determination of fully developed flow

yfyap

hi there,

i am interested in finding the entry length for several Reynolds numbers for flow in a straight rectangular duct. the criteria i used to determine the entry length was that the second derivative of the pressure with respect to the streamwise direction be less than certain predefined value of epsilon, (d^2P/dz^2 < epsilon). it was found that different values of epsilon give different entry length (note that the difference is significant).

questions:

what is the appropriate magnitude of epsilon? 1e-4? 1e-5?

other that the above mentioned criteria, what other criteria can i use?

thanks in advance.

regards, yfyap