Lethe 1.0: An open-source parallel high-order computational fluid dynamics software framework for single and multiphase flows

Lethe is an open-source Computational Fluid Dynamics (CFD) software framework with extensive multiphase and multiphysics capabilities. By leveraging the deal.II open-source framework, Lethe finite element solvers scale well on modern high-performance computers while possessing advanced features such as dynamic mesh adaptation, load-balancing, isoparametric high-order capabilities, and a fully-fledged Discrete Element Method (DEM) module. To facilitate contributions from the community, Lethe is extensively tested with continuous integration using over 450 unit and functional tests. Furthermore, Lethe contains 74 fully documented examples with pre-processing and post-processing steps to allow users to learn how to rapidly use and modify the framework. In this article, we give an overview of the simulation models available within Lethe and illustrate these capabilities with a selected list of examples including turbulent and multiphase flows.

Program summary

Program Title: Lethe

CPC Library link to program files: https://doi.org/10.17632/mc5trb4kd3.1

Developer's repository link: https://github.com/chaos-polymtl/lethe

Licensing provisions: Apache-2.0

Programming language: C++

Nature of problem: Single-phase incompressible flows of Newtonian and generalized Newtonian fluids. Granular flows of cohesive or non-cohesive spherical particles. Multiphase flows, including particle-laden (solid-liquid and solid-gas) flows and fluid-fluid (gas-liquid and liquid-liquid) flows. Multiphysics coupling with heat transfer.

Solution method: Lethe uses stabilized continuous Galerkin finite element formulations to solve the incompressible Navier-Stokes equations and other partial differential equations. Lethe utilizes the DEM to simulate granular flows. For particle-laden flow simulations, Lethe uses an unresolved CFD-DEM approach for flows containing numerous spherical particles ($>{10}^3$), while a resolved CFD-DEM approach is used for flows with few spherical or non-spherical particles (<100). For gas-liquid and liquid-liquid flows, Volume of Fluid (VOF) or Cahn–Hilliard (CH) models are used.

Additional comments including restrictions and unusual features: Lethe possesses both matrix-based and matrix-free CFD solvers for incompressible flows. The matrix-free solver efficiently simulates larger problem sizes with more than 1B unknowns, but only supports hexahedral (structured or unstructured) meshes, whereas the matrix-based solver supports both tetrahedral and hexahedral meshes. Lethe also supports dynamic mesh adaptation and load-balancing for hexahedral meshes. The load-balancing capabilities can also be used within the DEM and CFD-DEM modules and the CFD-DEM coupling.