From pet hair to barley: discover how Rocky’s fiber model expands the types of problems that DEM can now solve
Published on: May 28, 2020
DEM has become a widely used design tool, and the types of materials that are modeled are more varied than ever. Real-world particles are quite often elongated, such as hay or hair, and particle flexibility needs to be accounted for when designing equipment to deal with such materials.
Traditional DEM codes model particles using a clustered- or “glued-” sphere approach. Although this method is easier to implement due to simple contact-detection algorithms for spheres, it does have significant disadvantages, such as the inability to efficiently model large-scale simulations containing such high-aspect ratio particles. For example, to accurately model an elongated hay particle with an aspect ratio of 1,000, at least 1,000 spheres (diameter 1 mm) need to be glued together, resulting in high memory requirements and long solution times.
Read this post for a better understanding of how the Rocky DEM fiber model overcomes these modeling challenges and allows you to efficiently model particles with large aspect ratios, such as hay, barley, pet hair, nets, logs, and more.
Rigid vs. Flexible Fibers
In Rocky DEM, the fiber shape category encompasses particles that are mainly one-dimensional, i.e., the geometry of a fiber can be described by a line (or several lines) in a three-dimensional space.
Fibers can be rigid, or if they are composed of multiple segments, flexible. In this approach, a flexible fiber is built by connecting sphero-cylinders by means of bonds with elastic and viscous properties, as depicted schematically in Figure 1.
For flexible fibers, when the adjacent elements move, the relative movement between them produces linear and angular deformations on the virtual bonds. In response, forces and moments are induced and exerted on the adjacent elements to resist the normal, tangential, bending, and torsional deformations.
The example below shows a fiber compression test simulated to acquire the force-deformation response curve and the hysteretic behavior of the hay. Both characteristics are then used to calibrate the hay material for future DEM simulations.
Video 1. Fiber compression test.
Straight vs. Custom Fibers
The fiber model allows you to model straight fibers (a linear shape, as shown in Figure 1) or create your own custom fiber shape by importing a .txt file. With custom fibers, sphero-cylinder segments can be arranged arbitrarily forming complex multi-branched geometries, as exemplified in Figure 2. The branches can have different diameters and nonuniform flexibility, enlarging the range of applications that can be modeled using DEM.
Anisotropy, plastic deformation, failure criteria and breakage
Rocky DEM’s advanced fiber model includes plastic deformation effects, anisotropy, and an integrity failure model, which allow you to cover a wider range of fiber materials in more complex equipment and with higher fidelity.
It is possible to individually define the stiffness for each joint deformation: normal, tangential, bending, and torsion.Moreover, with the bilinear elastoplastic model, you can model the transition between elastic and elastoplastic regimes, including the option of adding a failure criterion that models the loss of joint stiffness without rupture, i.e., without element separation.
Video 2. Fiber modeled using the bilinear elasto-plastic model with the failure criterion.
Last but not least, fibers can break according to the tensile or shear stresses at the joint.
Video 3. Fiber breakage using the shear stress breakage criterion.
Flexible fiber application examples
Example 1: Pet hair in a vacuum cleaner
This simulation shows a 1-way DEM-CFD coupled simulation of a large number of pet hair strands inside a vacuum cleaner, depicting behaviors such as flexibility and inter-fiber interaction. The hair strands were modeled using flexible fibers with different diameters, and the fluid field, obtained in Ansys Fluent, was taken into account when solving the particle trajectory inside this multi-cyclonic device.
Video 3. Rocky DEM and Ansys Fluent coupled simulation using flexible fibers.
Example 2: Hay tedder performance
In this case study, the goals were to model a hay tedder and compare different operating conditions to ensure proper material handling. Flexible fibers were used to model the hay, and the multi-GPU capability allowed the customer to simulate a large number of fibers.
Video 4. Hay tedder machine simulation.
Example 3: Barley harvester performance
This example shows the simulation of a combine harvester header, including hay breakage and plastic deformation of the stems. Particles were modeled using the Rocky DEM custom flexible fiber particle type to model the crop.
Video 5. Harvester simulation.
Example 4: Wood chipper efficiency
In this example, the goal was to evaluate the chipping efficiency for different disk rotational speeds as well as to assessif clogging would happen when changing the wood tonnage. Wood branches were modeled using custom flexible fibers, and the wood chopping was modeled using the fiber breakage model.
Video 6. Wood chipper simulation.
Example 5: Fouling of heat exchangers
This study was performed by Sub-Zero. Rocky DEM was used, combined with Ansys Fluent, to understand heat exchanger robustness to fouling. This understanding is important in that it gives Sub-Zero more freedom to design more compact refrigerating systems with higher energy efficiency and carefree products.
In this case, the cotton linter fibers were modeled using flexible fibers. The fluid forces were taken into account using the Rocky’s one-way coupling with Ansys Fluent, and an adhesion model was added to mimic the electrostatic force acting on the fibers.
Example 6: Log unscrambler
This example shows a log unscrambler simulation using rigid custom fibers. The logs have different diameters along their length.
Video 7. Wood log unscrambler.
Example 7: Gummy bears on a net
This example shows gummy bears (modeled using the concave particle model) falling on an elastic net that is modeled using the custom fiber particle shape.
Video 8. Concave particle model example and custom fiber particle shape demonstration.
 Guo, Y., C., W., Curtis, J. S., and Xu, D. (2018). A bonded sphero-cylinder model for the discrete element simulation of elastic-plastic fibers. Chemical Engineering Science, 175:118–129
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.