Fix damping mass outlet

[bigfooted]

Proposed Changes

Only use the damping factor on the target massflow when dP is relatively large compared to P. This greatly improves the convergence rate and prevents stalling of the massflow rate.

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[pcarruscag]

In the incompressible solver, do we solve for gauge pressure or absolute pressure?

[bigfooted]

In the incompressible solver, do we solve for gauge pressure or absolute pressure?

We solve for gauge pressure, so our target massflow implementation is the same as in Fluent:
https://www.afs.enea.it/project/neptunius/docs/fluent/html/ug/node244.htm

[bigfooted]

our entire incompressible ideal gas:

[pcarruscag]

Then two equivalent incompressible cases with just a different pressure level will behave differently with this change, right?
Perhaps a criterion based on the range of pressures would be more invariant.

[bigfooted]

Then two equivalent incompressible cases with just a different pressure level will behave differently with this change, right? Perhaps a criterion based on the range of pressures would be more invariant.

Do you mean the reference pressure? Yes, if you choose a different reference pressure, and add 1 Pa or 1e6 Pa to it, it will behave differently. So I could take into account p_0 as well.

[bigfooted]

I checked and we never actually add a reference pressure anywhere, and the thermodynamic pressure is only used to compute the (incompressible ideal gas) density. So I think it's fine the way it is currently done.

[pcarruscag]

If you set up a case where you put all your pressures as absolute, and another where you put them as relative (to ambient, or to one of the inlets, or outlets) they will not behave the same with this logic.
One solution is to use a reference based on the difference between min and max pressure in the domain, this quantity should be the same regardless of whether you use relative or absolute pressures.
This is what the Venkat-Wang limiter does to scale the limiter constant.

[bigfooted]

OK, thanks, I understand. I can loop over all inlets and outlets and get the P_max/P_min ratio

[pcarruscag]

For preconditioning we already determine the maximum velocity, you can extend that to the the min and max pressure.
Note that what is invariant is max - min, not max / min

[bigfooted]

Is it sufficient here to subtract the minimum pressure in the domain and use that as the reference pressure, so we compare the update pressure dp to the local pressure (P-P_min_global)

[pcarruscag]

From the invariance perspective, yes, whether it is a good enough way of deriving a reference value, I don't know.

[pcarruscag]

Do we have a test that uses this?

[bigfooted]

Do we have a test that uses this?

Yes, the nozzle case that is failing in the regression tests.

[bigfooted]

The existing nozzle case in the regression test stalls at 99.90% target mass flow rate, the new implementation goes to 99.99%.
Not a large difference for the existing case, other cases show a more dramatic improvement.
For the target mass flow rate, the inlet pressure damping factor sometimes has to be lowered to 0.10 -> 0.04 or so.