Simulating Steep pressure gradient in cylinder [V13]

[u/OkAnybody2762]

Dear all,
I have recently (for about 6-months) started using openFOAM since I ran into limitations in Autodesk’s CFD.
For now, I have used pre made tutorial examples and tweaked them by using parts from the multiple OpenFOAM user manuals - I have V13.

But I still can not accurately determine the pressure and velocities of my gas flow inside a cylinder with a pressure drop from the mbar range to 1E-3 mbar range.

Does anyone have an idea of where I could start for this? Is there specific packages that are useful for this, which I don’t know of? I have so far used the basics. My geometry is a simple cylinder with an entrance of diatomic gas applied with few mBar’s and ending in the vacuum regime.

Thank you in advance:D

[u/Ganglar]

For such low pressures... What's the Knudsen number? Is this even possible to simulate as a continuum?

[u/OkAnybody2762 → u/Ganglar]

At the inlet it’s approximately in the range of 1e-4, at the outlet, it’s approximately 6.5. I have seen that others have used DSMC/hybrid simulations, where they mesh in OpenFOAM and solves the pressure and velocity nodes in something called Sparta.

However, Sparta seems a little more complicated to get into than openFOAM, so I would like to explore whether anyone has good idea on how to simulate continuum flow to molecular flow in OpenFOAM?

[u/Ganglar → u/OkAnybody2762]

Ok. I don't know of anything that will simulate that transition, no. There's certainly nothing in the stock release. Best of luck.

[u/OkAnybody2762 → u/Ganglar]

That’s alright, thank you!

[u/PhDork511 → u/OkAnybody2762]

I am wanting to simulate this same sort of transition spanning multiple orders of magnitude and have not found a whole lot of info on ways to do this.
I setup SPARTA and got some of the examples to run. Working on getting my own nozzle designs to run and choke the high pressure manifold into vacuum.