Gerris solves the Navier–Stokes equations in 2 or 3 dimensions, allowing to model industrial fluids or for instance, the mechanics of droplets, thanks to an accurate formulation of multiphase flows. Actually, the latter field of study is the reason why the software shares the same name as the insect genus. Gerris also provides features relevant to geophysical flows:
Flow types #1 to #3 were studied using the shallow-water solver included in Gerris, case #4 brings in the primitives equations and application #5 relies on the spectral equations for generation/propagation/dissipation of swell : for this purpose Gerris makes use of the source terms from WaveWatchIII. Lastly, one can note that the Navier–Stokes solver was also used in the ocean to study:
Gerrisbelongs to the finite volumes family of CFD models.
Type of grid
Most models use meshes which are either structured or unstructured. Gerris is quite different on this respect: it implements a deal between structured and unstructured meshes by using a tree data structure, allowing to refine locally the description of the pressure and velocity fields. Indeed the grid evolves in the course of a given simulation owing to criteria defined by the user.
Turbulent closure
Gerris mainly aims at DNS; the range of Reynolds available to the user thus depends on the computing power they can afford. According to the Gerris FAQ the implementation of turbulence models will focus on the LES family rather than RANS approaches.
Programming language, library dependencies, included tools
Gerris is developed in C using the libraries Glib and GTS. The latter brings in facilities to perform geometric computations such as triangulation of solid surfaces and their intersection with fluid cells. Moreover Gerris is fully compliant with MPI parallelisation. Gerris does not need a meshing tool since the local refinement of the grid is on charge of the solver itself. As far as solid surfaces are concerned, several input formats are recognized:
analytic formulas in the parameter file
GTS triangulated files; note that the Gerris distribution includes a tool to translate the STL format into GTS triangulated surfaces
bathymetric/topographic database in KDT format; a tool is also provided to generate such a database from simple ASCII listings
Among the various ways to output Gerris results, let us just mention here:
Graphical output in PPM format: images can then be converted in any format using ImageMagick, and MPEG movies can be generated thanks to FFmpeg.
Simulation files, which are actually parameters files concatenated with fields issued from the simulation; these files can then be re-used as parameter files, or processed with Gfsview.
Gfsview, a display software shipped with Gerris, able to cope with the tree structure of the Gerris grid.
Licence
CFD software, as any software, can be developed in various "realms":
As far as CFD is concerned, a thorough discussion of these software development paths can be found in the statement by Zaleski. Gerris was distributed as free and open-source software right from the onset of the project.
