sudden increase in diameter
 

How to design the simulation for a pipe flow in which diameter is suddenly increased from 5 mm to 60 mm ?

Geometry creation, mesh generation, setting boundary conditions, applying suitable solver settings, solving the model, post-processing.
For a more specific answer, you will have to formulate more specific questions. See also https://www.cfd-online.com/Forums/si...tml#post612025
In short: Don't expect people to put more effort into their answer than you put into your question.