Roughness
 

In Ansys Fluent, How can i use roughness inside pipe in my setup (also what is the used Model) ?
Secondly, If i want to start roughness after a distance from the inlet of the pipe -not from inlet directly- how can i do that ?

Check options in boundary conditions for all the walls; you can set roughness heigh and roughness constant (these parameters refer to sand grain roughness).
If you want to start roughness after some smooth pipe you need to create 2 different walls and set their roughnesses appropriately.

All thanks senior member "ghost82" , can i ask you how can i do that step of splitting wall into two walls ?. is it firstly in geometry , also how can i do ?