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Fluid not moving even after defining B.C.s and velocity

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I have modeled an electrolyte within a nozzle surrounded by air. I have also modeled a separate electrolyte domain (a meniscus) at the outlet of the nozzle. The meniscus was drawn on a work plane and then revolved to make a 3D part, Basically, the fluid should diffuse at the meniscus / air interface and generate a convective flux within the electrolyte.

The issue is that even after defining all the necessary B.C.s (and a fluid velocity), the fluid does not move. I can see this by plotting spf.U in the results sections after running the studies. At this point, I really have no clue what could be causing this. I am obtaining inaccurate results due to the fluid not flowing.

I have uploaded a file below. Can someone please take a look and see if the issue is apparent? I have tried a lot of different ways to fix this yet still get the same results. I have created the same model in 2D / 2D axisymmetric and had no issues.



5 Replies Last Post 20 gen 2021, 18:22 GMT-5
Robert Koslover Certified Consultant

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Posted: 4 years ago 19 gen 2021, 23:02 GMT-5

First, two caveats: (1) I can't actually run your model because I don't have at least some of the necessary license(s), and (2) I seldom do fluid models, so please don't trust my advice too much! That said, I took a look at your file anyway. Hmm! Now, again, I am probably just not sufficiently familiar with this type of model, but I just have to ask: where did you specify (or try to specify) a non-zero fluid velocity? Every place that I looked, where it seemed (to me) possible to specify a fluid velocity, it also seemed that the fluid velocity was being defined to be zero! But I probably just missed it (really, no kidding), so... where should I (or anyone else, who may wish to help you) look for it? You specified that velocity somewhere, right?

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Scientific Applications & Research Associates (SARA) Inc.
www.comsol.com/partners-consultants/certified-consultants/sara
First, two caveats: (1) I can't actually run your model because I don't have at least some of the necessary license(s), and (2) I seldom do fluid models, so please don't trust my advice too much! That said, I took a look at your file anyway. Hmm! Now, again, I am probably just not sufficiently familiar with this type of model, but I just have to ask: *where* did you specify (or try to specify) a *non-zero* fluid velocity? Every place that I looked, where it seemed (to me) possible to specify a fluid velocity, it also seemed that the fluid velocity was being defined to be zero! But I probably just missed it (really, no kidding), so... where should I (or anyone else, who may wish to help you) look for it? You specified that velocity *somewhere*, right?

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Posted: 4 years ago 19 gen 2021, 23:05 GMT-5
Updated: 4 years ago 19 gen 2021, 23:11 GMT-5

The fluid velocity is specified as the "outlet" B.C. of the laminar flow module - the actually velocity field (spf.U) is specified in the TDS2 module, but the two physics are coupled. I just plugged in an abritrary value for the fluid velocity to verify that it would give the same results (for example the actual outlet velocity is related to the diffusion rate as calculated in the stationary step, but this is not the issue)

The fluid velocity is specified as the "outlet" B.C. of the laminar flow module - the actually velocity field (spf.U) is specified in the TDS2 module, but the two physics are coupled. I just plugged in an abritrary value for the fluid velocity to verify that it would give the same results (for example the actual outlet velocity is related to the diffusion rate as calculated in the stationary step, but this is not the issue)

Robert Koslover Certified Consultant

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Posted: 4 years ago 19 gen 2021, 23:48 GMT-5
Updated: 4 years ago 19 gen 2021, 23:58 GMT-5

Ah, now I see it! Thank you. But of course I don't know enough about this kind of problem to actually help you. Sorry about that. So the same approach worked in axi-sym 2D, hmm? I'll bet you must have done something different there, but otherwise I have no idea. Maybe you should post that model too? Perhaps someone else here can figure it out. Best wishes and good luck!

p.s. You've got a rather complicated set of couplings and physics going on here. Are they set up in exactly the same arrangement/order as in your successful 2D models? Might there be a subtle difference that is to blame in the 3D case? If you haven't already, I suggest you compare the 2D vs 3D models closely, including all the bcs, solver configurations, etc. It might take some time.

-------------------
Scientific Applications & Research Associates (SARA) Inc.
www.comsol.com/partners-consultants/certified-consultants/sara
Ah, now I see it! Thank you. But of course I don't know enough about this kind of problem to actually help you. Sorry about that. So the same approach worked in axi-sym 2D, hmm? I'll bet you must have done something different there, but otherwise I have no idea. Maybe you should post that model too? Perhaps someone else here can figure it out. Best wishes and good luck! p.s. You've got a rather complicated set of couplings and physics going on here. Are they set up in exactly the same arrangement/order as in your successful 2D models? Might there be a subtle difference that is to blame in the 3D case? If you haven't already, I suggest you compare the 2D vs 3D models closely, including all the bcs, solver configurations, etc. It might take some time.

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Posted: 4 years ago 20 gen 2021, 11:12 GMT-5

Thank you for your input. I think the issue was that all time dependent solutions/dependent variables were being stored in the solution store I defined for the stationary solver. In my 2D model, I had separate solution stores / dependent variables for the stationary and time dependent variables. Now I am getting the error "Failed to find consistent initial values." Just replacing one problem with another, I guess.

I've tried a few fixes but haven't had any luck, so I'll keep troubleshooting.

Thank you for your input. I think the issue was that all time dependent solutions/dependent variables were being stored in the solution store I defined for the stationary solver. In my 2D model, I had separate solution stores / dependent variables for the stationary and time dependent variables. Now I am getting the error "Failed to find consistent initial values." Just replacing one problem with another, I guess. I've tried a few fixes but haven't had any luck, so I'll keep troubleshooting.

Robert Koslover Certified Consultant

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Posted: 4 years ago 20 gen 2021, 18:22 GMT-5

Sounds good. Methodical perseverance is often necessary! When you succeed, just be sure to tell all your coworkers that it was easy.

-------------------
Scientific Applications & Research Associates (SARA) Inc.
www.comsol.com/partners-consultants/certified-consultants/sara
Sounds good. Methodical perseverance is often necessary! When you succeed, just be sure to tell all your coworkers that it was easy.

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