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Confusion about subtracting two geometric entities

Posted Jan 21, 2021, 3:07 p.m. EST General, Geometry Version 5.3a 3 Replies

Vivek Gupta
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I have a question about using the Difference boolean operation when building a 3D geometry.

Let's say I am trying to create a final geometry composed of two parts: (1) a cube which is completely solid except for a hole in the center (denoted by C), and (2) a solid sphere (denoted by B) that fills the hole in the center of C.

So the steps I take to create this are (COMSOL model attached): 1) Create a solid cube (denoted by A). 2) Create a solid sphere inside the solid cube (aka B). 3) Do the Difference operation A - B = C, with the option "Keep input objects" checked.

Now my confusion is what are the end products remaining. Is it just B and C (which is what I want), or is it A, B, and C (which is not what I want). In other words, does "input objects" refer to both A and B, or just B? And is A removed automatically by default? If not, how would I just get B and C?

3 Replies Last Post Feb 1, 2021, 4:52 p.m. EST
Jeff Hiller COMSOL Employee

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Posted: 4 years ago Jan 21, 2021, 3:25 p.m. EST

Hi Vivek,

The input objects and A and B. You can test this in your file: after the dif1 step you can still use blk1 and sph1.

With that said, to create the geometry you describe in your second paragraph, you don't need to subtract the sphere from the cube, only to draw the sphere again. All you need to do is draw a cube and draw a sphere. The Form Union step will take care of everything else.

Best,

Jeff

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Jeff Hiller
Hi Vivek, The input objects and A and B. You can test this in your file: after the dif1 step you can still use blk1 and sph1. With that said, to create the geometry you describe in your second paragraph, you don't need to subtract the sphere from the cube, only to draw the sphere again. All you need to do is draw a cube and draw a sphere. The Form Union step will take care of everything else. Best, Jeff
Vivek Gupta
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Posted: 4 years ago Feb 1, 2021, 4:45 p.m. EST

Hi Vivek,

The input objects and A and B. You can test this in your file: after the dif1 step you can still use blk1 and sph1.

With that said, to create the geometry you describe in your second paragraph, you don't need to subtract the sphere from the cube, only to draw the sphere again. All you need to do is draw a cube and draw a sphere. The Form Union step will take care of everything else.

Best,

Jeff

Hi Jeff, thanks for your reply. So if I'm understanding correctly, once I create object B, the form union step will automatically delete the portion of object A that is occupied by object B?

>Hi Vivek, > >The input objects and A and B. You can test this in your file: after the dif1 step you can still use blk1 and sph1. > >With that said, to create the geometry you describe in your second paragraph, you don't need to subtract the sphere from the cube, only to draw the sphere again. All you need to do is draw a cube and draw a sphere. The Form Union step will take care of everything else. > >Best, > >Jeff Hi Jeff, thanks for your reply. So if I'm understanding correctly, once I create object B, the form union step will automatically delete the portion of object A that is occupied by object B?
Jeff Hiller COMSOL Employee

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Posted: 4 years ago Feb 1, 2021, 4:52 p.m. EST

The form union step will create two domains, one corresponding to the cube minus the sphere, the other corresponding to the sphere.

Best,

Jeff

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Jeff Hiller
The form union step will create two domains, one corresponding to the cube minus the sphere, the other corresponding to the sphere. Best, Jeff

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