I guess I was imagining it with the walls torn out as well, but you're right the op (of this comment chain) said top and bottom broken. If the walls are somehow firmly fixed forever no matter how much force they experience, and are not subject to thermal degradation, then we have a square pipe with 3m sides and infinite length. If the walls break down then it's also infinite diameter.
In terms of modelling it there's a FOSS option openfoam.org but I don't know how to use it and don't have time to mess about with it right now.