Using Transient Boundary condition
 

I plan to use FLUENT to simulate fluid flow and heat transfer in tubes/channels. But the inlet boundary condition is going to TRANSIENT , i.e., inlet flow rate varies with time. Flow rate can change from a very small value (hence laminar) to a very high value (hence turbulent) and a very high value to a very low value OVER A PERIOD OF TIME. Can FLUENT switch over from LAMINAR to TURBULENT and TURBUELENT TO LAMINAR automatically over a time period based on the transient boundary condition (inlet flow rate or velocity changing with time)?
If yes, can anyone please explain how it can be done.

You can use Automatic Case Modifications (See Fluent User's Guide 28.18) to enter commands after a fixed number of time steps. This will probably work for what you're trying to do, but it will bring its own issues with it. You'll need to initialize the turbulent variables when you activate the model and you may see unphysical results in the time steps right after model changes. It might be best simply to leave the turbulent model on for the entire simulation; it will increase the solve time a bit, but shouldn't affect accuracy.