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Old   July 2, 2014, 06:05
Default Lower Pressure than outlet
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rainer arenskrieger
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Hi,
I am currently running a simulation on the flow in porous media.
Right know I face quite strange effects when post processing my simulation.
I have boundary condition of
inlet: 5g/s a Fluid
outlet: static pressure : 101300Pa
Multiphase flow: begining: everything filled with Air, then Fluid gets filled inside cavity
Transient Simulation.

When I run the simulation for too long i get the error:
A wall has been placed at the following Domain Outlet to prevent flow inside...

If I run the simulation a bit shorter, so I stop before the error occurs, the pressure at the outlet is lower than the boundary pressure namely 83000Pa.

How is this possible? Any ideas how to prevent this?
I attach a picture of the solution a few timesteps before the error comes up.


Thank you for your help.
Attached Images
File Type: jpg Flow in Domain, Low Pressure.jpg (25.9 KB, 15 views)
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Old   July 2, 2014, 22:20
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Erik
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First of all, you should be using 0 Pa at your outlet, and a reference pressure of 83000Pa. You are getting recirculation, extend your outlet further away from your hump, perhaps 20*h. When you stop it early the model is not converged and you are getting rubbish.
600 psi is a heck of a lot of pressure drop though....
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Old   July 3, 2014, 04:01
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rainer arenskrieger
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Thank you for the advice.

I am running a transient simulation. The main purpose is to detect the flow front, and the pressure in the cavity during the injection.

When I inject the Fluid at the inlet the Air which is in the cavity at the beginning is supposed to be pushed out of the cavity with pressure (outlet pressure =0 )

When running the simulation now I get negative values for pressure which in my opinion is physically impossible for am mass conservation point of view.

As the cavity just has a specific length I think I would change the results if i make it longer.

Tell me if im getting something wrong.
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Old   July 3, 2014, 06:06
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Glenn Horrocks
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Do you mean negative absolute pressure or negative pressure? Negative absolute pressure is a worry in most cases (but note in some extreme cases negative absolute pressure is possible and really does happen) but negative pressure is normal - it just means the pressure is less than your reference pressure.
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a wall has been placed, error #001100279, multiphase flow, porous media

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