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vijeshravi December 7, 2017 11:54

Boundary Condition Mass Flow Rate does not match the CFX Solver result

I am trying to solve a simple Y-divider pipe problem. I have one Inlet and 2 Outlets. The Boundary Conditions that I give to the Inlet and the Outlets are Mass Flow Rate (0.02 kg/s) and the relative pressure (0Pa). The problem solves and the solution converges. When I check the Mass Flow Rate at the Inlet in the CFX result, I get a value at the inlet which is 100 times less than the actual input I give. I get 0.0002 kg/s whereas my input is 0.02 kg/s.

I tried calculating velocity based on Mass Flow Rate, Density, and Area and gave that as input. But, I still get the same wrong Inlet on the CFX result.

Please help me out what could be the possible thing that I could be doing wrong here? I tried increasing the convergence criteria, tried different gas. Nothing helped. I even forced the solver to take 1/4th of the actual inlet at one of the outlets and I get the same result.

evcelica December 7, 2017 13:36

sounds like you are doing something wrong when you typed in the value. Hard for us to tell what the problem is.
do you have the out file? That would be good to post.

vijeshravi December 7, 2017 13:59

2 Attachment(s)
Hello Evcelica! Thank you so much for the reply. I have attached the CFX out file here. Please take a look at it and help me out where I am doing wrong.
I have also attached the images of the Boundary Conditions and the CFX result that I was talking about.

ghorrocks December 7, 2017 22:47

How are you calculating the mass flow rate in the post processor?

Your output files show you converged until the imbalances are quite small, so that says it should have converged very close to your desired flow rate.

vijeshravi December 8, 2017 08:10

1 Attachment(s)
Yes! The solution converges pretty well with high accuracy. I used the expression massFlowAve(Mass Flow)@Inlet in the CFX post processor. Also, I generated a Mass Flow Rate Contour at the Inlet to check the mass distribution.

In my previous reply, along with the CFX out I have attached the images of the result. I will reattach the same here. Am I checking the results wrong?

Lance December 8, 2017 08:48


Originally Posted by vijeshravi (Post 674494)
I used the expression massFlowAve(Mass Flow)@Inlet in the CFX post processor.

If you only want the massflow rate you should use massFlow()@Inlet instead.

vijeshravi December 12, 2017 15:47

1 Attachment(s)
Hello Lance,

Thank you so much for the help. massFlow()@Region solved the problem. Though, one thing is confusing. I create a mass flow contour at the inlet and the value does not match the value that I get using the expression (massFlow()@Inlet). Any thought on that. I have attached the Inlet Mass Flow Contour.

Opaque December 13, 2017 08:49

The contour of mass flow works on the mass flows on each face of the mesh (specifically integration points on the faces).

massFlow()@Inlet is the summation of all those face mass flows.

They cannot be the same.

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