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Old   February 3, 2012, 22:53
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Haomin Yuan
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Dear ALL Foamers:

I have a very simple problem but is very difficult for me.

I am simulating a pressure driven compressible flow, ie, the inlet and outlet pressure are given, and velocity is calculated from the inlet and outlet conditions. but from the tutorial cases from compressible solver, velocity is given and pressure is calculated.

Do any of you have any ideas about how to set the boundary condition for pressure driven flow, OR any solver is suitable for this problem?

thank you in advance.
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Old   February 4, 2012, 16:58
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David Gaden
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I think velocity changes and pressure changes are related. So in solving for one, you are left with an unknown constant offset. That is why we need to fix a reference pressure when it is not set at a boundary condition, and I think the same is true for velocity. If the "inlet" and "outlet" are zero gradient for velocity, there is a continuum of possible flow solutions that give the same pressure distribution, even with no-slip conditions on the walls. In other words, your solution may "blow up" with the velocity ever increasing from inlet to outlet for no apparent reason.

/speculation
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Old   February 5, 2012, 12:14
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I had good results with the following settings for compressible, pressure driven flows:

Inlet
p - TotalPressure
U - pressureInletVelocity

Outlet
p - fixedValue (static pressure)
U - pressureInletOutletVelocity

I think, every compressible solver should be okay. I would start with rhoSimpleFoam for stationary flows and/or rhoPisoFoam for transient flows.
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Old   February 6, 2012, 10:03
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Are these BCs ok for an incompressible solver as well?

mad
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Old   February 20, 2012, 17:54
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Quote:
Originally Posted by dre View Post
I had good results with the following settings for compressible, pressure driven flows:

Inlet
p - TotalPressure
U - pressureInletVelocity

Outlet
p - fixedValue (static pressure)
U - pressureInletOutletVelocity

I think, every compressible solver should be okay. I would start with rhoSimpleFoam for stationary flows and/or rhoPisoFoam for transient flows.
I have been looking at the squareBend test case that comes with v2.0.1 as a rhoSimplecFoam test case. This is a high subsonic duct that is set up with a specified mass flow. I thought it would be straightforward to run this case as a pressure driven flow using pressure values from the converged test case. I'm not having any luck. Has anybody done this? What else would have to be done beyond replace the test case entries with the types above?

Test case ----> New case
Inlet
p - zeroGradient ----> totalPressure where p0 = value from test case
U - flowRateInletVelocity ----> pressureInletVelocity

Outlet
p - fixedValue ----> fixedValue (static pressure)
U - inletOutlet ----> pressureInletOutletVelocit

When I do this, the code bombs very quickly-2 iterations. I'm just starting to look at this more closely but I'm still clueless . The only thing I've tried beyond that is run slip walls on the duct, specify a small Pt, and turn off the turbulence model. I'm trying to make the simplest case possible to figure this out. Any tips would be appreciated.
Thanks
Tim
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