Difference between outlet and opening
 

I would like to know what are the main differences between a opening BC and outlet BC, and the influence of the choice in a simulation.
I guess the opening admits flux in both directions, while the opening admits only outwards, but are there other characteristics?

An opening allows flow in both directions. An outlet allows flow outwards only. An outlet is numerically simpler and if used appropriately will converge quicker. The difference may be small in some cases - but for cases with convergence difficulties the difference can be important.

Inspite that this thread seem too old I want to add that in outlet boundary the gradient of flow variable (u,v,w,p,k,epsilon....ect) to the direction normal to the outlet will be zero i.e. (dphi/dn)=0
where phi: any variable of (u,v,w,p,k,epsilon....ect)
n: vector normal to the boundary

regards
kbaker