OpenFOAM: The Open Source CFD Toolbox

OpenFOAM is a sophisticated open-source framework for performing computational fluid dynamics (CFD) simulations.

From the website: "The OpenFOAM (Open Field Operation and Manipulation) CFD Toolbox can simulate anything from complex fluid flows involving chemical reactions, turbulence and heat transfer, to solid dynamics, electromagnetics and the pricing of financial options. OpenFOAM is produced by OpenCFD Ltd, is freely available and open source, licensed under the GNU General Public Licence."