Bernoulli and Low Pressure

[splash]

It's my misunderstanding that weight is the force that holds us to the earth and not gravity. Boy, I learned incorrectly in like the 4 grade. This will take a long time to build on again.

 

[Cessna414JJB]

It's my misunderstanding that weight is the force that holds us to the earth and not gravity. Boy, I learned incorrectly in like the 4 grade. This will take a long time to build on again.

And you may run into someone that knows that the "popular" explanation of lift and bernoulli really suck... :insane: Teaching something wrong is not the solution...

 

[shdw]

If the low pressure air has momentum, it can make some headway against a high pressure area, but it will be continually decelerating.

It sounds possible, but it still doesn't seem likely if that makes sense. Do you have a source? I was always under the impression that blowing fast moving air over an area of still air would result in the still air loosing pressure. Most of this comes from study of airflow in meteorology though which very well could be completely different then that of air over a wing.

 

[splash]

weight is most definitely a force.

Here are the facts you decide.

A force, in the simplest sense, is a push or a pull. It's source may be gravitational, electrical, magnetic, or simply muscular effort.

Weight- The gravitational force exerted on an object by the nearest most-massive body (locally, by the Earth).

When I think of the force that pulls on an aircraft I think of gravity. When I think of how much gravity is pulling on a mass I think of weight because the farther we are from the center the less we weigh.

Cheers my friend.

 

[splash]

Pick up any elementary physics book and you won't see gravity included as a force except when multiplied by the appropriate mass, often expressed as "mg".

I just did :yup:. Conceptual Physical Science second edition (Hewitt, Suchocki, Hewitt)

 

[tgrayson]

Do you have a source?

Any book that has decent introduction to boundary layer theory. If you have nothing else, you might check out Aerodynamics for Naval Aviators, starting on page 56.

 

[EnRoute]

Man, it fascinates me to see how these threads can twist and turn and end up in places that one never would have imagined. Thank you all for your intriguing feedback.

So it looks like the word "vacuum" should not be any part of my lesson plan, understood, but I'm still a little confused on how the pressure differential alone creates lift. Am I to assume that because the pressure is lower on top of the wing, this then allows the higher pressure below the wing to overcome the lesser pressure lesser resistance of the upper surface, which is simply controlled by our AOA/elevator inputs?

Thanks again for your help

 

[tgrayson]

Am I to assume that because the pressure is lower on top of the wing, this then allows the higher pressure below the wing to overcome the lesser pressure lesser resistance of the upper surface, which is simply controlled by our AOA/elevator inputs?

Bingo.

Note that the "higher" pressure means higher with respect to the pressure on top of the wing, not necessarily higher with respect to ambient pressure.

 

[shdw]

Any book that has decent introduction to boundary layer theory. If you have nothing else, you might check out Aerodynamics for Naval Aviators, starting on page 56.

I finally bit the bullet and just bought this book yesterday and have been doing some reading but it still doesn't really clear up what was being discussed.

<tgray this paragraph isn't for you but for others arguing about the boundary layer>
It does however clear up the boundary layer portion discussed, but I don't think we were really talking about this. "This layer of air over the surface which shows local retardation of airflow from viscosity is termed the 'boundary layer'." Later it says, "the static pressure increases from the minimum to the high positive pressure at the aft stagnation point--an adverse pressure gradient." Now this confirms what you said that the boundary layer has a higher static pressure, but I wasn't referring to the boundary layer but instead the area where it no longer exists due to separation.

<tgray this section is for what we were talking about>
Specifically on page 59 it states, "if separation occurs the positive pressures are not recovered and form drag results." This is describing the separation area we are speaking of which the above paragraph shows is at a higher pressure pre-separation. However, it would seem as though the pressures are no longer positive, but instead negative and resembling a low pressure vacuum area.

Now the idea of a high pressure area, the boundary layer, separating and thus rapidly expanding would also leave me to theorize that this open area is at a lower pressure and now trying to fill itself with the surrounding air. Where am I confused, please help?

 

[splash]

Now the idea of a high pressure area, the boundary layer, separating and thus rapidly expanding would also leave me to theorize that this open area is at a lower pressure and now trying to fill itself with the surrounding air. Where am I confused, please help?

=vacuum.

:deadhorse:

 

[tgrayson]

However, it would seem as though the pressures are no longer positive, but instead negative and resembling a low pressure vacuum area.

Below is an image from John D. Anderson's "Introduction to Flight". It shows pressure coefficient on the vertical axis and chord position on the horizontal axis. Note these things:

1. The airfoil shows what appears to be a leading edge stall.
2. The airfoil that has flow separation shows less negative pressure over most of the airfoil, which would result in a loss of lift.
3. The airfoil that has flow separation shows a more negative pressure at the very rear of the airfoil. This is an example of a failure of "pressure recovery" and is the source of the drag on a stalled airfoil.
4. Nowhere on either airfoil is there anything resembling a vacuum.
5. The pressure on both airfoils increases continuously after the point of minimum pressure.

 

[tgrayson]

theorize that this open area is at a lower pressure and now trying to fill itself with the surrounding air

BTW, there isn't an "open area", if you're referring to the photograph you posted. What you're seeing is an effect of the technique used to visualize the stream lines, which is probably aluminum flakes scattered into the fluid. While the flow is laminar, the stream lines remain distinct, but once the flow becomes turbulent, the flakes become scattered and the distinct lines disappear. If you look closely at the dark areas, you can see well-distributed bits of brightness. There is a similar effect when you see smoke curl from the end of a cigarette.....the smoke rises in a thin straight line, then suddenly turns turbulent and mixes rapidly with the surrounding clean air becoming invisible.

 

[splash]

Increase airspeed and AOA and all the little specks of metal flakes with air will cease to exist in this area. Unlike other fluids, air can be compressed and squeezed into or taken out of. The picture shows the beginning minute stages of a vacuum being created on a micro level. The pressure is so great on the leading side and is forced to the edges where it rapidly changes into a great low pressure which in turn pulls the air away from the center of the backside even creating the true definition of the word vacuum.

 

[SteveC]

Increase airspeed and AOA and all the little specks of metal flakes with air will cease to exist in this area. Unlike other fluids, air can be compressed and squeezed into or taken out of. The picture shows the beginning minute stages of a vacuum being created on a micro level. The pressure is so great on the leading side and is forced to the edges where it rapidly changes into a great low pressure which in turn pulls the air away from the center of the backside even creating the true definition of the word vacuum.

Is that all based on speculation?

 

[splash]

Is that all based on speculation?

I speculate that air is stupid and will fill the easiest space available taking no definite shape or definite volume. The easiest space available is in the low pressure stream or in the burble just above where it tries its hardest to get to the vacuum (empty space) but simply can not because of the low pressure stream from all around. No it's not a total 100% empty space as in outer space.

I have never been in such a great detailed debate pass the point of a stall and the books I have seen never got to this point of addressing the meaning behind "separation".

Grayson gives an example of smoke from a cig but there is no force creating rapid pressure differences and metal flakes don't just disappear when mixing with air.

BTW, no I don't have a wind tunnel in my backyard or instruments to do such tests. So, yes it is my best guess of what happens if airspeed increases as AOA increases pass the critical AOA.

 

[BillErvin]

Increase airspeed and AOA and all the little specks of metal flakes with air will cease to exist in this area. Unlike other fluids, air can be compressed and squeezed into or taken out of. The picture shows the beginning minute stages of a vacuum being created on a micro level. The pressure is so great on the leading side and is forced to the edges where it rapidly changes into a great low pressure which in turn pulls the air away from the center of the backside even creating the true definition of the word vacuum.

Since you obviously feel you're correct in this assumption, Instead of arguing about it, I would just have to say "prove it", and write a paper explaining it.

 

[splash]

Since you obviously feel you're correct in this assumption, Instead of arguing about it, I would just have to say "prove it", and write a paper explaining it.

Feel is the key word here and I shall do my best for all of you by going pass all text books (ones I have seen) in one detail step pass them. Really been waiting for someone to really disprove it with simplicity and why this could not possibly ever happen.

 

[moxiepilot]

=vacuum.

:deadhorse:

I speculate that air is stupid

Feel is the key word here and I shall do my best for all of you by going pass all text books (ones I have seen) in one detail step pass them. Really been waiting for someone to really disprove it with simplicity and why this could not possibly ever happen.

There does come a point where one might recognize that they do not know something, admit it to themselves, then stop talking in order to learn something.

 

[SteveC]

I'm still hoping that someone will explain quantum string theory in a way that appeals to my common sense understanding of how the world works.

Could happen! irate: