Model of upstream reservoir-circular culvert-downstream reservoir in workbench
 

Could anyone please tell me if I want to model a geometry of upstream reservoir-circular culvert-downstream reservoir in workbench,

1. Should I consider them as 3 body 3 part or 1 body 1 part?
2. Should I apply different mesh for 3 parts? If yes, then how ?

I searched tutorials... didn't find what I want, may be I didn't search in proper way. It will be really great if anyone can suggest me any idea. Thanks in advance.