Discussion Forum

In general, the answer depends on SYMMETRY. There are TWO kinds of symmetry you MUST consider:
(1) Symmetry in the geometry.
(2) Symmetry in the physics (to include the expected solution of the problem)

If both the geometry and physics exhibit a symmetry with respect to a plane, you can generally cut the problem along that plane, and limit your geometry and model to one half of the original, since the solution in the other half follows immediately from the symmetry. To do this right, you will need to impose the correct boundary condition on the plane involved. If you have a good understanding of the physics, it shouldn't take you too long to figure out what boundary condition is required in any particular case. And in many cases, the appropriate condition will be available in one of the Comsol - supplied lists of BCs to choose from.

In some cases, there are multiple symmetries and multiple symmetry planes. Hope that helps. Good luck.

Thanks for your reply.

I have some questions because i am a beginner using comsol. How do i cut the geometry? Is it the same like to draw or build only the half geometry.

I have tried it like that. I have built my geometry, then i took a workplane and use the partion object function. After that i have selected under the boundaries in the physics menu the option symmetry. Then i have selected the half of the partioned geometry under the symmetry boundarie. But the problem is, that the solution i get isn`t correct.

Romol

1. Be aware that you will probably have to work through *many* examples, and *many* problems of your own, to become reasonably proficient with this tool. Work through some of the tutorials and other example models provided by Comsol. Take your time.
2. I seldom use the partition tool, but that's partly because it was not a part of older Comsol versions. If partitioning isn't giving you the geometry you want, you can either draw just the fraction of the part you actually want from scratch, or you can use Boolean operations (differences and intersections, especially) to eliminate any unneeded regions and to provide the problem geometry in the shape that you want. I use Boolean operations often, when I create my models.

Yes, adding blocks that you then subtract using a Boolean operation (a Difference feature) is a good way to create a part that is half of or a quarter of the original object.

Also, many of the physics interfaces in COMSOL Multiphysics provide predefined Symmetry and Antisymmetry boundary conditions that you can use on the symmetry boundaries.

Finally, there are data sets available that make it possible to visualize the full 3D solution for some cases: Use a Revolution 2D data set to take a 2D axisymmetric solution into a full 3D cylinder, or use a Sector 3D data set to take the quarter part and make it into 4 sectors using rotation so that they together form a full 3D object. You can also make these extensions so that they do not make up a full 3D object (using a revolution that is <360 degrees, for example) if you want to achieve a plot where you can "look into" the 3D structure by removing a "slice".

Best regards,
Magnus Ringh, COMSOL

Thanks for your replies.
I have tried the hints you gave. To understand i began with a very simple geometry - a cylinder.
Then i have applied a moment of 215 Nm and a little boundary load. I have simulate the full geometry and get results for the stress like expected. The stress increases radial. Then i have built only a quarter of the cylinder and defined the two cutting planes and selected these planes for the symmetry boundary like in the comsol video tutorial of the capacitive pressure sensor. The results i get are totally different. I have attached the mph-file. I hope you see the error and can help me.

Romol

Thanks for your replies.
I have tried the hints you gave. To understand i began with a very simple geometry - a cylinder.
Then i have applied a moment of 215 Nm and a little boundary load. I have simulate the full geometry and get results for the stress like expected. The stress increases radial. Then i have built only a quarter of the cylinder and defined the two cutting planes and selected these planes for the symmetry boundary like in the comsol video tutorial of the capacitive pressure sensor. The results i get are totally different. I have attached the mph-file. I hope you see the error and can help me.

Romol

Hi

But are you sure its a "symmetry" BC you should apply on the quarter edges ?
That will invert the torque per quarter too, no ?

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Good luck
Ivar

Hi Ivar,

i dont understand it. Everywhere i searched i found that you must define a symmetry plane. Why should i choose only the edges?

Romol

Hi

For me its rather an anti-symmetry plane you should use, make a small sketch and compare the two, and check the results

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Good luck
Ivar

Thanks Ivar! Now i understand it and it works.

Romol