A fluid flow can be categorized as fully developed flow if the fluid velocity profile in the pipe does not change anymore. Fully developed flow will occur if the condition of fluid in the pipe is quite.
The condition of fully developed flow in pipe is indicated by the ratio between the incoming length (Le) with pipe diameter (D). If the calculation result is:
Le / D ≈ 0.06 Re, so the fully developed flow in pipe is laminar
Le / D ≈ 4.4 Re1/6, so the fully developed flow in pipe will be turbulent
Assumption that viscosity of fluid is constant is used to analyze fully developed flow in pipe.