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Fluid flow mass balance not conserved
Posted 13.07.2015, 10:04 GMT-4 Fluid & Heat, Computational Fluid Dynamics (CFD) Version 4.4 3 Replies
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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?
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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
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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%).
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