# Connect RCC of different radius

Respected FLUKA expert:
I use RCC to simulate two kinds of optical fiber and find it said "Avoid touching surfaces"in lecture “05_Geometry_2016”,But how should I do if I want to connect two RCCs with different radius.
Best Regards,
xiongbp

Hi Xiong and welcome to the FLUKA forum!

As is suggested in the FLUKA course material you are referring to, it is advised to use as much as possible infinite bodies since they make tracking faster and generally help to avoid geometric errors through user definition or precision.

For your specific problem I assume the optical fiber you want to simulate is parallel to either of the geometry X, Y or Z-axis. Assuming for now it is the Z-axis, you can define two infinite ZCC cylinder bodies with different radius, called “F1” and “F2”. At the point where you want to connect both, you define an XYP infinite plane as cutter, let’s call it “C”. Using the combinatorial FLUKA geometry language we can then define the regions in the proper way: the declaration for the first optical fiber will be “+F1 + C”, the second one will be “+F2 - C”. This ensures the contact between the two fiber regions in the correct way. Does this answer your question?

Best regards,

Andreas

Dear Andreas,
Thank you for your help,I use three XYP to divide the two kinds optical fiber into three parts(z=0 to z=15 represent optical fiber with radius A,z=15 to z=215 represent optical fiber with radius B).Because ZCC is infinite cylinder,so I do not think it is suitable to use SPH for black hole and vacuum,should I use ZCC with large radius to express it ?

Hi Xiong,

Using the infinite ZCC bodies in combination with a SPH body to delimit the blackhole region is completely fine. The bodies can be infinite as long as the regions inside the simulation volume surrounded by the black hole are finite. This is certainly the case by using the three XYP planes you mention, which is a correct approach. Designating the XYP plane at Z=0 as “C1” and the one at Z=15 as “C2”, the first optical fibre region would then become “-C1+F1+C2”.

Best regards,

Andreas

Test.inp (3.6 KB)
Dear Andreas,
If I use SPH with large radius to define blackhole body,the region should be substract other bodys inside the SPH,but ZCC is infinite and will breakthrough the sphere,does it matter?
take the attachment as an example, I use “+void-jacket” to define Vacuum,is it right?but my file still found error in geometry,could you help me?`

Hi Xiong, as I mentioned before the intersection of bodies is not a problem. However you do need to make sure that the regions are all defined properly. You have indeed subtracted from the vacuum “void” region all of the volume enclosed by “jacket” which corresponds to the outer radius of the larger radius optical fiber. By consequence, not all regions inside the “jacket” body are declared properly. In particular, you need to add two more zones to the “void” region by using the union operator “|”:

| +void +jacket +plane1
| +jacket +plane2 -coat -plane1

This should eradicate the errors and ensure there are no more undeclared volumes in your geometry. Coming back to an earlier question you had in the 3d post: if you are only simulating the long fiber as you show, it is indeed better in this case to replace the SPH “blkbody” body by ZCC for computational efficiency.

Best regards,

Andreas

Dear Andreas
I understand this problem,thank you very much