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Conservation of Mass Broken

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History: We have a laminar fluid model with 1 inlet and 4 outlets which we are trying optimize flow equality to. Our boundary conditions contain an outlet volumetric flow rate of 100 [cc/min], an inlet pressure of 0 [Pa], and wall boundaries. Solving the model seen in Figure 1 kept resulting in a matrix singularity error. We tried many things to get away from this error. Adding fluid blocks to the outlets seemed to solve the problem and allows the model to run (Figure 2).

A surface integration of the outlet faces adds to 100% of the flow volume set in our boundary conditions (Figure 3). However, surface integration of the surfaces shown in Figure 4 only add up to a fraction of the total flow set in the boundary condition. The interesting thing is that even though the Figure 4 boundaries only add up to a fraction of the total flow rate, the ratio of flow imbalance between channels is equal.

My only explanation is that the surface integration is only taking a portion of the vector componenet of velocity at the Figure 4 boundaries, and therfore, the ratio is right but the total mass flow isn't accounted for. Is that the reason?

Thanks for your time,

Seth



4 Replies Last Post 08.02.2019, 21:43 GMT-5
Michael Rembe Certified Consultant

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Posted: 6 years ago 05.02.2019, 06:41 GMT-5

High Seth, do you already get any help?

It is not easy to find out the reason for your problem on this way. If your questions are still outstanding you can send me your model file. May be I can give you some hints.

Best regards Michael

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Michael Rembe
Rembe Consulting PartG mbB
www.rembe-consulting.de


High Seth, do you already get any help? It is not easy to find out the reason for your problem on this way. If your questions are still outstanding you can send me your model file. May be I can give you some hints. Best regards Michael

Michael Rembe Certified Consultant

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Posted: 6 years ago 08.02.2019, 04:06 GMT-5

Hi Seth,

do you have received my "private" messages?

Best regards Michael Rembe

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Michael Rembe
Rembe Consulting PartG mbB
www.rembe-consulting.de


Hi Seth, do you have received my "private" messages? Best regards Michael Rembe

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Posted: 6 years ago 08.02.2019, 11:44 GMT-5

Hey Micheal,

I just got to reading them and will get back to you as soon as I have some time to simulate what was discussed.

Thanks so much for the information. It was very detailed and helpful.

Talk to you soon,

Seth

Hey Micheal, I just got to reading them and will get back to you as soon as I have some time to simulate what was discussed. Thanks so much for the information. It was very detailed and helpful. Talk to you soon, Seth

Jim Freels mechanical side of nuclear engineering, multiphysics analysis, COMSOL specialist

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Posted: 6 years ago 08.02.2019, 21:43 GMT-5

Setting your pressure BC at the outlet will likely fix the problem. What you describe is mathematically ill-posed and, indeed, should yield an unstable solution Jacobian matrix and will not solve.

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James D. Freels, Ph.D., P.E.
Setting your pressure BC at the outlet will likely fix the problem. What you describe is mathematically ill-posed and, indeed, should yield an unstable solution Jacobian matrix and will not solve.

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