Discussion Forum

Hi

are you sure you didn't click the "difference" button ?

for me in V3.5a the intersection of two geometries not overlapping say's "operation results in empty geometry", but if you select the "difference" A-B then B dissapears if they do not intersect (in fact it always dissapears it's what you are asking for no ?).

To leave it there, the trick is to do a Control-C and Contorl-V to duplicate the object first, then you take the difference,

Note also that if you select carefully your geometries, in the desired order, and then hit the (+/-*) button (to manually check what you are doing, not going for the "quick buttons) it reorders your selection and proposes union (+) as default, you have to order them with this window open if you want to do a "difference (- operation)

Agee ?

Ivar

Hi Ivar,

Thank you for your reply. I just double checked, and I am now pretty sure that I did click the 'intersection', not 'difference'. Even I do the intersection operation by hit the (+/-*) as you suggested, the same thing happens.

I attached two images to show how that happened.

celina

hello

Well I tried before in 2D, did it again in 3D on a PC with V3.5a:

Intersection: BLK1*BLK2 when they do not intersect gives an error message resulting in empty geometry

when the volumes intersect I get the intersection as expected


Difference: BLK1-BLK2 when they do not intersect leaves only BLK1 (BLK2 dissapears)

when they intersect I get the difference (but the rest dissapears in this case BLK2 as above)

For me this is very logical.

From your images, are you using a mac ? or unix ?
there might be an issue there, or on the keyboard mapping ?

I would propoe to pass by COMSOL support, by documenting carefully (images plus fomulas, even screen snip/dumps of the fomulas, that helps. Do not forget to give them version and OS

Good Luck
Ivar

Hi Ivar,

The OS question is a really good call. I was using Mac, but I just tried the same operation on the same geometry objects in Windows, no problem at all. And yes I am using V3.5a

Celina

Hi there

Well then I would really suggest to inform COMSOL support as they should know such thing rapidly to improve it, ther could be other things related, and certanly several other having surprises.

By helping them improve the programme you are also helping us all end users (I'm not at all COMSOL, but a happy user) to get ane ven better product.

And I'm nut sure COMSOL is really following each discussion herein to tha last detail

I wish you some interesting simuations
Ivar

I too noticed some weird thing about intersection operation in COMSOL. I have two cylindrical objects of same diameter but different heights, created using COMSOL. The geometry is such that smaller object is imbedded The smaller object is completely solid but with a concave shape top face. The taller object has circular holes. The idea here is to create a lightweighted telescope mirror with undercut circular/triangular hole. The intersection of the two geometries should give me the desired object. When both objects are selected (Ctr+A), the intersection operation results in a composite object without removing the parts that are not common to both. For strange reasons, the result of 'intersection' is 'union'. If I try to take the 'difference' I get the error message 'operation resulted in empty geometry object'. Same time both objects also remain intact. That is somewhat puzzling. Did any one come across such a thing? There are some discussions about intersection on the forum. But that is little different.

Thanks,
-Ravinder