Learning Forum

[wayne.sexton]

The example below is a simplied Fluent model of a larger complex system I have. I use a fixed value fluid zone to drive the system flow with inlet & outlet pressure BCs, default air properties and viscuous model. I have found that the face averaged total pressure output (Fluent, results, surface intergals, area-averaged) at the Interior faces does not sum to the static and dynamic pressures for the same faces. At the pressure inlet and outlet BC, it sums as it should. The error appears to be related to hydrostatic pressures but the results are unchanged by the presence or not of gravity. Is unaffected by mesh density. Is also incorrect using mass-average outputs. I have run for excessive iterations. This only happens when using a fixed value fluid zone. Removing the fixed value setting and changing the inlet BC to velocity, Ptot output at the same interior faces does sum to the static and dynamic pressures. 

	Area Average	Area Average	Area Average	Calculated	Difference between
	Pstat	Pdyn	Ptot	Ptot=Pstat+Pdyn	Ptot Output & Calculated
--------------------------------	--------------------	--------------------	--------------------	--------------------	--------------------
	[Pa]	[Pa]	[Pa]	[Pa]	[Pa]
bc_inlet	-2.45	2.45	0.00	0.00	0.0
bc_interior#1	-2.71	2.45	1.09	-0.26	1.4
bc_interior#2	0.30	2.44	2.60	2.75	-0.2
bc_outlet	0.00	2.47	2.47	2.47	0.0

[Essence]

 

Hello,

Please refer this link:

/knowledge/forums/topic/mass-flow-weighted-averaging-vs-area-weighted-averaging/

Please note that here it is suggested to use mass flow weighted and not just the mass weighted.

 

[wayne.sexton]

Thank you for the response. Where do I find mass-flow weighted averaged values? Its not a drop down option under surface intergrals

[Essence]

You can see it in Expressions > Functions > Reduction > MassFlowAve

[wayne.sexton]

Thank you very much, this was very helpful! I intially thought this was an issue with the reporting of total pressure. However, using the expressions, I now believe the issue is with the reporting of static pressure from resports>surface-integrals>area-wt-avg (& mass-wt-avg). This only occurs at the interior faces surrounding a fixed-value fluid volume not at the doimain BCs. I believe a 2x factor is applied to the static pressure. Below is a snipit of a spreadsheet that illustrates my point. Please let me know if this is an issue or if there is a reason for this and its justification.