# Is circulation real?

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

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### Sailor AlSenior Member

Well, yes that is exactly what they are saying. When they say that below a certain speed (Mach .3 , Mach 1 or whatever) air stops behaving like an incompressible fluid, they are saying it changes from behaving like a liquid and starts behaving like a gas.
No, they are not. That's the fundamental difference between a liquid and a gas.
Correct. They can be ignored because they do not occur.
No. For a flow of liquids, it is ALWAYS true. That's not an opinion, it is a fact of science. Check any high school physics book. That's part of the definition of a liquid: it's what distinguishes liquids from gases.
Why, at any speed between the 3 kts of the usable wind speed over a sail to the Mach 3 of the Blackbird, does air change its behaviour from a liquid to a gas?

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### BarrySenior Member

Are you saying that if a fluid that is incompressible will not respond by expansion to increase its pressure ( in a controlled volume) with an increase in temperature?

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### Sailor AlSenior Member

Yes, in the sense of the gas laws and Bernoulli's theorem.
It will, of course, respond to temperature by its thermal coefficient of expansion, which for water, from a quick Google of the topic, is 210×10−6 , which is really, really tiny and accounts for some its divergence from a perfect fluid, along with its very slight non-zero viscosity and perhaps some other phenomena that I can't think of at the moment.

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### BarrySenior Member

Water is compressible but around 200 atmospheres the changes is less than 1%. So it is irrelevant with most normal design calculations unless the system is closed and is heated.

But water will expand due to an increase in temperature and significantly so. Prox 2-3% with 100* F change. (I will try to find a table for this and edit this post later) A power engineer (steam engineer) would take the expansion of water
into account in design for 0 quality portions of a system. ie all liquid phase, without vapor present ( and possibly other quality parameters)

But the gas laws do not apply to liquids.

So liquids are considered incompressible for the majority of design calculations. <1% as per DC's comment
(Incompressible) Water will expand in volume with changes in temperature. In a closed system, under pressure, the expansion and resulting pressure increase needs to be accommodated

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### Sailor AlSenior Member

Absolutely, that's why Bernoulli's Theorem doesn't apply to air!
Again, yes. But this whole thread is talking about air, a gas, not a liquid.

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### Alan CattelliotSenior Member

Look Sailor AI, every material is compressible, sorry to say that, but all depends on the external condition (adiabatic chamber in my example). I applied the Navier Stokes equation @Mach50 = hypersonic flow = enthalpie reconnection = chemical reaction. I also used in laboratory special small chamber to study the effect of water compressibility on chemical affinity. I also calculate drag and lift with Navier Stokes equation and made these result validate in wind tunnel or tank testing facility. So yes, my lord .. every material is compressible.

Regarding our external conditions and with the phase state curves, we choose to take account or not of compressibily. Read my word : under Mach 0.3, you can neglect the effect of air compressibility to calculate lift and drag of a wing. The airplanes you have flown have been calculated with this principle.

(Not to add too much of your distress, solid are also compressible. Piezzoelectricity is one phenomenon that result from that compressibility).

I very honorable to try to find simple and elegant equations to describe the lift and drag over a wing but that should be at least compatible with basic thermodynamics. With your concept, the temperature of air @the leading edge should increase, and cool down at the trailing edge. Sailor AI, you gave me no answer to this ? Do you believe that, under Mach 0.3, this phenomenon occurs ?

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### Sailor AlSenior Member

Trust me, I'm not distressed.
The facts water will compress under very high pressure, and that air's slight viscosity and low density means that it doesn't act as a perfect fluid do not distress me.
At sailing and airplane speeds, air behaves almost exactly like a compressible gas and not in any way like an incompressible fluid.

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### patzefranpatzefran

Sailor al is on my ignore list for a long time, useless to try to explain anything to him, waste of time !

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### rnlockSenior Member

At sailing speeds, air's compressibility can be neglected. At least on Earth. As I mentioned before, the dynamic pressure in air at 20 knots is something like 0.06 percent of atmospheric pressure. That is, negligible. At least until you get your other instrumentation fantastically precise. At the speed of a jet airliner, compressibility may not be neglected, if you want any accuracy. The dynamic pressure might be a third of atmospheric pressure. If not, Boeing wouldn't bother with the swept wings, supercritical airfoils, etc. They make things a lot more difficult. That doesn't mean air changed into a liquid. It would have to get much colder.

On Mars, if you were using a really efficient landsailer, in a severe windstorm, you might get up to the equivalent of about Mach 0.65, which would be high enough for compressibility to make a significant difference. (I'm assuming aerodynamic efficiency as good as the best landsailers on Earth, but much lower rolling friction, which might not be quite fair. Unless you could find enough water to make a frozen lake.) Then again, the speed of sound on Mars isn't a constant. CO2 is odd stuff. If we could keep emitting it indefinitely, aerodynamicists on Earth might have to do some calculations over, if they lived.

Only unobtainium is incompressible, and then only if you want it to be, though I admit I haven't tried putting any neutronium in a vise lately. If someone has some, I could try it, just for science.

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### BarrySenior Member

Which part of his theorem do you say that does not apply to air?
I will assume that you are referring to the rough premise " fast flow = low pressure and slow flow = high pressure"
Bernoulli's Theorem works just fine.
As an example, in a steady state, steady flow regime, a venturi exhibits the above premise. The higher velocity through the venturi produces a lower static pressure in the venturi.
So the Theorem applies. Both with liquids and gas

While the air that is influenced around a wing is not contained by a physical boundary/envelope, it has been a point in several discussions ( not on this forum) that the inertia of the air above and below the wing ( outside of the boundary layer) , provides a quasi containment
envelope.

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### Sailor AlSenior Member

That's a great question.
I think we will all agree that Bernoulli's theorem relates the pressure gradient in a fluid to its velocity gradient.
Bernoulli's theorem doesn't explain the pressure gradient in the air over the two sides of foil (which experimentally is shown to exist), since it doesn't provide an explanation for a velocity gradient.

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### Sailor AlSenior Member

Don't shoot the messenger. The message remains valid whilst it's useful and until disproven

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### Sailor AlSenior Member

Air is made up of approximately 78 percent nitrogen and 21 percent oxygen. (NASA)
At NTP, Nitrogen and Oxygen are gases whose behaviour varies minutely from the behaviour of the ideal gas of Charles' Law (PV=RT). (When subjected to impact at supersonic speeds this may not apply, but that does not apply to the current discussion.)
The fact that air is a compressible gas is the reason that Bernoulli's Theorem, requiring a incompressible fluids, should not be applied.
On what basis then do you claim that air's compressibility can be neglected?

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### Sailor AlSenior Member

I'm not sure where you sourced the 0.06% value, but I think you have incorrectly scaled the value. It's more like 6% than .06%,. that's 100 times bigger!)
( Air density , rho = ~1 kg/m^3, 20 Kt =~ 10 mps, Atmospheric pressure is ~1,000 kPa, Dynamic pressure of air at 20Kts ( 1/2 rho v^2) = ~50 kPa = 5% Atmos)
And 50kPa on a 100 m^2 sail would generate 5,000 Newtons of force... that would not seem negligible to the sailor!
But anyway, the argument is totally flawed, since the force of Dynamic Pressure is only achieved if the air is stopped, as in a pitot tube. A sail doesn't do that!
And also, unless you're sailing directly downwind with the mainsheet fully eased, the sail area presented to the apparent wind is only a fraction of the sail area (Area * sin( AWA)).

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### AJBJunior Member

No Al
Maximum sail force at maximum flow not stalled.

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