# Is circulation real?

Discussion in 'Hydrodynamics and Aerodynamics' started by Mikko Brummer, Jan 25, 2013.

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### Sailor AlSenior Member

Another believer. Welcome aboard.

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### Sailor AlSenior Member

I think that is trolling.

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### Sailor AlSenior Member

I'm not sure why you are shouting.
No, water IS incompressible.
No I'm not saying that at all. The keel is working in water (incompressible), the "lift" it generates counteracts the leeway component of the aerodynamic force on the sail.
The "drag" from the keel adds to the resistance.

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### Sailor AlSenior Member

Yes, they do. Even NASA says it. But why do they say that? Air is a gas, not a liquid. Gases are compressible. Why should air at under Mach 3 be incompressible????

Barry likes this.
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### DCockeySenior Member

Not Mach 3. He said under Mach 0.3 (zero point 3) = Mach 3/10.

If the free stream flow approach an airfoil, sail, etc is around Mach 0.3 or less then the density changes are negligible and can be ignored for calculating aerodynamic forces. This has been proven experimentally for a very long time.

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### Alan CattelliotSenior Member

That, I wouldn't SailorAI, I was just exploring the last sentence of the introduction you give in your link, after you speak about compressibilty : Hence the drag and the lift. That was my way in trying to push your view, and understand the meaning you give about compressibility, and pressure.

Liquid, solid, Gaz (and more states sometimes) are states of matter. I believe we can alter the state of matter by acting on the Pressure or the Temperature or its Volume. For instance, the Law of perfect gases has been heavily studied for all kinds of molecule. It can written in the form : P x V = n x R x T.
Where P is pressure, V is volume, n is molar number, R is Avogadro constant, and T is the temperature.
n = m / M,
Where
m is the mass of the molecules, and M their molar mass ( by addition of the atomic mass of each constituent of the molecule) .
The equation then is re-written :
P = rho / M x R x T,
Where Rho is the density.

Basically , its says that the pressure exerted by the molecule of gaz (in an adiabatic chamber) is proportional to the Density of the Gaz and its Temperature. Thermodynamics is essential to fluid dynamics.

As a pilot, we may be familiar with the aneroid anemometer ? Can we agree of some sort de concensus about the pressure ? Is it the pressure you are talking about ?

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### Alan CattelliotSenior Member

Thx

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### Sailor AlSenior Member

Yes, sorry, my mistake, I has just returned from yacht racing and was a bit tired, I should have left my response to the morning like now (Sydney UTC+10).
Mach 0.3 makes even less sense for two reasons
1. Modern passenger jets fly at Mach .7
2. Aeronautical texts say that air becomes compressible at Mach 1, and that's what I think is wrong. Why would it turn from a gas to a liquid at Mach 1?

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### Sailor AlSenior Member

I'm pretty good at following a discussion in 10pt Arial.
Agreed, actually two of the four: solid, liquid, gas and plasma and I have no problem with your deduction that pressure is proportional to density and temperature.
The gas laws, or specifically Charles' Law, states PV=kT (k is a constant). We don't have to complicate the matter by introducing Avogadro's number, molar mass and density to understand how the pressure of a gas responds to volume and temperature changes.
Yes, it is the pressure that I am talking about.
On what point do you disagree?

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### DCockeySenior Member

What text says air becomes compressible at Mach 1? Please provide a title and author.

Passenger jets are designed using analysis methods which include compressibility effects. Modern passenger jets usually cruise at Mach 0.8 to 0.85. And at those speeds the flow is transonic which means the flow over the top of the wing goes supersonic and a shock wave forms.

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### Sailor AlSenior Member

I could find the reference, but actually, it's not relevant. It was Alan Cattelliot who introduced Mach 0.3 into the argument:
Maybe we should ask him why air changes from a liquid to a gas at Mach 0.3.
Or, using the SVT's approach of viewing from the point of view of the air, not the object, why should air turn from a liquid to a gas when you move an object such as a wing, sail, propeller, etc., through it at Mach 0.3?

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### Sailor AlSenior Member

By "being right", I think you are saying SVT is right, and SVT relies on air being compressible.
I don't think you can have it both ways!
You can't have a degree of compressibility: either air is compressible and responds to pressure according to Charles' Law or it behaves like an incompressible fluid and responds to velocity change according to Bernoulli's Principle.
[EDIT]
I could have put that last sentence more clearly:
Either air is a) compressible, and its pressure responds to changes in temperature and volume according to Charles' Law or b) it is incompressible and its pressure responds to changes in velocity according to Bernoullis Theorem. Its pressure doesn't respond to changes in temperature, and it can't respond to changes in volume because, by definition, it's incompressible.
It can't be both!

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### BarrySenior Member

Where P is pressure, V is volume, n is molar number, R is Avogadro constant, and T is the temperature.

You should include a note that the pressure and temp are absolute and not PSI gauge and not Centigrade or Fahrenheit for T
So as to not mislead a uninformed reader that by increasing the temperature by 10 degrees to 20 degrees Fahrenheit will double the pressure. OR doubling the pressure that you read in a normal pressure gauge ie psig, that the volume will not
be halved.

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### Sailor AlSenior Member

It's not unreasonable to assume the readers of this forum are well aware of these basic tenets of physics.

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### DCockeySenior Member

No one is saying air changes between a liquid and gas depending on Mach number. To suggest that anyone is doing so implies a fundamental shortfall in knowledge of the subject.

Both liquids and gases are compressible. Increase the pressure and the density of both increase. Liquids are not exactly "incompressible" though the change in density due to a change in pressure is generally much less for liquids than for gases.

"Incompressible" is used to describe a fluid flow where density changes due to pressure variation can be ignored. For flow of liquids this is almost always true. For flows of gases this is true if the Mach number is low enough. How low is low enough depends on the geometry and accuracy required but Mach 0.3 is typically low enough.

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