Discussion Forum

Hi,

It will be easier to understand if you upload your model.

Regards

--
Dima Apter
www.a2z-consulting.com

Hi Dima,

Thanks for your response. Attached is the original file that I downloaded from this forum. It was originally posted at: www.comsol.com/community/forums/fsi/thread/18200 under "simple model of "inflating a cavity" with a fluid".

I comandeered this model as it does most of what I wanted and solves the CFD portion very quickly. There is however something wrong in the structural mechanics computation but I didn't care about that.

To reproduce the problem that I experienced, open the model. There will be an error message but just ignore. (or fix the problem)

To ensure that file is not corrupt, solve the CFD portion. It should be fine. Next, In the Model Builder go to the Geometry and delete all the geometry features. Now draw a simple rectangle. If you can, make the left the inlet, the right boundary the outlet, and the remaining two boundaries the walls.

I was not able to do that. It's almost as if COMSOL saves the boundary assignments. Dima, if you can change the boundaries freely, please tell me what you did.

Also I don't understand why I can't override the "not applicable" boundaries. I sure hope that you can help.

Thanks,

Art

I cannot say much about this specific model because I do not have the Laminar Flow interface and thus cannot open this model. In general, however, COMSOL will often require that you set a couple of nodes in tandem in order to create a boundary condition. It may also require that you include/exclude certain regions from the modeling domain in order for them to be applicable as boundary conditions.

To designate a terminal on an electrical conductor, for example, I'd have to make certain that the "exterior" domain contacting the terminal boundary is not part of the model domain (i.e., it is not included in the list of selected domains for the main physics node). Then I'd have to make sure the boundaries are designated as Magnetic and Electric Insulation (happens by default), and then finally I could add a "Terminal" sub-node to the Magnetic Insulation node and designate the desired boundary as a terminal.

You may need to designate a similar combination of conditions to specify boundaries in your flow problem. The combinations of conditions mostly make sense once you've figured them out. Until you've learned them all, however, it's just a lot of trial and error, and also a lot of very carefully following example problems to see what works. Unfortunately, you probably won't find much documentation for this sort of thing.

Hi KG,

Thanks for taking the time to respond. I appreciate your perspective. BTW, did you try to open the model that I posted? It may not require the CFD module. Worth a try. :-)

There may be some quirks like you describe for the electrical models but I think that COMSOL made some big changes in how boundaries are handled in going from V3.5 to V4.XX.

I think that COMSOL has dumbed down the boundary assignment feature because folks were overriding internal boundaries and getting unrealistic results. In V3.5 it was a piece of cake to override boundaries.

When I open models created originally in V3.5, I am free to reassign boundaries as I wish; models created in V4.XX don't allow me to redraw the geometry and then reassign boundaries.

This really isn't good because prefer to use tried-and-true models rather than always creating a new one from scratch.

In looking through the forum postings, it appears that I'm not alone in having these problems.

Anyone please weigh in.

Thanks,

Art