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No solution after switching from creeping to laminar flow regime

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Hello everyone

I have a problem, where I cannot find a solution to. My problem I want to solve, is a narrow channel (µm range), where in same parts species are generated (transport of diluted species). The flow is then stopped, the species accumulate in the channel and are then flushed away by restarting the flow. So, as my Re number is roughly 1.2 I decided to start with a creeping flow regime to make thinks easier. After a while it works with my geometry well. But now, when I switch to laminar flow (as the Re has to be << 1 for the creeping flow, I want to use laminar flow to have a more realistic model) the solver does not find any solution. I tried all kind of different solver conditions and tried to find the answer here in the forum, but the solver does not convergate and find no solution. I uploaded the file. Hope some of you can help me out here, as I really do not know what the problem is.

Thanks a lot



5 Replies Last Post 2019年5月7日 GMT-4 上午9:56
Jeff Hiller COMSOL Employee

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Posted: 11 months ago 2019年4月29日 GMT-4 下午5:35

Hi Richard,

I didn't try to run it with modified settings, but noticed that you only have one element through the thickness of those flat channels. That wouldn't be enough to capture boundary layers. To keep the number of degrees of freedom low, you can use a swept mesh here. Actually the cylindrical regions also lend themselves well to a swept mesh approach.

There could be other issues as well.

Best,

Jeff

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Jeff Hiller
Hi Richard, I didn't try to run it with modified settings, but noticed that you only have one element through the thickness of those flat channels. That wouldn't be enough to capture boundary layers. To keep the number of degrees of freedom low, you can use a swept mesh here. Actually the cylindrical regions also lend themselves well to a swept mesh approach. There could be other issues as well. Best, Jeff

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Posted: 11 months ago 2019年5月2日 GMT-4 上午7:05

Hi Jeff,

thanks for your help. I will defenitly try to you use the swept mesh. I took a look at the crosssection of the channel, to check how many elements are in the flat channel. It looks like there are not so many, as you mentioned. But would this not also influence the result for the creeping flow? But for the creeping flow the solution can be computed. Anyway, I will try to take a look at the swept mesh, maybe it helps. Thanks :)

Hi Jeff, thanks for your help. I will defenitly try to you use the swept mesh. I took a look at the crosssection of the channel, to check how many elements are in the flat channel. It looks like there are not so many, as you mentioned. But would this not also influence the result for the creeping flow? But for the creeping flow the solution can be computed. Anyway, I will try to take a look at the swept mesh, maybe it helps. Thanks :)


Jeff Hiller COMSOL Employee

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Posted: 11 months ago 2019年5月2日 GMT-4 上午8:38
Updated: 11 months ago 2019年5月2日 GMT-4 下午5:11

Sure, but they are different equations (one linear, one nonlinear) so one could well solve on a mesh while the other doesn't on that same mesh.

Jeff

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Jeff Hiller
Sure, but they are different equations (one linear, one nonlinear) so one could well solve on a mesh while the other doesn't on that same mesh. Jeff

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Posted: 11 months ago 2019年5月3日 GMT-4 上午2:58

Hi,

so I tried a different mesh and also a Swept mesh... (see picture) but I sitll could not find a solution. Any other ideas? When I try to solve this geometry only with a stationary step and with the standard solver, comsol does find a solution. But with the same solver condition (and same mesh) with the transport of diluted species, it does not find one. I really do not know what to change to get it to work.... any suggestions? Thanks a lot!

Hi, so I tried a different mesh and also a Swept mesh... (see picture) but I sitll could not find a solution. Any other ideas? When I try to solve this geometry only with a stationary step and with the standard solver, comsol does find a solution. But with the same solver condition (and same mesh) with the transport of diluted species, it does not find one. I really do not know what to change to get it to work.... any suggestions? Thanks a lot!


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Posted: 11 months ago 2019年5月7日 GMT-4 上午9:56

Hey,

I am still strugling with my problem for the solution. Meanwhile I tried adabtie remeshing and also different finer meshes. I also tried to change my inflow conditions as I thought this might interrupts with the stationary solver, but this also did not lead to a convergence in the solver... I have really no idea anymor. I thought that this is not such a big problem, as I thought that when it works with creeping flow conditions, it might not be a big problem to switch to a laminar flow. So if anyone has an idea to solve that problem, I would be really thankfull!

Hey, I am still strugling with my problem for the solution. Meanwhile I tried adabtie remeshing and also different finer meshes. I also tried to change my inflow conditions as I thought this might interrupts with the stationary solver, but this also did not lead to a convergence in the solver... I have really no idea anymor. I thought that this is not such a big problem, as I thought that when it works with creeping flow conditions, it might not be a big problem to switch to a laminar flow. So if anyone has an idea to solve that problem, I would be really thankfull!

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