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Fluid flow mass balance not conserved

Posted Jul 13, 2015, 10:04 a.m. EDT Fluid & Heat, Computational Fluid Dynamics (CFD) Version 4.4 3 Replies

Garrett Jeffries
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I'm having issues getting COMSOL to strictly adhere to a specified inlet flow rate, and for some reason the resulting outflow from my system is also incorrect.

I'm working on a 2D axis-symmetric fluid flow model where I have an inlet/outlet and fluid passes through a porous bed on the way. I'm using Free and Porous Media Flow physics with a stationary solver, where I specify the inlet flow rate as a velocity, and the outlet as a zero pressure boundary (backflow suppressed). The problem I'm having is that my specified flow rate ~= inlet flow rate ~= outlet flow rate (according to a line integration over the inlet/outlet boundaries).

The error in the most basic geometry I set up (flow through a pipe with porous bed somewhere in the middle) gave me an error around 1%, but in the more complex system I've been developing with several inlets/outlets the problem is compounded and I end up with an outflow several orders of magnitude greater than my inflow. I've checked my math backwards and forwards and I do not believe that's the issue here. Is there a common problem with how inlet/outlet boundaries can be set up that would create this issue?

3 Replies Last Post Jun 30, 2016, 12:36 p.m. EDT
Lasse Murtomäki
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Posted: 9 years ago Jul 14, 2015, 3:05 a.m. EDT
This is just a suggestion:

I had a similar problem which was solved with a denser mesh. Fluid dynamics is not the strongest part of Comsol - if I may say so. Especially, if you have fine features, errors pile up high quite fast. You must have realized that when you choose physics defined mesh, fluid dynamics creates a very dense mesh by default.

br
Lasse
This is just a suggestion: I had a similar problem which was solved with a denser mesh. Fluid dynamics is not the strongest part of Comsol - if I may say so. Especially, if you have fine features, errors pile up high quite fast. You must have realized that when you choose physics defined mesh, fluid dynamics creates a very dense mesh by default. br Lasse
Garrett Jeffries
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Posted: 9 years ago Jul 14, 2015, 1:33 p.m. EDT

This is just a suggestion:

I had a similar problem which was solved with a denser mesh. Fluid dynamics is not the strongest part of Comsol - if I may say so. Especially, if you have fine features, errors pile up high quite fast. You must have realized that when you choose physics defined mesh, fluid dynamics creates a very dense mesh by default.

br
Lasse


Thank you for the suggestion! I had previously toyed around with more refined meshing and only in the situations where I use Extra Fine or smaller (all calibrated for fluid dynamics) do I get any changes in my measured flow rates, however the changes are almost negligible. In a case I just ran the new measurement was actually further from my target rate than with a coarser mesh, but the change was very low (<1%).

[QUOTE] This is just a suggestion: I had a similar problem which was solved with a denser mesh. Fluid dynamics is not the strongest part of Comsol - if I may say so. Especially, if you have fine features, errors pile up high quite fast. You must have realized that when you choose physics defined mesh, fluid dynamics creates a very dense mesh by default. br Lasse [/QUOTE] Thank you for the suggestion! I had previously toyed around with more refined meshing and only in the situations where I use Extra Fine or smaller (all calibrated for fluid dynamics) do I get any changes in my measured flow rates, however the changes are almost negligible. In a case I just ran the new measurement was actually further from my target rate than with a coarser mesh, but the change was very low (
FJNB
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Posted: 8 years ago Jun 30, 2016, 12:36 p.m. EDT
I had a similar issue. I solved it using a different inlet boundary condition. Instead of velocity there is one called 'Mass flow' which forces the flow rate to be accurate. Basically it does the line integration for you. Quite handy.
I had a similar issue. I solved it using a different inlet boundary condition. Instead of velocity there is one called 'Mass flow' which forces the flow rate to be accurate. Basically it does the line integration for you. Quite handy.

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