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HYBIRD is a numerical code that combines a Lattice-Boltzmann Method (LBM) solver for fluid dynamics with a Discrete Element Method (DEM) solver for the solution of the dynamics of a particle ensemble. The code exists in two main forms:

• One with both DEM and LBM parallelized using OpenMP loops. This version is therefore optimized for a few processors, sharing memory.
• One where DEM runs on CPU, and LBM runs on GPU. This version is to be preferred for full DEM-LBM simulations

Fluid dynamics with LBM

The Lattice-Boltzmann Method, commonly abbreviated as LBM, is a recently developed tool for the simulation of fluid systems. It is popular because of its superior performance on parallel architectures, and is one of the most promising tools for the future development of Computational Fluid Dynamics (CFD). LBM is generally successful in handling intricate boundary conditions and is an optimal tool for the coupling of fluid and particles. Beyong the wiki, information and a complete overview of the method can be found in Kruger et al. (2017), while a more practical guide is provided by Mohamad (2011).

Granular dynamics with DEM

While LBM solves the Navier-Stokes equation on an Eulerian, fixed grid, DEM follows a Lagrangian approach. A group of spherical particles is treated as a mobile set of points, which can navigate through the fluid domain. The two solvers are coupled by the transmission of hydrodynamic interactions between fluid and particles, via an Immersed Boundary Method (IBM). The discretization in time is explicit, and a Gear 5th order predictor-corrector scheme is used. The contact mechanics are based on a soft-contact approach. Two spheres are allowed to overlap slightly, and a repulsive force is applied to both elements. Two models are available for the resolution of the contact: a Linear model and a more accurate but less intuitive Hertzian model. On the tangential direction, frictional forces arise whenever two surfaces are in contact.