Bernoulli's Principle and Static Gas Pressure

[Thomas2]

And remember your previous claim: that the front part is pushed down while the back part is pulled up? That one was refuted by the force/pressure diagrams. The lift is not distributed the way you think it is, for the same reason you think a horizontally symmetrical and flat wing will not produce a net lift.

The question is whether the 'pressure' being measured is the pressure that the wing experiences. As I have indicated before in this thread, the 'pressure taps' in the wing will indicate a lower pressure for a tangential airflow because molecules from the air in the pressure tap are sucked into the airflow by means of viscous friction. The material surface of the airfoil will not experience this effect as its molecules can not move.

 

[Andrew Mason]

Not really. The reason the gas cools upon expansion can be seen in the first law of thermodynamics.

If P decreases, the energy per unit volume of the fluid decreases, so the temperature must decrease. This was a response to Thomas2's statement:

But this would correspond to a decrease in temperature as the flow speeds up, which I don't think will be observed here

AM

 

[Thomas2]

That is exactly why a refrigerator works: By allowing a gas at high pressure to pass through a constricted space and accelerate! The accelerated gas has lower temperature and draws heat from the surroundings.

I don't actually think that a refrigerator would work with an incompressible gas (and incompressibility is the standard assumption in connection with applications of Bernoulli's principle).

 

[Q_Goest]

I don't actually think that a refrigerator would work with an incompressible gas (and incompressibility is the standard assumption in connection with applications of Bernoulli's principle).

Correct. As stated previously, the simplified Bernoulli's equation assumes isothermal conditions.

 

[russ_watters]

The question is whether the 'pressure' being measured is the pressure that the wing experiences. As I have indicated before in this thread, the 'pressure taps' in the wing will indicate a lower pressure for a tangential airflow because molecules from the air in the pressure tap are sucked into the airflow by means of viscous friction. The material surface of the airfoil will not experience this effect as its molecules can not move.

Yes, I know: hence your claim about most of the lift being generated on the back third of the wing (and, hence: your misunderstanding of what is going on in a venturi tube). I guess its just a coincidence to you that the results in both cases are exactly what aerodynamics predicts. But regardles, once again, wind tunnel tests prove that the pressure profile measured and predicted is real because it accurately models the lift on the wing. Once again, your model fails because it doesn't match reality.

 

[russ_watters]

Regarding refrigerators: compressibility is largeley irrelevant as a cause of temperature fluctuations because the working fluid is undergoing a phase change. That's where most of the energy comes from.

 

[Andrew Mason]

I don't actually think that a refrigerator would work with an incompressible gas (and incompressibility is the standard assumption in connection with applications of Bernoulli's principle).

There is no such thing as an incompressible gas. If you are talking about a gas in a pipe, which is what I thought you were talking about, the gas compresses. Bernoulli's equation works quite well for a gas in a pipe, with a slight modification to factor in the energy that goes into the work of compressing the gas or the work done in expanding it.

If you are talking about air flow around a wing, the air is considered non-compressible because there is nothing to compress it against (like a pipe wall) so the air just moves out of the way and creates a compression wave that propagates away much faster than the wing speed.

AM

 

[hydrologyphys]

How Bernoulli's Equation Explains Airflow

An airfoil (wing of an airplane) is designed such that the upper surface of the wing is longer than the bottom. (a curved line is longer than a straight line of same horizontal displacement)

Now, think of a bend in a river. The water on the outer edge of the bend travels a longer distance over the same period of time as it takes the water on the innder edge to round the bend. Since velocity = distance/time, the water on the outer ede of the bend must travel at a faster velocity in order to travel a larger distance over the same period of time. Say the water traveling on the outer edge is traveling at velocity v2 and the water at the inner edge is traveling at v1. From this we have v2>v1. Similarly, v2 represents the velocity of the air flowing on top of the wing and v1 is the velocity of air flowing under the wing.

Now use Bernoulli's Equation: p1 -p2 = [1/2]rho(v2^2 - v1^2) where p is pressure, rho is the greek symbol representing density and v is velocity.
Density (rho) remains constant in a given system. (Assume the density of air does not change as it passes around the wing). THEN since v2>v1, p1>p2.

So the pressure on the bottom of the wing is greater. A resultant force is created due to the pressure different. (keep in mind pressure=force/area). So the force is acting perpendicular to the wing, thus holding the wing up.

Another way to derive this force is by considering the difference in velocities. Since
v2>v1, then V2 must be ACCELERATED from v1. There must be a net force inbetween the pressures in order to accelerate the wind.

 

[arildno]

The equal transit time principle is FALSE.
There is no natural law that says particles traveling along the outer bend must arrive at the same time as those traveling around the inner bend.

Utilization of the equal transit time principle yields totally false pressure distributions.

 

[russ_watters]

No need to open a 2.5 year old thread...