Granulare Medien

Not quite a hopper, but a hole in the floor.

A Novel Approach to the Simulation of Particles on a Large Size-Range.

A very tiny simulation. 36 balls are being thrown into a container. The rolling of the particles leads to a final configuration, which is virtually flat. 

By means of molecular dynamic simulation, each individual ballast stone in the ballast track can be simulated in the model.  It is investigated to what extent it is possible to define contact force laws, material parameters and geometry variables in such a way that the resulting computational model reproduces the essential behaviour of the real system. Furthermore, possible areas of application of molecular dynamic simulation are mentioned.

The walls of the hopper are not as steep as in the other examples. The flow ends when an arch forms spontaneously and the hopper is blocked.

We have a hopper, filled with particles. Additional to the repulsive forces due to collisions we have ceohesive forces. You can find more information in this publication.

A piston is compressing with a certain force onto an assembly of granular materials. No gravity is acting. The Material is very soft, so there are oscillations.

Here a sand pile is build from a line source to demonstrate the importance of static friction in sandheaps. At time step 420 the friction coefficient is quickly reduced to zero. The pile melts and the granulate flows like a liquid. 

The time evolution of the force network of a sand pile which is built from a point source is shown. The width of the blue lines is proportional to the strength of the forces. The red arrows are the forces onto the ground. Note that the avalanches either go left or right and therefore lead always to slight asymmetries of the heap.

For the simulation, we have calculated the stress tensors inside the pile. In this movie, the major principal axis of the stress (crosses) and the pressure on the ground (arrows) are shown. 

This heap is build using 4500 slightly elongated Particles. The first frames shows the evolution of the heap, the bottom frame shows the pressure onto the ground. You can observe the evolution of a dip below the apex of the pile

This heap is the same as the other examples. In the top frame you can see the stress tensors inside of the pile. The second frame shows the forcenetwork; small forces are black, medium forces are red and strong forces are yellow. The 8 small frames show the axis of the major principal axis in the different layers of the pile. The last frame shows the pressure distribution. 

A small movie, showing a tinkertoy-setup involving dominos and a seesaw. This was produced for the open day of the university 1998 and aims to impress non-scientists. It also demonstrates the versatility of our program.

This work is a contribution to the under- standing of the mechanical properties of non-cohesive granular materials in the presence of friction and a continuation of our previous work (Roul et al. 2010) on numerical investigation of the macroscopic mechan- ical properties of sand piles. Besides previous numer- ical results obtained for sand piles that were poured from a localized source (‘‘point source’’), we here consider sand piles that were built by adopting a ‘‘line source’’ or ‘‘raining procedure’’.

The findings on the pressure distribution under conical piles has increased the interest in the stress distributions in static granular packings.

Since stresses inside granular systems are very difficult to access experimentally, we have performed DEM (Discret Element Model) simulations with polygonal particles. The shape and size of the particles can be specified arbitrarily in the simulation; the influence of these parameters can be determined.

The motion of a sliding particle, influenced by friction, in a rotating drum is investigated. A differential equation is formulated for general friction laws. Assuming a constant coefficient of friction, the equation is exactly solvable. For a velocity dependent coefficient of friction, perturbation methods may be used. The nonperturbed system is solved and with the help of the averaging method, the perturbed system can be examined for periodic motions.

The motion of a sliding particle, influenced by friction, in a rotating drum is investigated. A differential equation is formulated for general friction laws. Assuming a constant coefficient of friction, the equation is exactly solvable. For a velocity dependent coefficient of friction, perturbation methods may be used. The nonperturbed system is solved and with the help of the averaging method, the perturbed system can be examined for periodic motions.

Mit der Methode der Mittelwertbildung 

• testen wir verschiedene Reibungsgesetze
• suchen wir nach periodischen Bahnen
• untersuchen wir die Struktur des Phasenraums

Zur Zeit sind wir daran interessiert, den Einfluss der Reibung in Simulationen für granulare Systeme besser zu verstehen. Das Fernziel ist eine 3-dimensionale Simulation für nicht-sphärische Partikel.

The effect of the particle shape on the bulk-stress-strain-relations for triaxial compression of granular media is investigated via the molecular dynamicsmethod. It is found that crucial properties exhibited by experimental granular media cannot be reproduced by round particle simulations, but only by theuse of elongated particles.

The most time consuming part in molecular dynamics simulations is the collision detection. Usually, this problem is solved by restricting theshape of the particles to spheres. I will present an algorithm, originally developed for virtual reality visualizations by D.Baraff and M.C.Lin, that enables us to use complex polyhedra  (up to 920 faces and more). The expected run time is  O(N), where N is the number of particles in the simulation. Neither complexity nor shape of the particles affect the run time.