Calculating Entrance Lengths in Hydrodynamics

[pauleck]

Hi everybody,
first of all, sorry for any grammatical mistakes, I am a non-native English speaker.
So here is my concern,

the dimensionless (hydrodynamic) entrance length is defined as:

z+=zh/(Re*Dh)

,where zh is the hydrodynamic entrance length, Re is the Reynolds number, and dh is the hydraulic diameter of the duct.

If zh exceeds the total channel length L, do I have to assume zh=L for calculating z+??

Thank you very much

 

[bigfooted]

If zh exceeds the total channel length L, do I have to assume zh=L for calculating z+??

No. The entrance length is longer than the channel length, and the dimensionless entrance length is longer than the dimensionless channel length. zh is not limited to the actual channel length. If zh>L it just means that at the end of the channel the flow hasn't fully developed yet.