ANSYS BLOG

December 26, 2023

# Unlocking the Power of CFD-DEM Coupling: Modeling Fluid-particulate Systems

Coupling computational fluid dynamics (CFD) and the discrete element method (DEM) expands the range of granular-fluid systems that can be addressed through numerical simulations. Learn how CFD-DEM coupling works, the types of coupling available in Ansys Rocky, and how you can unlock the power of CFD-DEM by coupling Ansys Fluent and Rocky.

## What is CFD-DEM Coupling?

CFD-DEM coupling is a promising alternative for modeling granular-fluid systems because it can capture the discrete nature of the particle phase while maintaining computational tractability.

This type of coupling is a numerical simulation technique used to model the behavior of fluids and particulate systems, in which CFD is used to simulate the fluid flow while DEM is used to simulate the motion and interactions of discrete solid particles within the fluid. When these two methods are coupled, they allow for a more comprehensive understanding of solid-fluid interactions.

## Why Can’t I Simulate Granular-fluid Systems with CFD Alone?

There are two main approaches to deal with solids in CFD: the Eulerian approach and the Lagrangian approach.

In the Eulerian approach, both the fluid and particles are treated as continuums, so constitutive equations for inter- and intra-phase interactions are needed. However, finding general equations for granular systems is hard due to the changing nature of how solids flow.

In addition, prescribing a particle size distribution can increase computational cost, as different phases are required to model different particle sizes. Moreover, due to the continuum interpenetrating approach, no individual particle information is available — which might be the very data you are seeking.

In the Lagrangian approach, the fluid is still treated as a continuum, but the particulate phase is treated in a discrete fashion. Each particle (or parcels modeling a group of particles) is tracked along the domain by the result of forces acting on them.

However, because particle-to-particle interactions are not solved, this approach is limited to very dilute flows, which is not the reality in most industrial applications.

## What Are the Benefits of CFD-DEM Coupling?

The benefits that make the CFD-DEM coupling a valuable technique are:

• Realistic simulations
• The motion of every particle is simulated.
• The computation of all forces acting on particles and cases with unique, nonspherical particles can be more accurately solved using Rocky’s precise shape representation.
• The adhesive/cohesive materials can be modeled using one of the adhesion models available in Rocky.
• Using Rocky’s multi-GPU (graphics processing unit) capabilities for the DEM solver with Fluent’s distributed parallel option for solving CFD equations enables the solution of real-scale applications.
• Optimization
• There is no limitation on the particle size concentration, and particle size distribution is easily prescribed without increasing CFD solver computational costs.
• Different particle shapes can be simultaneously used in the DEM solver without increasing the CFD cost.
• Increased accuracy
• Because convective heat transfer between particles and fluids can be solved, along with the conductive heat exchanged during collisions, processes that involve temperature changes (such as drying) can be modeled with increased accuracy.
• Comprehensive analysis
• As the complete time history is available for all particles inside the domain (e.g., velocities, temperatures, and contact data), the extensive set of post-processing tools available in Rocky enhances the level of information that can be extracted from the coupled simulation, providing better insight into your problem.

CFD-DEM coupling also includes a broad range of application, such as:

• Slurry mills (mining industry)
• Cyclones, desanders, and drill cutting removal (oil and gas industry)
• Pneumatic conveyors (multiple industries)
• Wastewater management (waste disposal industry)
• Grain drying and sorting (agriculture and food industries)
• Tablet and candy coating (pharmaceutical and food industries)
• Biomass reactors (energy industry)
• Fluidized beds and catalytic reactors (chemical and nuclear industries)

## Exploring CFD-DEM Coupling in Ansys Rocky

It is possible to use the CFD-DEM coupling method using just Ansys Rocky, without the need for an external CFD solver, through one of the three methods below:

1. One-way constant (unresolved): This method should be used when the solution in the CFD domain can be reduced to a homogeneous fluid velocity, pressure, and density fields for the entire DEM domain. In this case, the user sets the fluid and flow characteristics directly in Rocky.

2. LBM: This method is useful for predicting particle-induced flow, in which particles induce fluid motion but are not affected by fluid forces. This strategy also allows the user to define a lattice of airlike fluid and simple flow conditions.

Weerasekara, N., Bharadwaj, R., Potapov, A., Daroz, V., Hanhiniemi, J.. SPH-DEM techniques to solve complex flow structures in AG/SAG mills due to slurry and rock particle flows. Comminution 23.

3. SPH-DEM: This Lagrangian mesh-free method is useful for accounting for the fluid effect on particles in problems with high solid content and free surface flows, especially involving splashing or surface fragmentation, by discretizing the fluid into a set of fluid elements.

## Using Ansys Rocky and Ansys Fluent in CFD-DEM Coupling

The coupling between Rocky and Fluent emerges as one additional option among the comprehensive set of capabilities for solving fluid-particulate systems available in Ansys tools. This expands the range of granular-fluid systems that can be modeled.

There are currently two modes to couple Fluent and Rocky: the one-way and the two-way approach.

In the one-way approach, the fluid field affects the particle flow, but the particle flow does not affect the fluid field. This method is particularly useful for simulating dilute flows.

In the two-way approach, the fluid flow calculated in Fluent affects the flow of particles in Rocky, while particles also affect the flow properties in Fluent.

In both approaches, all particles are tracked in a Lagrangian frame of reference by the DEM solver, explicitly solving the equations that govern translational and rotational particle motion along with the energy balance on the particle. These equations consider the forces and torque on particles due to the fluid phase.

The main advantage of the Fluent-Rocky coupled approach is that cases in which particles have unique, nonspherical shapes — such as tablets — can be accurately solved, as particle-particle and particle-boundary interactions are solved and all forces acting on particles are computed on the Rocky side.

Rocky’s precise shape representation, combined with its many correlations for computing the fluid forces on particles, increases the accuracy of the models.

In addition, as each individual particle is tracked by the DEM solver, the complete history of all particles inside the domain is available. This includes velocities, temperatures, contact data, and more. Bringing together the extensive post-processing tools available in Rocky increases the level of information that can be extracted from your coupled simulation, providing better insight into your problem.

Rocky’s multi-GPU capabilities, when used in combination with several GPU cards and Fluent’s distributed parallel option for solving CFD equations, enables you to utilize all your hardware capabilities to solve real-scale applications.

Learn more about CFD-DEM coupling in Ansys software through an on-demand webinar or watch this how-to video.

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