Discover how Fluent and Rocky coupling can be leveraged for efficient CFD + DEM simulations. Here, we modeled an aerated tank, simulating the dynamics of granules commonly used in water treatment processes.
In water treatment processes, improving mixing efficiency and controlling particle dynamics are key for achieving high-performance. One promising approach is to combine Computational Fluid Dynamics (CFD) with Discrete Element Method (DEM) using tools like ANSYS Fluent and Rocky DEM. This 2-way coupling allows engineers to capture the mutual interactions between fluid and solid phases in complex systems, such as aerated tanks.
In water treatment applications, mixing is often enhanced by aeration. Air bubbles rising from diffusers at the tank bottom induce turbulence and circulation, which in turn promote mass transfer and suspension of solid particles. When solid carriers or granules are introduced (e.g., in Moving Bed Biofilm Reactors or aerobic granular sludge systems), their movement strongly depends on the flow field. Conversely, the presence of particles alters the local flow through drag, momentum exchange, and turbulence.
This interaction cannot be easily captured accurately with CFD alone or DEM alone. That’s where 2-way coupling comes in:
Case Study: Aerated Mixing Tank with Granules
Here, we modeled a square-shaped water treatment tank with aeration at the base. Air bubbles rise. Granules are suspended in the liquid. The modeling objectives are:
The first step is to set up a Fluent case with the desired geometry. In this simple design, we have air coming from the bottom of the domain as bubbles.
Three phases are selected here. The phases are described below:
The particle phase will be reserved as the DEM phase used by Rocky once the coupling is established.
An initial volume of water is patched as initial condition, and only air flows through from the mass flow inlets at the bottom. Once the model is set up in transient mode, we can run for a few iterations to make sure it can converge on its on before we move it over to Rocky.
In Rocky, we need to first enable coupling, by right clicking in Study and enabling the 2-way coupling with Fluent. Note that we also need to create a particle in Rocky, with a prescribed density and size distribution.
Rocky also allows for easy transfer of boundaries from Fluent. We would also need to load the corresponding case file. To initialize using the data file generated in Fluent, check the Use Data Initialization option in the Fluent tab in the 2-Way Fluent (Data tree), and load the data file.
The proper modules would also need to be selected. Here, I chose the CFD Coupling Particle Statistics, Contacts Overlap Monitor, Inter-Group Collision Statistics and the Inter-particle Collision Statistics.
Particles are added into the system for the first 1 second at a rate of 0.04 tons/h, and the simulation is run for a total of 3 seconds. While the simulation is in progress, we can visualize the flow field from Fluent as it interacts with the DEM particles.
For more detailed visualization, we can employ EnSight tools to view the particle dynamics within the tank, colored by different key parameters.
Details about the simulation setup are shown in this video:
By leveraging the 2-way coupling of Fluent and Rocky, engineers can model aerated tanks with high fidelity. This approach captures the essential physics of bubble-driven mixing, particle suspension, and fluid–solid interaction. As a result, it becomes a powerful tool for designing and optimizing advanced treatment processes.
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