Seminar on Modeling May 2015 - Laurent Orgogozo: RichardsFOAM : high performance computing for the resolution of Richard

The infiltration of rain waters through soils are determinant for numerous engineering applications (water ressources, environment, geotechnics, ...) as well as in various natural phenomena, such as for instance weathering of continental surfaces (e.g.: Goddéris et al., 2012), which is a key process of the carbon cycle (Walker et al., 1981). A classical way to quantify such flows in variably saturated porous media consist in the numerical resolution of the Richards equation, 3D, instationnary and non-linear (Richards, 1931). However, the study of the hydrological behaviour of soils under evolving conditions (climate changes, land use changes, ...) requires modellings at large spatial scales (km 2 and beyond) and on large time scales (decades, century). The massively parallel computing is a major way of dealing whith such large scale problems (see for example Miller et al., 2013). We present here a massively parallel solver for Richards equation, RichardsFOAM.

This solver has been developed in the framework of the open source CFD tool box OpenFOAM ® (Jasak, 1996, Weller et al., 1998). RichardsFOAM is able to solve large scale problems due to the good parallel performances of OpenFOAM ® (with RichardsFOAM, about 90% of parallel efficiency with 1024 cores both in strong and weak scaling). These performances will allow us to propose mechanistic modellings of water fluxes at the relevant space and time scales for the study of weathering processes (> km 2 , > decades).

A detailed study of the parallel performances of RichardsFOAM will be presented (strong and weak scaling, I/O's impacts, numerical stiffness impacts) as well as an example of application on a field data set. The associated scientific perspectives will be discussed.