I suppose the context is something like; there are two inlet pipes and at some point in time one of the pipes opens or closes.

You can treat both of those as different inlets and write separate UDFs. When you want the inlet closed you can simply set the mass flux to be zero by using something like a step function.

Thank you Atharva Somshekhar Modi. I found a way around it.

The aim is to develop a digital twin for the model, so I used one inlet and variables as input.

I will implement a switching logic for boundary conditions in Twin Builder

The most important steps to write a UDF are:

1- knowing the principle of writing the code.

2- the equations that deal with the issues.

3- knowing where to define it exactly in the fluent.