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Monitor Point expression value different from same expression in CFX-Post

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Old   March 13, 2018, 16:05
Default Monitor Point expression value different from same expression in CFX-Post
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David VanBronkhorst
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Running an oil flow simulation (laminar flow) with Pt specified at inlet and Ps at outlet. The mass flow is a desired output.

I set up a monitor point with the expression:

massFlow()@Inlet

During the solver run it is reported on a graph with no units specified along the Y-axis. The value of the monitor point expression is about .009 which is not what I expect for mass flow. So I stop the solution and open the results.

In CFX-Post, using the function calculator I see mass flow at the Inlet = .021 lbm/sec (which is what I expected).

Why is the monitor point expression different from the function calculation in CFX-Post? Is this a unit issue? How can I tell which units the solver monitor presents the monitored expression? Or is there something else I am missing? I am not strong on CEL, have I erred somewhere in that simple expression?

Thanks,
David
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Old   March 13, 2018, 16:37
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Gert-Jan
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Quite convinced it is in kg/s. So, 0.021 lb/s*0.453592 = 0.0095.

BTW, there is no need to make a monitor for the massflow in CFX-Pre beforehand.
It is in the Solver manager by default. Just create a new graph there on the fly, and select FLOW>Boundary>Inlet>P-Mass.
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Old   March 13, 2018, 16:42
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Ah, yes, thank you - I have found the built-in mass flow monitor under Montors>Momentum and Mass>Mass Flows

kg/s makes sense, too, since this is most likely the setting for 'solution units'. Thanks for the info, much appreciated.
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Old   March 13, 2018, 16:51
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Glenn Horrocks
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First of all check you are comparing the same thing - is the monitor point expression giving you the same iteration as the results file?

There is a fundamental difference in the way expressions are calculated in the solver versus CFD-Post. In the solver it uses the full integration points to evaluate expressions, but in CFD-Post only the nodal values get put in the output file so calculations are done on nodal values only. This means there can be differences between the solver and CFD-Post, but the difference should be small on any reasonable mesh and should approach zero with mesh refinement. Note that the solver calculated values are more accurate.
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