Say you divide your pipe into 3 sections: inlet1->out1, inlet2->out2, inlet3->out3.
Run the first section of your pipe. Then from the Physics tab, go to Zones > Profiles... > Write... and select out1 as your surface and the value that you want to export to the second section (say xyz velocity).
For your second section of the pipe, import the profile from Physicas tab, Zones > Profiles... > Read and use this value in the boundary conditions for inlet2.
Repeat for the third section of the pipe.
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