Fluid mechanics - Poiseuille Equation

[simonre7]

The question is as follows:

Determine the maximum velocity at which the poiseuille Equation may be used for
a) water flowing in pipes diameter 0.001m, 0.015m & 0.20m, stating f, pressure drop/length in each case.
b) Re-work question a) using air instead of water.

I am a bit confused as to how i reach the maximum velocity in each lengths of pipe any help would be greatly appriciated guys.

thank you in advance.

 

[Astronuc]

Think in terms of transition from laminar to turbulent flow - Poiseuille's law (or the Hagen-Poiseuille law) is valid for laminar flow. The maximum velocity is that velocity (or speed) where the flow begins transition from laminar to turbulent.