Periodic Pipe flow
 

While meshing a pipe for simulation as periodic, is it safe to consider computational domain dimensions used in the case of periodic channel?

ANSYS recommendation states that the size of the computational domain for a straight channel with channel height H is taken as 4H in the stream-wise direction and 1.5H in the span-wise direction. For all considered Reynolds numbers, the computational grid is unchanged in the stream-wise and span-wise directions with a uniform grid-spacing of 0.05H and 0.025H, respectively. Can this rule, while determining stream-wise length, be considered for straight pipe as well?

For LES/DNS, pipe length matters a lot for reasons having to do with physics.

For RANS, I've (frequently) simulated as short as 1 pipe diameter on nice grids.