Ansys Fluent (CFD) and Rocky DEM coupling for modeling fluid particulate systems
Published on: February 19, 2021
Updated from original publication on March 14, 2019
The coupling between Ansys Fluent (CFD) and Rocky DEM emerges as one additional option amongst the comprehensive set of capabilities for solving fluid particulate systems available in Ansys tools, enlarging the range of granular-fluid systems that can be modeled.
In this post, you will learn how the Ansys Fluent-Rocky DEM coupling solution works and under what conditions it is best to employ it. At the end of the article, you will also see some industry examples of the Ansys Fluent-Rocky DEM coupling solution in action.
How does the Ansys Fluent-Rocky DEM coupling work?
There are currently two modes to coupling Fluent and Rocky DEM solutions: 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, in turn, affect the fluid field. This method is particularly useful for simulating dilute flows.
In the two-way approach, the fluid flow calculated in Ansys Fluent affects the flow of particles in Rocky DEM while Rocky-calculated particles also change the flow of the fluids in Ansys Fluent.
In both approaches of the Rocky-Fluent coupling, all particles are tracked in a Lagrangian way 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 take into account the forces and torque on particles due to the fluid phase.
In the 2-way coupling, given the pressure and fluid velocities computed by Ansys Fluent, Rocky DEM computes the volume fraction of the particulate phase and the momentum and energy exchanged between particles and fluid phases.
These terms are then transferred to the CFD solver, which uses this information to solve the equations that govern the fluid flow, updating the pressure and fluid velocities. This two-way exchange of information continues for each new time step until the full granular-fluid model is revealed (Figure 1).
When should Ansys Fluent-Rocky DEM coupling be used?
The main advantage of the Fluent-Rocky DEM coupled approach is that since particle-particle and particle-boundary interactions are solved and all forces acting on particles are computed on the DEM side, cases in which particles have unique, non-spherical shapes ‒ such as tablets (Figure 2) ‒ can be accurately solved.
Rocky’s precise shape representation (including custom convex and concave shapes, flexible fibers, and shell particles) combined with its several laws for computing the fluid forces on particles, increases the accuracy of the models. By the same token, adhesive/cohesive materials can be modeled using one of the adhesion models available in Rocky DEM.
In addition, as each individual particle is tracked by the DEM solver, the complete history is available for all particles inside the domain (this includes, for example, velocities, temperatures, and contact data). Bringing together the extensive post-processing tools available in Rocky, it builds up the level of information that can be extracted from your coupled simulation, providing better insight into your problem.
Moreover, the association of Rocky’s multi-GPU capabilities to combine several GPU cards for the DEM solver with the Ansys Fluent distributed parallel option for solving the CFD equations, brings into reality the simulation of cases with a huge number of particles.
Finally, Rocky features such as adhesion/cohesion modeling, detailed particle collision statistics, energy spectra analysis, and breakage modeling can be simultaneously used with the Fluent-Rocky coupling solution, broadening the range of cases that can be numerically modeled and expanding your analyses to the next level.
Examples of Ansys Fluent-Rocky DEM coupled simulations:
Rings suspended in the fluidized bed
In this coating equipment, small spherical particles are fluidized due to the air flowing from the bottom while some “ring” shaped particles are kept suspended due to the collisions of the small particles against the ring surface. Particles are colored in Rocky by their initial position. This simulation helped engineers to predict scenarios in which the rings were not suspended and fall near the gas inlet, blocking the gas flow and collapsing the bed.
Flexible hair strands in a cyclonic vacuum cleaner
In this simulation, performed by Bissell, a large number of hair strands are modeled using Rocky’s flexible fiber particle shape and the one-way coupling with Fluent. A drag model adequate for long slender particles was adopted to incorporate the effect of fluids on the hair fibers. Using Fluent-Rocky coupling as a tool for hair modeling enables conceptual device testing, reducing the number of lab-tested prototypes and, therefore, minimizing development time and cost.
Pharmaceutical tablet coating device
In this study, Rocky DEM was fully coupled with Ansys Fluent to study the drying process in a tablet coater. Custom particle shapes are used to accurately predict the tablet heating as hot air flows within the equipment, by taking into account both convective and conductive heat transfer. The coating liquid was injected and allowed to dry via convection heat from the hot air stream. During this process, the solute that was present on the coating liquid was incorporated to the tablet, and coating homogeneity was then investigated by measuring the final particle mass variability.
Fine particles in human airways
The coupling framework between Rocky DEM and Ansys Fluent accurately predicts the transport and deposition of cohesive pharmaceutical powders in human airway with different shapes, such as spherical particles or multi-element flexible fibers. More details on how this multiphysics modeling adds to understanding fine-particle transport in human airways can be found in this post.
Rocky DEM flexible fiber is used to model grass in this simulation of a riding lawnmower. One-way coupling between Fluent and Rocky is adopted to incorporate the fluid forces on grass particles near the rotating blades. A breakage model allows engineers to model the cutting of the grass.
Sugarcane bagasse pneumatic separation
This simulation, performed by researchers from the University of Campinas, shows the 1-way coupling CFD-DEM technique as a powerful tool to enhance the sugarcane bagasse pneumatic separation process understanding. Different particle shapes for the bagasse are used and the air flow rate is varied in order to evaluate different operational conditions. The use of Ganser drag law takes into account the particle shape and alignment with the flow and allows the correct prediction of the separation efficiency.
This blog post shows how the CFD-DEM coupling provides an intermediate approach between using the sub-particle resolution for the fluid and the mesh resolution for both fluids and particles.
 Haghnegahdar, A., Bharadwaj, R., Feng, Y., Sarkar, S., Targeted Pulmonary Drug Delivery in COPD Patients, AIChE 2019 Annual Meeting – November 10-15, 2019 – Orlando, USA.
 Almeida, E., Spogis, N. and Silva, M.A., Computational Study of the Pneumatic Separation of Sugarcane Bagasse, 6th International Conference on Engineering for Waste and Biomass Valorisation – May 23-26, 2016 – Albi, France.
Global CAE Specialist, ESSS
Lucilla holds a BE (Chemical) undergraduate degree, an M.Sc. in Chemical Engineering and a Ph.D. in Nuclear engineering from the Federal University of Rio de Janeiro. She joined ESSS in 2008 and has spent 5 years focused on applying CFD to solve common engineering problems in the Oil and Gas industry, dealing with turbulent and multiphase flow simulations. Since 2013, she is an Application Engineer for Rocky DEM, supporting users, working on consultancy projects and validating models implemented for the CFD-DEM coupling.