CFD question: ventilation, inlet or outlet?

[RatonLaveur]

Hi gents,

I will soon model the extraction flow of a chamber, but i wonder, what boundary condition should I use for the extraction face?
The underlying interrogation is whether Inlet and Outlet are just names or if they are actually really specific to allow good calculation or calculation at all.

Since we're here, if i want to model a vacuum chamber in that case, how can I reconcile the absence of (physical) inlet?

[thschrader]

screenshot? FC-file?

[RatonLaveur]

Hi ok, simple screenshot to support my question:

Case1: Ventilation in a chamber

2020-05-11 21_45_20-FreeCAD 0.19.png (63.27 KiB) Viewed 2719 times

I want to apply suction (extraction) to the left "outlet" and allow airflow to come in from the top "inlet".
In this case, what constraints should I use? I'm asking because I am not quite sure how the computation will treat it. For example, the most sable simulations are with a fixed flow at the inlet and a static pressure at the outlet (Flow-Pressure). But you can also simulate Pressure-Flow for example and Pressure Pressure however they are not recommended from what I am reading.

Hence the question, how should I configure the constraints in that case?

Case2: Vacuum chamber

2020-05-11 21_44_53-FreeCAD 0.19.png (62.65 KiB) Viewed 2719 times

Here I'd like to suck-in air from the only outlet to the left. The main idea is to see from a transient point of view how the sucked flow will behave. For example, if trying to use such a vacuum chamber for a Physical Vapor Deposition of a very delicate sample, I want the sample to be unaffected during the venting of the atmosphere. In such a case I would apply the same answer to the "inlet" as for case 1, but how can I deal with the absence of outlet condition? The simulation will simply not run. Any idea?

[thschrader]

Case1:
When you know the extraction rate at the outlet, you can calculate the volumetric flow through the outlet face.
This value you can use as the inlet boundary condition (for incompressible fluid).

case1_suction.FCStd

(33.96 KiB) Downloaded 91 times

Case2:
I think this is a case for a compressible solver, see shocktube in openfoam-tutorials.
You can run the shocktube directly in the bluecfd-core console. A fast pressure wave runs
through the tube in milliseconds.

test.JPG (57.72 KiB) Viewed 2691 times

[RatonLaveur]

Hi tschrader, thank you for the info.

I was thinking indeed that the flow of extraction (linked to a pump obviously) would be the official "inlet" but that's what I wanted to confirm.
I find "inlet" and "outlet" can be rather in-intuitive in some CFD definitions. In such cases as described here, it seems more like the "input" face and the "output" face, then again Output and Input are very specific terms...

At any rate your answer was spot-on thanks a lot.

Regarding the vacuum, ok I understand there is no current way to do it all in FreeCAD (and that's fine ).

[oliveroxtoby]

Hi tschrader, thank you for the info.

I was thinking indeed that the flow of extraction (linked to a pump obviously) would be the official "inlet" but that's what I wanted to confirm.
I find "inlet" and "outlet" can be rather in-intuitive in some CFD definitions. In such cases as described here, it seems more like the "input" face and the "output" face, then again Output and Input are very specific terms...

It should also work if you set the outlet as a velocity outlet and the inlet as a pressure inlet.

Regarding the vacuum, ok I understand there is no current way to do it all in FreeCAD (and that's fine ;) ).

The other case would, indeed, need compressible flow. Since a low-mach option is not yet available in CfdOF, HiSA would be your only option at present. You can do this in CfdOF by choosing 'Compressible', 'High Mach Number' in PhysicsModel.

[RatonLaveur]

It should also work if you set the outlet as a velocity outlet and the inlet as a pressure inlet.

Hello Oliver, thanks for dropping by with some more knowledge. Is that to say that essentially the Solver doesn't really care what the Inlet and Outlet are? it does not prioritize?
So it would be safe to assume that as long as I choose the right couple of (inlet;outlet) boundary conditions (i.e. FlowPressure or PressureFlow) it will manage?

[Macer1902]

Hi guys,

I´m a student from Germany and I´m currently dealing with the simulation of water jet cutting. There is a pressure (e.g. 1000 bar water pressure) on one inlet side and ambient pressure 1 bar (air) on the inlet 1. The outlet is the small diameter at the end (outlet). When I start the simulation there is always an error.
Now my questions are:
1) is it even possible that there is water pressure on the inlet and air pressure on inlet 1?
2) are my limits correct? I have Inlet, Inlet 1, Outlet and all other pages as "Wall" or do I have to specify the symmetry area as such?
3) what do I have to set for the solver to get a plausible result?

Feel free to edit my file. Thank´s for helping me.

greetings from germany and a nice weekend.

[RatonLaveur]

Hello Macer,

I think you need to open a whole new topic for your problem. This is a Multi-Phase (water and air), Viscous, Compressible model...at least. In CfdOF's current implemention, lest I am mistaken, Multi-Phase is automatically Transient. These are all keywords that entail heavy computation.

Please: open a new topic on your very interesting problem. Describe it in as much detail as possible, but more importantly, describe what you are trying to achieve. What result do you want. It might be possible to reduce the problem to simpler chunks.

I think the CFD wizards around these parts may help you achieve your goals if they are so inclined

[thschrader]

This is a very ambitious project.
From Bernoullis equation you can estimate the velocity at the nozzle to appr. 450 m/s.
At the smallest diameter the velocity reaches up to 1600 m/s. The fluid needs only 1,6*E-4 seconds
to run through the “needle”.

What are your domains of interest? Maybe you can subdivide your model.
I do not understand the air-inlet. Only 1 bar? How will you press the air into water with 1000 bar?

For a two-phase model you need the interfoam-solver. But I have never seen a simulation using
interfoam at such high speed. But maybe you can use an average density for the water-air (and sand?) mixture and run a fist steady-state simulation with simplefoam (by neglecting the air-inlet).
Good luck

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