Quote:
Originally Posted by winkosmosis
By the way, airplane wings don't create lift with the Bernoulli effect. That's one of those persistent myths. It sounds good when you read it in a textbook and when you use the paper strip demonstration, but it doesn't stand up to scrutiny.
Google "bernoulli vs newton" or "bernoulli myth" and you'll find a lot of articles and discussions. Planes generate lift by angling the airfoil.
Edit: Here's a link. Two myths I learned in college: bathtub drains and airplane wings — The Endeavour |
Hi Wink,
Another approach to getting a grip on what actually happens with air flow is to look at vacuum aspirators...
They take high pressure air and convert it to a vacuum due to the presence of high velocity air flow.
To answer your question on whether there is a higher than atmospheric pressure at the front of our cars, the answer from me would be, yes.
I think of the air as being a very loosely knitted soft foam. Or another analogy is a series of tiny balls that are all attached to each other by very soft springs.
If you move your car forward then the front of the car does compress the springs somewhat as it moves through the air. The faster the movement, the more this compression takes place.
Back to a simple test. If you can find an old water manometer, or for that matter an old Dwyer water manometer dial indicator, you could mount a forward facing tube out through the grill and measure this pressure as you drive.
Now here's the cool part...
If you could drive fast enough and you place another tube through the roof line of the car that faces straight up, it would measure.... a pressure reduction over atmospheric. This reduction would increase the faster you go.
And yes, you would see the same effect, except greater if you mounted the same two manometers on a model air plane and flew it. The manometer mounted on the top of the wing would be an indicator of the wing's lift.
The stagnation pressure is really measuring the force of the air at zero velocity. In otherwords, where the air hits the leading edge of the wing and there is no movement either over the wing or under this air velocity is essentially zero because it is stuck in position at the front of the wing and not moving relative to the aircraft.
Any movement of air that is accelerated either over or under the wing will create a pressure decrease or vacuum in the air. I can demonstrate this over and over in the lab. This is worth repeating. Any time the velocity of the air is increased, there is a general decrease in atmospheric pressure. This phenomena is used to make all manner of air flow calibration equipment.
Here's a strange fact:
PC computer hard drives use the Bernoulli Effect to great effect. How?
The head above the spinning platters must stay very close to the surface to read the digital information. Since the platter is spinning so quickly, it rides in a small pocket of air only microns from the platter surface, even if you pickup the PC which this is all happening. What controls this small air gap? Right, the high velocity air trapped between the platter surface and head. The head is literally *stuck* to the surface of the platter from the high level vacuum present.
Jim.
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