Question

Which of these is the best practice regarding outlet boundaries?

(a) Outlet boundaries should be at the exact outlet of the geometry

(b) Outlet boundaries should be set as close as possible to the inlet boundaries

(c) Outlet boundaries should be set as far as possible to the physical geometry

(d) Outlet boundaries should be set as close as possible to the physical geometry

This question was addressed to me in an interview.

Question is taken from Boundary Conditions in chapter Boundary Conditions of Computational Fluid Dynamics

Answer 1

Right option is (c) Outlet boundaries should be set as far as possible to the physical geometry

Easiest explanation: At the outlet, usually we know only little about the flow properties. For this reason, we should set the outlet boundary conditions as far as possible from the region of interest. Otherwise, error may propagate upstream.