OpenFOAM

A library of computational fluid dynamics solvers covering compressible and incompressible flow, turbulence, heat transfer, multiphase flow and other continuum mechanics fields. It has no graphical interface of its own: you define the case in text files, run from the command line, and post-process in ParaView. That is precisely why it is treated as a de-facto standard in research and industry — the solver itself can be modified. The trade-off is a significant initial investment in learning.