Does it be necessary to define any given boundary in the initialisation setup or do I have to define only one, e.g. inlet.

Thanks Gernot

Initialization is not for a single boundary but for the whole solution process. You can think of the initalization as an initial guess of the solution you expect.

But do I have to initialize every given input in the initialisation setup or can I choose which inputs I do?

You can initialize the solution from one bc, all zones or you enter an individual initial guess, which think of suits your solution best. Just have a look at the initialization panel in Fluent. So to answer your question: you can choose which inputs to make, but have your solution in mind!

Thanks for helping me, Mario. Have a nice day. cheers Gernot