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Boundary condition to simulate a compliant volume

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Old   November 11, 2020, 03:25
Lightbulb Boundary condition to simulate a compliant volume
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Hello world from sunny South Africa,

I am looking for help to solve what seems should be a relatively simple problem. I am new to openFOAM and have googled this particular thought but not come up with many results other than programming a new BC which seems quite daunting.

I would like to define a BC that essentially acts as a compliant isentropic volume. In other words, consider the boundary as a gas balloon/accumulator/lung etc that expands and as it expands its pressure rises as the volume expands/stretches. The pressure will rise (compressing the gas) because of the compliance of the volume walls. I am not interested at this point to consider thermal effects as the process happens slowly and at low pressures (5cmH20 to 40cmH20). I define the volumes compliance as (change in volume/change in pressure). EG if the volume changes by 50ml then the pressure goes up by 5cm.

The hope is to model a lung without the computational overhead of:
-Modelling the actual volume as a 'static' mesh and running a compressible solver
-Modelling a moving mesh system like a piston in a cylinder

The following BC's showed potential but thought I would reach out and see if im missing something simpler:
- codedFixedValue (not sure this has the ability to look at dV/dT)
- externalCoupled (seems most likely to work)
- Possibly using an energy BC like gradientEnergy (doubt this will work)
- plenumPressure (too defined for my use)
- syringePressure (too defiend for my use)

I think the most likely is attempting a pseudo-custom BC and emplying externalCoupled which I assume will involve a Python script to pull data from the OF results, make a calculation and then return/apply the boundary pressure based on the volume of air that has moved through the boundary.

Would appreciate some insight!
Thanks.
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Old   January 19, 2021, 03:09
Cool Any thoughts?
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Bump? Any thoughts?
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Old   November 3, 2021, 01:31
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Intruiging that this question has not come up before and seemingly no-one here can suggest a good solution.
If anyone is interested, I did manage to get a solution working using the external coupled solver but its very inefficient due to the built in wait time delays in the OpenFOAM external solver routine. I have also found that the postProcessing routine seems to fail in calculating the correct flow through a boundary when using an adaptive mesh (which of course leads to a failed volume calculation)
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Old   November 27, 2021, 18:18
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In the OpenFOAM.com line, there seems to be a boundary conditions for this:

plenumPressure

https://www.openfoam.com/documentati...ary-conditions
https://www.openfoam.com/documentati...d.html#details
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