An Introduction to Discrete Element Modeling

ANSYS BLOG

May 24, 2023

An Introduction to Discrete Element Modeling

What physics are involved with DEM?

Almeida: DEM measures body forces such as gravity, fluid, and electrostatic or magnetic fields. It also tracks surface forces such as contact and adhesion or cohesion, like liquid bridging between particles.

Can you explain how DEM methods work?

Almeida: The core of every DEM code is to detect particle collisions and compute the contact force. With the soft-sphere method, particles are rigid and any deformation at contact is modeled as an overlap. Normal and tangential contact forces are computed as a function of the overlaps. DEM tracks every particle in the domain, and we can learn each particle's position at the next time step. When needed, we can also measure adhesion cohesion and the breakage of particles.

Why does particle shape matter in DEM?

Almeida: Particle shape can affect the flow of particles. Spheres do allow for simple contact detection and a single point of contact. But sphere shapes are pretty rare in the real world, so we need to be able to model other shapes. Ansys Rocky, for example, enables this modeling capability.

How do real particle shapes differ from spherical particles?

Almeida: Real particle shapes - like polyhedrals, for example - differ in their packing density, the linear and rotational modes of transport, the dilating during shear and interlocking, and the strength of the materials. Note that real particle shape modeling can increase computational demand (cost) due to contact detection and overlap requirements.

How does GPU computing help DEM simulation?

Almeida: Remember, DEM tracks every particle at every time step. So, graphics processing unit (GPU) computing speeds up simulation significantly without sacrificing accuracy. For example, using a 4x A100 GPU can complete a solver 700X faster than one central processing unit (CPU) core.

Why might someone use coarse-grain modeling?

Almeida: Even if you have GPU processing, it may not be feasible or practical to get accurate results if you have millions of particles. Commonly, people use coarse-grain modeling to reduce the total number of particles by using larger particles to represent a group of smaller particles.

When is CFD coupled with DEM?

Almeida: When we need to find ways to account for fluid forces and flow on particles, computational fluid dynamics (CFD) can be effectively coupled with DEM. This coupling can be used with both mesh-based approaches and any particle shape.

When is smoothed particle hydrodynamics (SPH) coupled with DEM?

Almeida: This coupling is increasingly used in hydrodynamics. It can capture the complex-free surface flows with no diffusion errors, and dynamic body interaction is easily handled. It's suitable when splashing or surface fragmentation is involved, for example.

SPH-DEM is often used with a Lagrangian meshless approach to capture flow dynamics by discretizing them into a set of fluid elements. Particles are interpolated using a kernel function to compute smooth fields for local variables.

What are the advantages of the SPH-DEM coupling approach?

Almeida: This coupling naturally captures complex free-surface flows with no diffusion errors, and dynamic body interaction is easily handled.

Find out how Ansys Rocky can help with DEM.

Lucilla Almeida is a CAE specialist at Ansys and is responsible for quality assurance of Ansys Rocky. She holds a master's degree in chemical engineering and a doctorate in nuclear engineering. She focuses on applying CFD and DEM to solve engineering problems.