Lethe (pronounced /ˈliːθiː/) is an open-source computational fluid dynamics (CFD), discrete element method (DEM) and coupled CFD-DEM software which uses high-order continuous Galerkin formulations to simulate single and multiphase flows.

Lethe contains a family of solvers that are based on deal.II, a finite element library. Through deal.II, Lethe uses Trilinos, for its sparse linear algebra routines and p4est for its distributed adaptative quadtrees and octrees. Lethe is named after the river of forgetfulness, which, according to Wikipedia:

is one of the five rivers of the Greek underworld[,] the other four [being] Acheron (the river of sorrow), Cocytus (the river of lamentation), Phlegethon (the river of fire) and Styx (the river that separates Earth and the Underworld). […] The shades of the dead were required to drink the waters of the Lethe in order to forget their earthly life.

Lethe is described here. It originally began as a weekend project, but slowly grew into a robust and efficient CFD, DEM and CFD-DEM software. Lethe is under active development.