Mass flow inlet boundary condition

John10 · December 11, 2022, 15:34

Hello,

I am simulating a steady state case using k-epsilon and the material used is Air ideal gas.

I am trying to fix the mass flow at the domain inlet however when I check the results the velocity is lower than expected and the density higher, since I am missing one variable which I cannot input into CFX-pre. The mass flow is a bit higher than expected.

Using mass conservation and ideal gas law if I provide mass flow, temperature and density everything is fixed while now I only have the first two.

Could it also be a convergence problem? The RMS for mass is in the order of e-7 while momentum e-5 using a fine mesh.

Thanks in advance for any suggestions!

ghorrocks · December 11, 2022, 16:26

Please attach your output file and an image of what you are modelling.

Opaque · December 12, 2022, 10:28

How are you checking the mass flow at the inlet?

massFlow()@Inlet ?

or

areaInt (density * Velocity dot Normal)@Inlet

Though theoretically the same, numerically they are different.

The massFlow()@Inlet must be EXACT what you provided.

December 11, 2022, 15:34	Mass flow inlet boundary condition	#1
John10 New Member Join Date: Dec 2022 Posts: 1 Rep Power: 0	Hello, I am simulating a steady state case using k-epsilon and the material used is Air ideal gas. I am trying to fix the mass flow at the domain inlet however when I check the results the velocity is lower than expected and the density higher, since I am missing one variable which I cannot input into CFX-pre. The mass flow is a bit higher than expected. Using mass conservation and ideal gas law if I provide mass flow, temperature and density everything is fixed while now I only have the first two. Could it also be a convergence problem? The RMS for mass is in the order of e-7 while momentum e-5 using a fine mesh. Thanks in advance for any suggestions!

December 11, 2022, 16:26		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,830 Rep Power: 144	Please attach your output file and an image of what you are modelling. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

December 12, 2022, 10:28		#3
Opaque Senior Member Join Date: Jun 2009 Posts: 1,862 Rep Power: 33	How are you checking the mass flow at the inlet? massFlow()@Inlet ? or areaInt (density * Velocity dot Normal)@Inlet Though theoretically the same, numerically they are different. The massFlow()@Inlet must be EXACT what you provided. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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