# DEM Application

WARNING: this page is not finished, we are writing it still...

The DEM Kratos Team

## Theory

This application solve the the equations.... Mathematical approach to the problems.

Nothing numerical

### Integration Schemes

Forward Euler Scheme

### The contact laws

Concept of indentation HMD, LSD

##### Normal Force Laws

##### Tangential Force Laws

##### Damping Force Laws

(restit. coef)

## Numerical approach (implementation)

Structure of the code (Strategy, Scheme, Element, Node, Utilities, functions frequently used like FastGet,...)

### DEM strategies

##### Non-cohesive materials Strategy

##### Continuum Strategy

### DEM schemes

### DEM elements

##### Spheric Particle

##### Spheric Continuum Particle

##### Spheric Swimming Particle

## Benchmarks

- brief description of the solved problem, if it is a benchmark that can be found in literature, insert the link to the reference or, at least a reference). - reference with a link the location in which you describe all the theory behind Numerical approach.

### Elastic normal impact of two identical spheres

Check the evolution of the elastic normal contact force between two spheres with time.

### Elastic normal impact of a sphere with a rigid plane

Check the evolution of the elastic normal contact force between a sphere and a plane.

### Normal contact with different restitution coefficients

Check the effect of different restitution coefficients on the damping ratio.

### Oblique impact of a sphere with a rigid plane with a constant resultant velocity but at different incident angles

Check the tangential restitution coefficient, final angular velocity and rebound angle of the sphere.

### Oblique impact of a sphere with a rigid plane with a constant normal velocity but at different tangential velocities

Check the final linear and angular velocities of the sphere.

### Impact of a sphere with a rigid plane with a constant normal velocity but at different angular velocities

Check the final linear and angular velocities of the sphere.

### Impact of two identical spheres with a constant normal velocity and varying angular velocities

Check the final linear and angular velocities of both spheres.

### Impact of two differently sized spheres with a constant normal velocity and varying angular velocities

Check the final linear and angular velocities of both spheres.

References:

Y.C.Chung, J.Y.Ooi. *Benchmark tests for verifying discrete element modelling codes at particle impact level* (2011).

## How to analyse using the current application

### Pre-Process

GUI's & GiD

##### D-DEMPack

D-DEMPack is the package that allows a user to create, run and analyze results of a DEM simulation for discontinuum / granular / little-cohesive materials. It is written for GiD. So in order to use this package, you should install GiD first.

You can read the D-DEMPack manual or follow the D-DEMPack Tutorials for fast learning on how to use the GUI.

##### C-DEMPack

Continuum / Cohesive

##### F-DEMPack

Fluid coupling

### Post-Process

## Application Dependencies

The Swimming DEM Application depends on the DEM application

### Other Kratos Applications used in current Application

FEM-DEM

## Programming Documentation

The source code is accessible through this site.

## Problems!

#### What to do if the Discrete Elements behave strangely

In the case you notice that some discrete elements cross walls, penetrate in them or simply fly away of the domain at high velocity, check the following points:

In the case of excessive penetration:

**Check that the Young Modulus is big enough**. A small Young Modulus makes the Elements and the walls behave in a very smooth way. Sometimes they are so soft that total penetration and trespass is possible.

**Check the Density of the material**. An excessive density means a big weight and inertia that cannot be stopped by the walls.**Check the Time Step**. If the time step is too big, the Elements can go from one side of the wall to the other with no appearence of a reaction.**Check the frequency of neighbour search**. If the search is not done frequently enough, the new contacts with the walls may not be detected soon enough.

In the case of excessive bounce:

**Check that the Young Modulus is not extremely big**. An exaggerated Young Modulus yields extremely large reactions that can make the Elements bounce too fast in just one time step. Also take into account that the stability of explicit methods depends on the Young Modulus (the higher the modulus, the closer to instability).

**Check the Density of the material**. A very low density means a very small weight and inertia, so any force exerted by other elements or the walls can provoque big accelerations on the element.**Check the Time Step**. If the time step is too big, the method gains more energy, and gets closer to instability.**Check the restitution coefficient of the material**. Explicit integration schemes gain energy noticeably, unless you use a really small time step. In case the time step is chosen to be big (but still stable), use the restitution coefficient to compensate the gain of energy and get more realistic results.

## Contact

Contact us for any question regarding this application:

-Miguel Angel Celigueta: maceli@cimne.upc.edu

-Guillermo Casas: gcasas@cimne.upc.edu

-Salva Latorre: latorre@cimne.upc.edu

-Miquel Santasusana: msantasusana@cimne.upc.edu

-Ferran Arrufat: farrufat@cimne.upc.edu