3. Granular Overview

Note

Feature requires a Momentum Granular license.

Algoryx Momentum can simulate granular material together with multi-body dynamics. The Granular functionality can mainly be found in the Granular Ribbon Menu and requires a Granular license in order to function.

3.1. Granular Simulations

Algoryx Momentum can be used to simulate granular materials such as pellets, pills, grains, soil, rocks, etc. together with articulated rigid body dynamics. The simulations can be conducted both inside and outside the SpaceClaim environment. External simulations run with higher computational performance in a minimal AGX Dynamics <http://www.algoryx.se/products/agx-dynamics/> instance that can record simulation data to files that can later be loaded in a post processing environment.

The created granular flows can be divided into two different types:

Both material types can intermix and interact with other rigid multibody systems in the scene.

Attention

For systems with more than 10000 granular particles/bodies, it is recommended to run an external simulation.

3.2. Interacting Granular Material with Articulated Rigid Body Systems

Algoryx Momentum support simulation with strong coupling between the granular material and other rigid multibody systems.

3.3. Control of Granular Flows using Emitters and Sensors

Flows of granular material can be inserted in the simulation by adding Emitter objects. Sensors can be defined to extract data from specific locations in the granular flow or to alter the mechanical properties of the particles.

3.4. Spherical Granular Bodies

The granular bodies are represented as spheres with 6 Degrees of Freedom (6-DOF) using Hertzian contacts laws, Coulomb friction and rolling resistance. The Material Pairs Material Pairs interface is extended with options for controlling Adhesion, Rolling Resistance and Twist Resistance in the granular contacts.

The granular contact laws are transformed to nonsmooth dynamics formalism that AGX Dynamics rest upon. High-velocity impacts are modeled with impulse transfer, that may propagate instantly through the contact network and the articulated rigid multibody system. This may be considered as a time-implicit version of the discrete element method (DEM) and is referred to as a nonsmooth DEM (NDEM).

The theory for the contact model can be found under the Granular Contact Model section.

Attention

When using granular flows are consisting of Rigid Bodies, the rolling resistance part of the contact equations are omitted; The resistance is instead modelled via the shape irregularity.

3.5. Rigid Bodies

6-DOF Rigid Bodies consists of bodies created from Templates based on existing user-defined Rigid Bodies in the scene. These are used in body distributions.

Key differences compared to Spherical Granular Bodies are:

  • Uses the regular Rigid Body construction pipeline for defining body properties such as material and mass properties and solid/geometry composition.

  • Supports arbitraty shape representation and composition with the option for primitive detection for increased collision detection and contact generation performance.

  • Rigid Bodies uses a linear-elastic contact model with Coulomb friction. It does not support the rolling resistance equations. The rolling resistance phenomena is modelled via shape regularity.

  • Can use both the Direct and Split solver in the contacts as well as the Iterative solvers. ( Note that a performance warning will be given upon usage of DIRECT contact materials used in emitted Rigid Body contacts. )

  • Some features are not supported for the Rigid Bodies pipeline, for example collision shape coloring.

Note

The Rigid Body computation pipeline is more complex than the Spherical Particles, which results in increased memory footprint and required computational power.

Note

By default the Rigid Body Templates have convex detection enabled to convert convex shapes trimeshes to convex meshes. This increase collision detection performance and reduces the number of contacts for faster computational times. This is relevant when simulating large sets of bodies. The setting can be toggled under Granular Settings. To visualize what type of collision shape your Rigid Body Templates have been converted to, use the Color By Collision Shape feature.

Attention

The Rigid Body pipeline currently has some limititations compared to spherical granular bodies:

  • Does not support the non-linear Hertzian model in contacts, instead uses the elastic foundation model.

  • Does not support rolling resistance forces. Use shape irregularity instead.

  • Does not support body coloring.

  • Does not support some sensor functionality, for example: Teleportation, contact export and coloring.

  • Does not support lifetime settings.

3.6. Numerical Integration and Solver

Large scale granular simulations, using either spherical particles or Rigid Bodies, results in a large contact set and thus a many equations that need to be solved. A Parallel-Gauss-Seidel solver that partitions and solves the domain in different sub-domains is used to achieve high-performance computation that scales over many cores. See Stable Simulations for more information about iterative solvers. If enabled, this solver will also work for regular rigid body contacts that uses an iterative material.

Attention

Granular contacts will always be solved using the iterative solver, regardless of solver settings in the material pair!

3.7. External Post Processing

External simulations generated journal files (.agxJournal) that contain simulation data. These files can be loaded into a post processing environment where rudimentary analysis can be performed.