure.

The air on the front face of a bus is very unlikely to be turbulent. It is in a region of favorable pressure gradient and not generally moving very fast along the surface and is likely to be laminar over a great portion. Of course obstructions like windshield wipers may create local turbulent regions.

1. Why does wind enter a car window?

Wind enters a car window due to the difference in air pressure inside and outside of the car. When the car is moving, the air outside is moving faster than the air inside, creating a lower pressure inside the car. This lower pressure causes air to rush into the car through any open windows, including the car window.

2. Is it dangerous for wind to enter a car window?

No, it is not dangerous for wind to enter a car window. In fact, some people enjoy the feeling of wind blowing through their hair while driving. However, it is important to make sure that any loose items in the car, such as papers or hats, are secured to prevent them from flying out of the window.

3. Can wind entering a car window affect fuel efficiency?

Yes, wind entering a car window can affect fuel efficiency. As the wind enters the car, it creates drag which can make the car work harder to maintain its speed, thus using more fuel. This is why it is recommended to keep windows closed while driving on the highway to improve fuel efficiency.

4. How can I reduce the amount of wind that enters my car window?

One way to reduce the amount of wind that enters a car window is to partially open the opposite window. This creates a balance of air pressure on both sides of the car, reducing the amount of wind that enters. Additionally, using a sunroof or wind deflector can also help to reduce wind entering the car window.

5. Does the size and shape of a car window affect how much wind enters?

Yes, the size and shape of a car window can affect how much wind enters. The larger and more angled the window, the more wind it will catch and funnel into the car. This is why some cars with smaller, more curved windows may experience less wind entering compared to cars with larger, flatter windows.

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