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Backflow into inlets with multiple inlets

Posted May 14, 2019, 3:09 p.m. EDT Fluid & Heat, Computational Fluid Dynamics (CFD) Version 5.4 3 Replies

Claus Mcintyre
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Hi everyone,

I am simulating laminar flow through a thin geometry with two inputs on one side and one outlet on the other side (see image for the geometry). When initializing the inputs via velocities I get the expected results - see the corresponding image (the upper inlet has 10x the velocity of the one below). But with pressure as inputs, the simulation results in a (back)flow between the two inputs.

Obviously I enabled "suppress backflow" for both inlets but this only seems to account for the "own" input and not the other one. I tried to add weak constraints but these can only be applied to a volume. Is there an option to define equations on a surface?

An option I found is to take fans as input and output but these require pressure curves and the system will not be fan-driven.

I also added the MPH file.

best regards Claus

3 Replies Last Post May 15, 2019, 9:24 a.m. EDT
Michael Rembe Certified Consultant
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Posted: 6 years ago May 15, 2019, 8:41 a.m. EDT
Updated: 6 years ago May 15, 2019, 9:15 a.m. EDT

Hello Claus,

I downloaded your files and I think, your problem can be solved! Do you already get a sufficient solution?

I give you some hints from my point of view: 1.) Use a mesh, that is fine enough to resolve the lower thin flow domain. There should be more than one elements in y direction. 2.) May be the pressure/pressure BCs don't result in a stable solution. In this case you can combine the velocity inlet BC with ODEs in such a way, that the velocities are calculated to give the appropriate pressures.

If I can help you please contact me. You can also use my e-mail adress: "michael.rembe@rembe-consulting.de".

Best regards Michael

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


Hello Claus, I downloaded your files and I think, your problem can be solved! Do you already get a sufficient solution? I give you some hints from my point of view: 1.) Use a mesh, that is fine enough to resolve the lower thin flow domain. There should be more than one elements in y direction. 2.) May be the pressure/pressure BCs don't result in a stable solution. In this case you can combine the velocity inlet BC with ODEs in such a way, that the velocities are calculated to give the appropriate pressures. If I can help you please contact me. You can also use my e-mail adress: "michael.rembe@rembe-consulting.de". Best regards Michael
Michael Rembe Certified Consultant
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Posted: 6 years ago May 15, 2019, 9:09 a.m. EDT

Claus, did you check if your flow is laminar or turbulent?

Regards, Michael

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


Claus, did you check if your flow is laminar or turbulent? Regards, Michael
Michael Rembe Certified Consultant
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Posted: 6 years ago May 15, 2019, 9:24 a.m. EDT
Updated: 6 years ago May 15, 2019, 9:40 a.m. EDT

Claus, I checked your model. All boundary conditions (BC) as pressure BC can not work! Because there are differences between the pressures of the two inlets the fluid must flow out of the "inlet" with the lower pessure! There is a small pressure gradient! It is a nunmerical problem. Because water nearly is incompressible in reality backflow is prevented by pumps or valves.

May be you can use an average pressure for both inlets. Solve for the steady state flow. Then change the pressures and solve for the transient flow. This will give you a more smoother solution.

What do you want to evaluate with your simulation? Best regards Michael

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


Claus, I checked your model. All boundary conditions (BC) as pressure BC can not work! Because there are differences between the pressures of the two inlets the fluid must flow out of the "inlet" with the lower pessure! There is a small pressure gradient! It is a nunmerical problem. Because water nearly is incompressible in reality backflow is prevented by pumps or valves. May be you can use an average pressure for both inlets. Solve for the steady state flow. Then change the pressures and solve for the transient flow. This will give you a more smoother solution. What do you want to evaluate with your simulation? Best regards Michael

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