Tuesday 30th Nov, 2021

Supporting software for simulation

Peter Rizkalla, LEAP Australia’s Product Manager of Rocky DEM, explains how the program can be combined with other software to maximise the benefits of simulation.

Rocky is a 3D discrete element method (DEM) program that quickly and accurately simulates particle behaviour within bulk materials handling systems.

One of the programs key features is its integration with the engineering simulation software Ansys for finite element analysis (FEA) and computational fluid dynamics (CFD).

Rocky’s integration into the Ansys Workbench framework, allows Rocky users to share common geometry across multiple systems or to transfer the results from one solver to another allowing multi-physics simulations to be conducted. A user can start with any geometry in Ansys Spaceclaim and share this between Rocky DEM, Ansys Fluent and Ansys Mechanical.

DEM-CFD coupling with Rocky DEM and Ansys Fluent

In many cases, the solution of the discrete particulate phase does not accurately describe how a system performs, as there are external factors that can influence the overall bulk flow behaviour.

For example, consider the fish feeding device that distributes fish food pellets radially from a central location. If we were to solve this using DEM alone, air resistance would be ignored, and the pellet trajectory distance would be overestimated. By taking the drag force into account, we can estimate the landing location with a reasonable degree of accuracy. To complicate matters even more, an appropriate drag law must be selected that takes particle shape and alignment with the flow into account.

For dense flow systems on the other hand, where the feedback of the fluid flow on the particle flow is important, a two-way coupling approach must be used. In both approaches, the particles are solved in DEM using Lagrangian particle tracking by 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.

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In the two-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 and temperature in cases where heat transfer is important. This two-way exchange of information continues for each new time step until the full granular-fluid model is revealed.

There are many applications requiring a fully coupled two-way solution such as fluidised beds, pneumatic and hydraulic conveying, slurry mills and slurry transport. Multiple domains with non-conformal and moving meshes are used.

Custom particle shapes are used to accurately predict the tablet heating as hot air flows within the equipment, by considering both convective and conductive heat transfer. This simulation allowed engineers to predict the residence time and temperature distribution of particles for different operational conditions.

Using the right solution for the right application

There are already many comprehensive multiphase models within Ansys Fluent, making model selection little overwhelming or sometimes confusing.

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 can be accurately solved.

Rocky’s precise shape representation combined with its laws for computing the fluid forces on particles, increases the accuracy of the models. Adhesive and cohesive materials can also be modelled using one of the adhesion models available in Rocky DEM.

As each individual particle is tracked by the DEM solver, the complete history is available for all particles inside the domain. This builds up the level of information that can be extracted from a coupled simulation and providing better insight into a problem. In cases where particles are too small to model individually, Rocky provides a coarse-grain modelling approach allowing a ‘pseudo’ or scaled-up particle size to represent many smaller particles.

To ensure accuracy is maintained when particles are larger than the CFD mesh cell, a new volumetric diffusion mapping approach has been implemented.

DEM-FEA coupling using Rocky DEM and Ansys Mechanical

Imagine you’re tasked with designing a bucket conveyor system or bracing support frame for a transfer chute hood. How would you approach such a project?

One way would be to start with a known design, run some hand calculations with some 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, not to mention the potential safety risks associated with any physical testing.

Alternatively, high-fidelity simulation tools like Rocky DEM and ANSYS Mechanical can optimise the process and design parameters in a virtual environment.

DEM allows users to evaluate particle trajectories and bulk flow phenomena and interactions including inter-particle and boundary forces. FEA allows users to determine the impact and severity of these forces on the underlying structure helping to troubleshoot systems subject to failure and optimise their layout.

By coupling the two simulation techniques, engineers can make less assumptions about loads acting on structures. A typical example is the load acting inside a bin or silo during discharge. With the absence of DEM, the assumption commonly made is to treat the internal load as a hydrostatic pressure which varies with height. Experience has shown that this can lead to an overestimation of the loads, therefore leading to an overly conservative design.

During a simulation, Rocky DEM calculates the loads on each node of the boundary mesh. These loads are then exported as a pressure field for further analysis using Ansys Mechanical, which uses either a profile preserving or force conservative mapping algorithm. This is accomplished via Ansys’ external coupling feature within Ansys Workbench allowing dissimilar meshes to be used for the DEM and FEA models.