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DEM-FEA: realistic representations of granular materials loads on equipment

Updated from original publication on: June 5, 2019

Imagine you’re tasked with designing a bucket conveyor or a loader at a given throughput. How would you approach such a project?

One way is to start with a known design, run some hand calculations with due assumptions, and perform a field test. Then in the likely event it fails on the first trial, make design changes based on your best assessment and try again. This physical prototyping approach involves a lot of time, cost, and physical effort.

Alternately, you can try what engineers throughout the world are doing – use high-fidelity simulation tools like Discrete Element Method (DEM) and Finite Element Analysis (FEA) to optimize the process and design parameters virtually.

DEM and FEA basics

Today, DEM is an integral tool across many industries that handle bulk material like rocks, soil, powdered chemicals, potato chips, and pharmaceutical tablets. By taking into account all the forces acting on each particle in the bulk system, DEM brings insight into how these materials would perform within the given equipment over a range of process conditions. With its ability to handle large particle counts of real shapes and sizes to provide a quick and accurate prediction of process performance, Rocky DEM is used across multiple industrial sectors, including mining, heavy machinery, agricultural, chemical, and pharmaceuticals.

FEA is widely used for structural analysis in the civil, automotive, and aeronautic sectors. For a given load, FEA software such as Ansys Mechanical solves for an equilibrium condition in the structure. For transient simulations, the equilibrium conditions account for both deformation and kinematic energies, while for static simulations, only the deformation is considered.

Coupling Principle

During a simulation, Rocky DEM tracks the particle loads on each node of the geometry mesh. These loads are then exported as a pressure field for further analysis using Ansys Mechanical, which discretizes the geometry and then solves for the equilibrium condition as discussed above. Figure 1 shows how the loads are exported from the DEM simulation for static structural analysis.

static structural simulation
Figure 1. Exporting loads from Rocky DEM for static structural simulation within Rocky DEM.

Why use Rocky DEM for DEM-FEA coupling?

Choosing Rocky DEM for structural analysis provides great value, as can be seen from the following highlights:

Both static and transient structural analysis problems can be solved using Rocky DEM and Ansys Mechanical. With the latter program, engineers can simulate transient cases while incorporating geometry motion and time-varying loads on boundary elements. This animation shows how the instantaneous bulk material loads on a bucket excavator are captured.

Rocky DEM is fully integrated into Ansys Workbench (Figure 2) and does not require any other external software for coupling. The tight integration between Rocky DEM and other component applications within the Ansys Workbench platform makes the setup of complex multiphysics simulations easy. The two programs together enable engineers to realistically model granular materials and their complex behaviors by including different physics in the analysis, evaluating a wider range of scenarios, generating results that are closer to real life.

Rocky DEM ANSYS Workbench
Figure 2. Integration of Rocky DEM into Ansys Workbench.

As parameters are managed at the project level, it is simple to define multiple parametric cases and effortlessly evaluate key design parameters and operational conditions to assess equipment performance or perform what-if studies with a set of design points.

This integration also allows engineers to use design exploration systems to perform automated design explorations such as run DOEs, generate response surfaces with well-defined input and output parameters as shown in Figure 3, or even run goal-driven optimizations.

response surface
Figure 3. Design of experiments within Ansys Workbench to generate a response surface and assess the impact of geometry and process changes.

Rocky DEM can replicate complex motions within its UI, including combined motions and particle-induced free body motion with 6 degrees of freedom. No external coupling software is needed! 

The animation below shows the instantaneous loads on the blades and the shaft of harrow equipment as it plows the soil. Note that the assembly has free body motion, enabling it to move according to the soil topology – just like it would in reality.

Erosion is a complex phenomenon and depends on surface conditioning and particle interactions. Rocky’s wear model, which correlates volume losses with the work due to friction forces, has been extensively correlated with a wide variety of materials and is frequently used by the mining industry to simulate wear.

In addition to predicting the wear itself, users can actually predict wear surface modification as shown in Figure 4 . 

Figure 4. Design of experiments within Ansys Workbench to generate a response surface and assess the impact of geometry and process changes.
Figure 4. Design of experiments within Ansys Workbench to generate a response surface and assess the impact of geometry and process changes.

With a wear model that accounts for the particle loads, taking into account the real particle shape and size distribution, designers can increase equipment lifetime and reduce costs related to replacements and production stops. The video below shows an excavator bucket simulation using Rocky DEM coupled with Ansys Mechanical to accurately predict stresses and strain on the excavator bucket.

Solving real-life problems

Armed with these state-of-the-art features, Rocky DEM has helped clients across the globe improve their equipment and process design at a fraction of the cost associated with trial and error experimentation.

Vale, for example, one of the largest producers of iron ore in the world, faced reduced production efficiency when crushed ore jammed the moving screens at the base of its hoppers. This also increased the maintenance downtime caused by frequent grid cleaning. Using Rocky DEM and Ansys FEA coupling to capture the broad size and shape distribution of the incoming feed, Vale engineers accurately characterized the loads induced on the screens, allowing the company to implement effective design changes (Figure 5).

Roller screen before (top) and after (bottom) with design changes based on Rocky DEM-Ansys FEA simulations.
Figure 5. Roller screen before (top) and after (bottom) with design changes based on Rocky DEM-Ansys FEA simulations.

Vale engineers then virtually optimized the tilt angle, rotation speed, spacing, and profile of the roller disks – leading to significant design improvements.

After the changes were implemented, production increased by 11.4%, saving $100 million in just over 3 months!

Saurabh Sarkar

Applications Engineer, Rocky DEM

Dr. Saurabh Sarkar is an Applications Engineer for the Rocky DEM Business Unit. Prior to joining ESSS, Dr. Sarkar worked as an Adjunct Faculty at Rutgers University and an on-site Consultant at Sunovion Pharmaceuticals where he supported drug formulation and process development activities. He obtained his Ph.D. in Pharmaceutics from the University of Connecticut where his focus was understanding and optimization of different pharmaceutical unit operations using DEM and CFD tools in projects with multiple industrial and government collaborators. He is a Senior Member of the AIChE and serves as an expert reviewer for several journals.

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