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Is the inflowing air into the moving car against Bernouli's law?

05/07/2007 8:40 AM

I think of this phenomenon for a long time, but no apropriate answer to accept.

One that I learned from Bernouli's Law that the fluid pressure is lower at faster moving side, and the fluid will flow from higher pressure to the lower side.

The outer air of the moving car is faster than inner air, should the inner air flow through car opening to outer side.

But, according to our experience, the air flow is INTO the car. More precisely, flow into the car at backside openings, and exhausted at the frontside openings.

Any suggestion will be highly appreciated. Shine my darkness side, please.

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Anonymous Poster
#1

Re: Is the inflowing air into the moving car against Bernouli's law?

05/07/2007 3:48 PM

I don't understand your question. Air that is captured by an opening in the front of a car goes in, not out when the car is driving forward. Look at a radiator. Fans on the radiator pull air in from the front to the backside of the radiator. If air went forward when you drive then the fan would be working against airflow. Also, brake ducts work the same way, scooping in air from the front of the car and exhausting it behind the scoop onto the brakes.

Try driving down the road and hold a funnel out the window. Either way you hold it, air will pass through it in the same direction as the air that goes around it.

On second thought, get someone else to drive or don't do it around me. ;-)

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#2
In reply to #1

Re: Is the inflowing air into the moving car against Bernoulli's law?

05/07/2007 4:50 PM

Really sorry for unclear question. What I mean is a situation when you sit in the back seat of a bus and the driver is smoking. No windows were opened, except for a little opening neatly beside you and beside the driver.

As long as the car in moving, you feel the wind flowing in through the window while you notice that the driver smoke goes outside in the same time.

Hope this illustration make it clear. Thanks any way.

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#3

Re: Is the inflowing air into the moving car against Bernouli's law?

05/07/2007 6:03 PM

Guest (in reply #1) has a good point - you must clarify any such problem.

Not offering any solutions, I have noticed (as a smoker in hired cars (in which I shouldn't be smoking!)) that there seems to be a critical level of "window opening". Given all other thing equal (say, for example, all other windows closed), if the window is up above the said level, smoke (and ash) goes out; near the level, it seems to go out and back in again; well below the level, it comes straight back in.

A car body (unless it's very old) is designed to reduce drag. The pressure at any point near the surface of a moving car is a complex function of which bit of car, the airspeed (and direction relative to the motion of the car), what windows are open (and how wide), and all sorts of other stuff.

Having said all that, the observation I mentioned above seems to hold for most cars, at most moderate speeds. Maybe someone out there has a suitable aerodynamic model that can help explain what's going on? (Please ?)

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#4

Re: Is the inflowing air into the moving car against Bernouli's law?

05/08/2007 12:11 AM

The reason for smoke exiting the drivers window might be as simple as the vehicle being positively pressurized by air inlet vents (defrost, heater and A/C vents), and/or enhanced by the venturi effect of the open window. Picture a boat moving through water, and the wake it creates. The wake is pushed away from the front of the boat by the bow, and is drawn in to meet behind the boat by the stern. Air is affected by the bus in a similar way.

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Anonymous Poster
#5

Re: Is the inflowing air into the moving car against Bernouli's law?

05/08/2007 12:44 AM

To me, Moving Cars internal Air (moving) Pressure becomed lower causing the outer air to enter into (for relatively higher pressure out side). Due to geomatery (not absulute linear / car shape) and wind directions effect the stability / equilibrium in air pressures.

Let us wait and see more detailed answers.

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Anonymous Poster
#6

Re: Is the inflowing air into the moving car against Bernouli's law?

05/08/2007 1:40 AM

I have a pickup with a cap on the back and a door cut out between the cab and the bed. There is a gap in the floor between the bed and the cab of about 1.5 inches. When sitting still, with the wind blowing from (not sure about this) any direction, a nasty draft comes up through the opening. My assumption is that the distance over the top of the pickup/cap is longer than the distance underneath and, as such, mimicks a crude airfoil shape. Pressure is lower on top of an airfoil than on bottom, which is what makes it fly. The higher pressure underneath the truck moves toward the lower pressure inside the truck, which is probably higher than the pressure on top of the truck, which is why my cigarette smoke will go out a hole in the very top of the back of the cap even when wind is blowing directly towards it.

I have a vent window on the door in the cab. My cigarette smoke goes out the vent window. That is because, when opened, the vent window displaces the airstream outward away from the door, thus causing a partial vacuum to form immediately behind the window. The higer pressure air inside the cab moves towards the lower pressure partial vacuum behind the vent window and carries the smoke with it. Any other aerodynamics besides pickup camper smoke theory is over my head though I've seen several completely different explanations for Bernoulli's Law.

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#7

Re: Is the inflowing air into the moving car against Bernouli's law?

05/08/2007 1:46 AM

The answer lies in the relative pressures around the outside of the vehicle. As was stated previously, the amount each window is opened affects the results significantly, as direct flow through the vehicle occurs under certain conditions.

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Anonymous Poster
#8
In reply to #7

Re: Is the inflowing air into the moving car against Bernouli's law?

05/08/2007 8:54 AM

The confusion here is with turbulence versus vacuum

Bernouli's principle is based on fast moving air is a low pressure area - like a vacuum bag, the car interior is actually in a pressurized state with the exterior as a low pressure area. The flow of air in to the cabin from the vent helps equalize this.

The best example is an aircraft wing with high velocity air flowing over the top of the wing resulting in lift. The top of the wing is a low pressure zone.

This is the same for the skin of the car.

The shape of the vehicle disturbs flow affecting velocity of air and resulting in various pressure changes around the vehicle

When you open your window, the smoke is extracted because it is being pulled in to a low pressure zone or vacuum.

If you open your window all the way, the velocity of air is disturbed by the change in the shape and added turbulence forcing air in to the car. With the windows open, the pressure zones equalize and now you are just dealing with high velocity air without restriction entering the cabin of the car.

This is simple physics...

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#9

Re: Is the inflowing air into the moving car against Bernoulli's law?

05/08/2007 11:08 AM

This is a thought provoking observation- especially in the recent quest to build more fuel efficient vehicles. Most vehicles on the road today are extremely non- aerodynamic- being a compromise of shapes necessary to both contain people and freight- as well as displace air- which is necessary to move that container through the air.

It is interesting to me that large effective frontal area is the very thing that a designer wants to avoid when designing an aerodynamic shaped vehicles. Yet, a sail on a boat has a huge frontal area that actually uses Bernoulli's Law to pull it through the air. This suggests to me that there must be an aerodynamic shape that can be applied to "people containers" that - by passive design- essentially eliminates effective frontal area, and may even reverse "aerodynamic drag".

There are already many clever devices that seem to defy logic when dealing with aerodynamics. I read (years ago) in a popular Science magazine of a bow-shaped wing applied to a dry-lake sail boat that was a near omni-directional sail. Consider the toy Aero bee- a gyro stabilized flying cylinder that sails thru the air like a smoke ring- almost unimpeded. And the high efficiency air scoop developed in auto racing that gulps more air into it than imagined due to its tiny mouth and bell-shaped interior. AS well , there is the "Condon Effect" that exploits boundary layers on on surfaces moving thru the air to dynamic advantage. I think that using a combination of these ideas will evolve into totally new shapes for automobiles that will increase their efficiency many times, and- as well- their appearance. We seem to identify beauty with shapes that imitate nature. Bernoulli's Law is right, we just have not applied it to its ultimate extrapolation.

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Anonymous Poster
#10

Re: Is the inflowing air into the moving car against Bernouli's law?

05/09/2007 9:55 AM

Bernoulli stated that the total pressure is the sum of the dynamuic pressure and the static pressure. As the dynamic pressure rises, the static must decrease, hence the lift on a wing. But this simple equation does not take into account turbulent flow. If the flow separates, there are large eddies and flow disturbances. Similarly, when the flow separates from an airplane wing, the pressure distribution is disturbed, and the the wing 'stalls', often with dire consequences if there is not enough altitude to recover. In reality the bernoulli equation is a 29 variable nonlinear partial differential equation. A lot more than is usually quoted.

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