## Introduction to CFD-DEM for modelling fluid-particle systems

Granular materials are everywhere! In fact, they are present in about 70% of all industrial processes. In many cases, the granular material does not exist in isolation - their behavior is significantly influenced by the liquids and gases in which they are found. These particle-fluid, or multi-phase flows, are encountered across industries including energy, agricultural, mining, oil and gas, chemical, and pharmaceutical. Coupling numerical methods like CFD (Computational Fluid Dynamics) and DEM (Discrete Element Method) is one of the ways to model these particle-fluid systems.

How does CFD-DEM work?

In CFD-DEM simulations, the fluid phase is solved using CFD, while the particulate phase is handled by DEM. The two methods are coupled through the exchange of information at each time step. Here is a simplified overview of the process:

1. Particle Motion Calculation (DEM): Each particle's position, velocity, and acceleration are updated based on the forces acting on it, including collisional forces, and any body forces like gravitational forces.
2. Coupling: Particle position information is passed to the CFD solver.
3. Fluid Flow Calculation (CFD): The fluid domain is discretized into a mesh, and the fluid flow equations are solved based on the volume fraction of the particles, to obtain velocity, pressure and other fluid properties. The hydrodynamic forces acting on the particles are also calculated.
4. Coupling: The drag and lift forces on the particles are passed on to the DEM solver.
5. Iteration: Steps 1-4 are repeated iteratively for each time step until the simulation is complete.

Here is an example of how the coupling works for Altair CFD (AcuSolve) and DEM (EDEM) tools.

You can find more information on this short 15 min video: https://altair.com/resource/go-with-the-flow-when-dem-meets-cfd

AcuSolve-EDEM coupling

Here are some examples of CFD-DEM coupling applications using Altair tools (in additional to the ones shown above):

You can find a self-paced e-Learning course for the AcuSolve-EDEM coupling here: https://learn.altair.com/course/view.php?id=176

Tutorials on the coupling can be found here: https://help.altair.com/hwcfdsolvers/acusolve/topics/chapter_heads/particle_simulation_r.htm

If you are interested in more examples of AcuSolve-EDEM coupling, check these other articles:

EDEM-Fluent coupling

Altair EDEM can also be coupled with Ansys Fluent to model fluid-particle systems.

Here is a self-paced e-Learning course on the EDEM-Fluent coupling: https://learn.altair.com/course/view.php?id=171

Tutorials on the coupling can be found here: https://community.altair.com/community/en/edem-fluent-tutorials?id=kb_article&sysparm_article=KB0115859

Other coupling

Altair EDEM is extremely customizable, which can be used to write your own coupling with any other open-source or proprietary CFD code that you like. This can be achieved through EDEM API (Introduction to EDEM API).

Multiphase Particle-in-Cell (MP-PIC) Approach

If you have very small particles and a very large number (billions) of them, CFD-DEM coupling may not be the right approach. In such cases, MP-PIC may be a better fit, wherein the particles and their interactions are calculated stochastically as opposed to them being deterministic in DEM.

Barracuda Virtual Reactor is one of the tools that uses an MP-PIC approach for such large scale systems. Barracuda is one of Altair's partner products which can be accessed through the Altair license. More information about Barracuda Virtual Reactor can be found here: https://cpfd-software.com/barracuda-virtual-reactor-product-sheet/

Smoothed particle hydrodynamics (SPH)

If you are interested in performing a particle-based fluid modelling, SPH is the right approach. This approach does not require any mesh and is very well suited for sloshing free-surface flows.

Altair nanoFluidX is an SPH solver that can help with such simulations: https://www.youtube.com/watch?v=-yxYgi_MBeQ