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?
Consider first the formulation and the Reynolds number you want to solve.
Pipe is a full confined flow problem, you can consider only the streamwise direction as periodic. Grid spacing in periodic direction (both channel and pipe) depends on the Re_tau number, cannot be take unchanged. A non-uniform grid is required for the wall normal direction.
https://www.computationalfluiddynamics.com.au/convergence-and-mesh-independent-study/
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