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Old   November 28, 2016, 22:41
Default Issue with targeted mass flow at outlet
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Hi all,

I'm having an issue with my boundary conditions for an incompressible steady flow computation.

My domain is quite simple: one inlet, one outlet, and a flow domain with some corners in between.

I know the mass flow rate I want, and if I initialise my domain with a mass flow inlet and a pressure outlet (initial guess of 0 Pa), everything works well and I get final pressures of around 100 Pa at the inlet and -2 kPa at the outlet.

I'm now trying a pressure outlet with an initial guess of -2 kPa for the pressure, and a targeted mass flow rate (with the same magnitude as for the mass flow inlet from before). From what I understand, I should be able to change the inlet BC to a pressure inlet, and get similar results to before. However, I end up with something like a 800 kPa pressure difference over the domain and velocities 2 orders of magnitude higher.

Can anyone shed some light on what is happening here? I feel like I'm missing something obvious but it's got me stumped.

Kind regards,
Chris
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Old   November 29, 2016, 14:44
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Lucky Tran
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Is it a static versus stagnation property problem?

Is the inlet pressure a stagnation pressure or static pressure?
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Old   November 29, 2016, 22:15
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Hi LuckyTran,

Possibly. The total pressure at the inlet in the mass flow inlet case is around 100 Pa (as the velocity is not large), which is not far from the initial guess of 0 Pa of total pressure in the pressure inlet case. I don't really see how this could result in such a huge discrepancy between the two cases.

Kind regards,
Chris
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Old   November 29, 2016, 22:34
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Just to clarify; and you can forget about the initial pressure. What are the actual boundary conditions?

The first case was...
a mass-flow inlet with specified mass-flow rate and inlet temperature
+
a pressure outlet, what was the outlet pressure that you specified? Did you specify -2kPa here?

The second case was...
a pressure outlet w/ targeted mass-flow set to the same mass-flow as the first case
+
a pressure inlet and inlet temperature. What is the inlet pressure you specified? Was it 100 Pa?

If that is all correct. Pay attention to the density in the simulation.

Are you using an incompressible-ideal gas equation of state? Or are you using an ideal gas? Or constant density?
For an incompressible ideal gas, the density is computed using only the temperature and operating pressure (which is a user input). Just some ideas....
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Old   November 29, 2016, 23:22
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Hi LuckyTran,

For the first case
- Mass flow inlet with specified mass flow (final total pressure around 100 Pa)
- Pressure outlet with 0 Pa gauge static pressure (this ends up at -2 kPa at the conclusion of the simulation)

For the second case
- Pressure inlet with 0 Pa gauge total pressure
- Pressure outlet with targeted mass flow and -2 kPa gauge static pressure

The density is constant throughout the domain. The operating pressure is 1 atm.

Kind regards,
Chris
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Old   November 29, 2016, 23:47
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Quote:
Originally Posted by clgs.1903 View Post
- Pressure outlet with 0 Pa gauge static pressure (this ends up at -2 kPa at the conclusion of the simulation)
This is impossible. You cannot specify 0 Pa pressure and have it end up at -2 kPa. If you specified it to be 0 Pa, it will stay fixed at 0 Pa.
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Old   November 30, 2016, 00:15
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Hi LuckyTran,

My apologies, the outlet pressure is indeed -2 kPa, I had re-run this case with slightly modified conditions (-2 kPa instead of 0 Pa at the outlet) and was confusing the two. I'm still having the issue with the pressure inlet BC case.

Kind regards,
Chris
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