Stirlingengineforum.com

Ideal Gas Law vs. Lennard-Jones (Real Gas) simulation comparison:

Ideal Gas Law:

https://youtu.be/LiEpu315REc

Lennard-Jones:

https://youtu.be/mpDcrlHr-kI

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Worst simulations ever!

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Fool wrote: ↑Tue Nov 05, 2024 7:46 am .

Worst simulations ever!

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Well, moron, feel free to present a Lennard-Jones simulation that appears any different.

No, not "ideal gas law" simulations. REAL gas behavior simulations that include Van der Walls forces and Lenard-Jones potentials, and temporary dipole bonding etc. that is different for each type of gas.

You know, the REAL forces of ATTRACTION between the gas molecules.

All that requires a lot of processing power but produces more accurate, more realistic simulations. Unlike the completely cartoonish and misleading "ideal gas law" simulations that are all the same and don't require a lot of processing because it is fake, a gross over-simplification.

A Lennard-Jones simulation of the Coanda effect in an air stream.


https://youtu.be/4o-6ciIjSOM

A lennard-Jones simulation apparently modeling a gas under increasing pressure.

https://youtu.be/-rZ6AYxre2U

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Wasn't the Coanda Effect.

https://en.m.wikipedia.org/wiki/Coand%C4%83_effect



It isn't a gas. You didn't read the description:

This third crystal formation simulation shows again 1542 particles in a rectangular enclosure, interacting with a Lennard-Jones potential and subject to viscous damping.

Quit wasting your time being ignorant. Go build something.

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Fool wrote: ↑Tue Nov 05, 2024 1:26 pm .

Wasn't the Coanda Effect.

The heading reads: "An improved simulation of the Coandă effect". Aside from that, it looks like Coanda effect to me.

It isn't a gas. You didn't read the description:

This third crystal formation simulation shows again 1542 particles in a rectangular enclosure, interacting with a Lennard-Jones potential and subject to viscous damping.

Quit wasting your time being ignorant. Go build something.

Well, I said "apparently". However, I've been watching many many Lennard Jones simulations where gas .molecules assembling themselves in a regular order are described as crystal-like. It would also be a bit odd to describe a crystal as "viscous". I did read the description accompanying the video which mentions "gas" as well as Van der Walls. Lennard-Jones simulations are most frequently associated with gas behavior.

At any rate, Lenard-Jones applies primarily to gases, but also liquids and solids or generic "particles". It is how all particles behave, transitioning between phases.

Regardless, here is another where the description specifies that the simulation is of a "gas" being compressed, and looks virtually identical:


To quote:

description
Symmetric pistons., Nils Berglund

This is a variant of the piston simulation • Under pressure: in which a gas of particles interacting with a Lennard-Jones potential is compressed symmetrically from both sides. ... The compression results in the particles arranging themselves in more an more closed-packed patterns...

https://youtu.be/mBGaFP9HsQA

This is the first video described above as: "the piston simulation in which a gas of particles interacting with a Lennard-Jones potential is compressed ..." Using just one piston.

https://youtu.be/P2Vw9CWD8YM

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So far all the Lennard-Jones simulations you've supplied do not look gas like, unless it is liquefied gas, or solidified. Perhaps one day those students will learn what phases it is, or they are, simulating,.and explain that correctly to the viewers. Until then you will need to figure it out from words like, crystal, or 2r sigma, or just watching how it all bunches up and falls to the floor, and freezes.

To correctly model a gass in all phases it appears that both the kinetic, and Lennard-Jones, mathematics must be used. It doesn't look like they are there yet, maybe. We'll see and play with it.

Play with the following you see what I mean. It does show some kinetics associated with a gas, but I've never seen dry nitrogen act that way in any enclosed box. It just acted uniformly as a gas. No clumping or any noticable effect of gravity. No sloshing. Even water vapor in a hot box doesn't act like that.

https://physics.weber.edu/schroeder/md/ ... ivemd.html

Water dropped onto a hot pan acts a little like that, until it completely boils away.

Maybe dripping back from condensing on a cold top, as in an ice vacuum boiler, might look like that. Which then again is at or near liquefaction. Kinetic theory only works when far away from that range.

The Lennard-Jones potential was published in 1924. That is probably why it was taught to me as part of the classical theory of material science and thermodynamics.

Computer simulations were in there infancies. Computer power went from mainframes to desktops. Yes I was in college for both. My early programs were using cards, and latter ones using the first desktop models, like the HP 9816, using Pascal programming language, the predecessor to C. I also learned how to program simulations on an analog computer. Yes, using integrators, amplifiers, and patch cords. That was excellent.

Modern computers have way more power than those, so simulating lots of points is easier, but they still become resource hogs. Big data sets requiring many many calculations, memory, and storage. Virtual memory was new in 1984. Learned about linked lists and data structures after I was out. Still learning.

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Fool wrote: ↑Wed Nov 06, 2024 3:50 am .

So far all the Lennard-Jones simulations you've supplied do not look gas like, ...

Only in your mind, because of your insane prejudice and idiotic insistence that gas particles have no mutual attraction but only "expand forever".

Thanks for the link though. Kind of interesting to play with.


You only THINK the simulation. "doesn't look like a gas" because of a wrong idea implanted in your mind that gas particles ONLY repel, never attract.

You're such a fucking moron dozens of detailed references, video, simulations, whatever, makes no impression on your pea sized brain.

You will probably go to your grave muttering on your death bed "gases only push, never pull, push never pull, escape velocity, expand forever... never pull. never ever ever... only expand, never contract....."

Even the preset for "gas" using few molecules shows "clumping" or "stickiness".

Increase the number of molecules a little and the attractive force increases, (more clumping).

But hey, it wasn't long ago I was arguing that the gas particles in a Stirling engine should benefit from having a straight line-of-sight path between the hot plate and the piston.

I had the same "kinetic theory" picture of the gas ricocheting between the hot plate and piston at the speed of light, or maybe only the speed of sound.

Well, now I think that idea is out the window.

In our gravity well at 15 psi gas particles would barely leave the hot plate before colliding and "sticking" to another gas molecule a billion trillion times. No chance whatsoever of a gas molecule making it from the hot plate to the piston in the split second it takes for an engine cycle.

All "pressure" in an engine cylinder is indirect, transfered via other gas molecules, so line-of-site between the hot plate and piston is pretty irrelevant.

Unless, of course, in your engine, there is only one trillionth of an inch between the hot plate and the piston.

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Straight line heater to piston, hot to cold, unimpeded, is definitely out the window, in the trash. I never considered it. The only way that would happen would be if there were very few molecules in play. Very low density, low pressure. Then nothing would be between to hit. Not true of any engine, or vacuum we have here on Earth. There will always be plenty of molecules to hit. In outer space maybe not.

Yes, the mean free path, compared with human perception, is extremely short. The molecules bouncing off the hot plate hit molecules a very short distance away. But they bounce off them too. If a gas is hot enough, (for helium it's about 30 K, number guessed. Possibly lower. Pressure and density matter.) there will be very little sticking. Yes a small percentage of the molecules will be moving slowly enough to 'stick' until they get knocked free again by the onslaught of many more hotter molecules. It is the Bell curve statistical theory, and 'boiling velocity'. It is also likely that above a certain temperature for each substance, there is zero sticking, just bouncing. Pretty hard to get data showing what is happening.

Yes gravity is pulling all gas particles downwards. The density at the top of a container is ever so slightly lower than the bottom. That should be high enough to measure, or cause a very minor effect. That is, if the proper test methods are used.

The Galileo thermometer demonstrates that quite well:

https://en.m.wikipedia.org/wiki/Galileo_thermometer

It also demonstrates how temperature effect liquid density. Nite: It is a closed and sealed in glass system. Glass has a very low coefficient of thermal expansion. The fluid has a higher value. The two together allow the phenomenon to be observable.

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Fool wrote: ↑Wed Nov 06, 2024 12:52 pm ...
Yes a small percentage of the molecules will be moving slowly enough to 'stick' ...

"Stick" = molecular attraction (to near infinity).

"Bounce" = very close range molecular repulsion (overlapping electron orbits)

...at an infinite distance, the attractive force in the Lennard-Jones potential is essentially zero because the "r^-6" term approaches zero

I think this may provide a molecular explanation for why the gas in a Stirling engine is easier to compress, but also why high rapid compression provides a benefit.

When the gas is expanded with added heat the molecular distances between molecules.begin to attract more than repel.

So the gas is easier to compress, up to a point.

But at maximum compression the electron orbits get close enough to repel strongly, or overlap resulting in a very strong and sudden "explosive" repulsion.

The electrons are basically moving in an orbit at the speed of light or some such thing. Rapid compression forces the molecules together like spinning tops that when in close contact fly apart suddenly.

https://youtu.be/fUYl4F6vkg0

Something more than just "adiabatic bounce" I think. Otherwise, logically, what would be the benefit from putting work into compression?

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A ping pong ball hits your paddle without being attracted to it. It pushes back in the form of a bounce. Imagine 6.022x10^23 more ping pong balls, per mile.

Yes, none of the simulations use molecular spin.

Electrons don't spin around an atom. Quantum mechanics doesn't work that way. It's called an electron cloud. And energy levels.

Quantum mechanics and relativity/lightspeed don't mix well. Nor gravity and quantum theory.

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I’m not sure where you all were concerned with gravity, but it certainly plays a role in these engines, or at least can be used to one’s advantage.

Fool wrote: ↑Wed Nov 06, 2024 11:32 pm .

A ping pong ball hits your paddle without being attracted to it. It pushes back in the form of a bounce. Imagine 6.022x10^23 more ping pong balls, per mile.

Yes, none of the simulations use molecular spin.

Electrons don't spin around an atom. Quantum mechanics doesn't work that way. It's called an electron cloud. And energy levels.

Quantum mechanics and relativity/lightspeed don't mix well. Nor gravity and quantum theory.

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Yes, well that's the problem with modeling, it doesn't necessarily reflect a realistic picture. The spinning tops is, of course, an analogy.

You can picture the electrons themselves as spinning tops.

The Lennard-Jones potential references attribute the effect (sudden strong repulsion when the electron clouds overlap) to the Pauli exclusion principle.

To simplify, using the orbital model, when the electron orbits overlap, the molecules electrons, in effect, have to share the outer orbit. The electron "cloud" is too crowded so the electrons try to jump into a higher orbit, or put another way, the negative charges of the electrons repel. Or the electron cloud wants to expand.

I think, though, Quantum mechanics is itself just another model. But pretty much any model is going to be more accurate than the classical Kinetic, ideal gas law .model of gas particles only repelling, never interacting, "expanding forever" that "Fool" is so in love with and thinks represents reality.

You have the gross over-simplification of the kinetic theory of gases.

Van der Walls is more accurate.

Lennard-Jones is more accurate still, giving a much more realistic picture of gas behavior.

Quantum mechanics gets more granular.

The orbitals can be viewed as standing waves. A string can only vibrate with a certain number of nodal points. Whole numbers or "quanta".

https://youtu.be/I93gZ_zfOpg

String Theory anyone?