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cfduser9 November 4, 2011 15:48

Operating Pressure Issues
I am attempting to model fluid flow through a body in space. I have a fluid body with an inlet and an outlet. The inlet is choked, has a specified mass flow rate, and non-zero gauge temperature, while the outlet has a gauge pressure of zero. The operating pressure is zero.

1.) Is this the correct way to represent a space situation? Simply setting operating pressure and outlet gauge pressure to zero?

2.) I keep getting "Divergence Detected in AMG solver" errors. However, when I solve at an operating pressure of pressure of 101325 Pa, the simulation works fine. Why does it do this, and how can I prevent it? I'm pretty sure I want 0 operating pressure.

Thanks for any help.

delaneyluke November 7, 2011 02:51

What is your mass flow rate? Most probably at vacuum pressures, your flow will not obey continuum physics and will be a free molecular flow. Calculate the flow Knudsen number to find out. A free molecular flow cannot be modeled directly with CFD. If you're only interested in velocities and not pressure drop, then you could model a free molecular flow using particle tracking.
Note: If your density is modeled with an ideal gas formulation you cannot specify your operating pressure as zero. That makes your density zero.
Vacuum pressures are generally specified with a very low pressure (not zero).


sandeep_tu November 9, 2011 07:38

When you model your density through incompressble then you cannot specify as zero for operating pressure (i.e the reason you are getting the error-"Divergence Detected in AMG solver" ).

As you said you are pretty sure you want to use 0 operating pressure then shift the density modelling to ideal gas.

cfduser9 November 26, 2011 16:32

My mass flow rate is .005 kg/s. Regardless, my operating pressure is not zero. I've set it to be 50 Pa, and I still get a temperature divergence. I am using ideal model for the gas.

Still no luck. It diverges regardless of how much I fiddle with the under relaxation factors. My outlet is made up of 32 smaller outlets. I really just want to know if the flow through each of those is even. I'd prefer not to have to treat it as a molecular flow.

roukaia November 27, 2011 15:33

doubt concerning gauge total pressure plz help
i'm simulating the compression stroke of ic engine in FLUENT 13. i'm getting pressure only 22 bar at the end of compression stroke (ideal gaz). experimentaly it is around 48bar. why the pressure is not rising the compression ration is about 18
i am using these conditions for intake pressure inlet
gauge total pressure 0
operating condition 10 1352 pascal

exhaust pressure outlet

but when i used gauge total pressure 101352 i get 50 bars
am i right ? iam really confused about the result because pressure abs=pressure gauge+pressure operating conditions

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