CFX Simulation of flow in a Pump

stephec1 · March 19, 2020, 18:03

I have steady state setup in CFX for a centrifugal pump rotating at 1450rpm, inlet BC of 0atm rel. total Pressure and 4.167kg/s Outlet BC with SST turbulence model. Ref pressure is 1 atm. Refined the grid to a Yplus of less than 5 for both the impeller and the volute. The problem arise when I run the simulation cause the RMS residuals be dropping and then suddenly increases to until it fails. The RMS P-Mass residual would drop and suddenly rise and be constant till it fails. On the Out File it would be saying a wall has been placed at the outlet if I run with only the impeller and at the inlet if I run with the impeller and the volute interfaced. Anybody who knows what might be causing the failure please help.

ghorrocks · March 20, 2020, 00:39

Failures like this are common caused by something getting convected to the outlet boundary, and the outlet boundary cannot handle it. Usually it is a recirculation, but it could be other things. Have a look at the results file just before it crashes and pay close attention to the outlet.

If it is a recirculation causing problems at the outlet you have many options, the most reliable being to extend the outlet boundary further downstream to a location beyond the recirculation.

stephec1 · March 20, 2020, 05:18

What is most surprising is the wall forms at the inlet of the pump not at the outlet

ghorrocks · March 20, 2020, 05:22

Oh, in that case the recirculation is hitting the inlet boundary. This happens as well, and the fix is the same - move the inlet boundary further upstream to a location where there is no back flow.

Opaque · March 20, 2020, 07:49

Just a thought, have you try running with a mass flow inlet condition instead.

It seems you are running incompressible fluid, correct? Then, pressure level does not mean much, just the change between inlet and outlet.

Hope the above helps,

Muhammad Asif · March 20, 2020, 23:51

Hello Everyone,
I encounter the Informational problem while updating setup to go to solution.
Snapshoot attached,
Thanks in advance for Kind reply.

ghorrocks · March 21, 2020, 05:13

Do not post new questions on existing threads. For a new question, post a new thread.

But the error message in your case says exactly what the problems is - you cannot run the simulation until you set it up in CFX-Pre (the setup step).

March 20, 2020, 00:39		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	Failures like this are common caused by something getting convected to the outlet boundary, and the outlet boundary cannot handle it. Usually it is a recirculation, but it could be other things. Have a look at the results file just before it crashes and pay close attention to the outlet. If it is a recirculation causing problems at the outlet you have many options, the most reliable being to extend the outlet boundary further downstream to a location beyond the recirculation. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

March 20, 2020, 05:22		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	Oh, in that case the recirculation is hitting the inlet boundary. This happens as well, and the fix is the same - move the inlet boundary further upstream to a location where there is no back flow. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

March 20, 2020, 07:49		#5
Opaque Senior Member Join Date: Jun 2009 Posts: 1,803 Rep Power: 32	Just a thought, have you try running with a mass flow inlet condition instead. It seems you are running incompressible fluid, correct? Then, pressure level does not mean much, just the change between inlet and outlet. Hope the above helps, __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

March 21, 2020, 05:13		#7
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	Do not post new questions on existing threads. For a new question, post a new thread. But the error message in your case says exactly what the problems is - you cannot run the simulation until you set it up in CFX-Pre (the setup step). __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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March 20, 2020, 05:18		#3
stephec1 New Member Calvin Stephen Join Date: Feb 2020 Posts: 4 Rep Power: 6	What is most surprising is the wall forms at the inlet of the pump not at the outlet