Gerris is a Free Software program for the solution of the partial differential equations describing fluid flow. The source code is available free of charge under the Free Software GPL license.
Gerris was created by Stéphane Popinet and is supported by Institut Jean le Rond d'Alembert. Note that the original developers of Gerris have now moved development to Basilisk which can do most of what Gerris can do, and more.
A brief summary of its main features:
 Solves the timedependent incompressible variabledensity Euler, Stokes or NavierStokes equations
 Solves the linear and nonlinear shallowwater equations
 Adaptive mesh refinement: the resolution is adapted dynamically to the features of the flow
 Entirely automatic mesh generation in complex geometries
 Secondorder in space and time
 Unlimited number of advected/diffused passive tracers
 Flexible specification of additional source terms
 Portable parallel support using the MPI library, dynamic loadbalancing, parallel offline visualisation
 Volume of Fluid advection scheme for interfacial flows
 Accurate surface tension model
 Multiphase electrohydrodynamics


Have a look at the introduction for more information.