Lower Pressure than outlet

rainer_kit · July 2, 2014, 07:05

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.

evcelica · July 2, 2014, 23:20

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....

rainer_kit · July 3, 2014, 05:01

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 i´m getting something wrong.

ghorrocks · July 3, 2014, 07:06

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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July 2, 2014, 23:20		#2
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,186 Rep Power: 23	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....

July 3, 2014, 05:01		#3
rainer_kit New Member rainer arenskrieger Join Date: Jun 2014 Posts: 12 Rep Power: 12	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 i´m getting something wrong.

July 3, 2014, 07:06		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,852 Rep Power: 144	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.