Please see the figure below. Hope the information is clear.
Now imagine we have flow comes into a larger tube (Tube 1) from left, and then moves into a smaller tube (Tube 2) and exit Tube 2 from the right. By mass balance, we can easily find out that the Reynolds number between Tube 1 and Tube 2 is:
So by picking the right D1 and D2, we can easily have laminar flow in Tube 1 and turbulent flow in Tube 2. In this case, what model should we use? Laminar model or turbulent model? It seems neither would work and I couldn't figure out how I should model this problem.