Dr. Volkhard Buchholtz

Function

Diploma student, graduate student

Info

Logos Verlag Berlin, Germany

Publications

Molecular Dynamics of comminution in ball mills

Bulk Solids Handling, 20, 159-172

(2000)

Research Projects
Abstract

We investigate autogenous fragmentation of dry granular material in rotating cylinders using two-dimensional molecular dynamics. By evaluation of spatial force distributions achieved numerically for various rotation velocities we argue that comminution occurs mainly due to the existence of force chains. A statistical analysis of these force chains explains the spatial distribution of comminution efficiency in ball mills as measured experimentally by Rothkegel [1] and Rolf [2]. For animated sequences of our simulations see url http://www.mss.cbi.uni-erlangen.de/index.php?p1=research&p2=articles&r=granular/RotatingCylinder/Comminution

Materials
 

Molekulardynamische Untersuchung von Zerkleinerungsprozessen in Kugelmühlen

Schüttgut, 6, 11-24

(2000)

Research Projects
Abstract

Molekulardynamische Untersuchungen eignen sich zur Simulation des Verhaltens makroskopischer Mengen granularen Materials unter bestimmten, technologisch relevanten Beanspruchungen. Wir untersuchen das Zerkleinerungsverhalten von Mahlgut in einer Kugelmühle, insbesondere im Hinblick auf die Effizienz als Funktion der Drehzahl und auf die räumliche Verteilung von Beanspruchungen. Die Untersuchung der Verteilung von Kraftketten liefert eine Erklärung des Experiments von Rothkegel [1] und Rolf. Für animierte Sequenzen der Simulationen s. url: http://www.mss.cbi.uni-erlangen.de/index.php?p1=research&p2=articles&r=granular/RotatingCylinder/Comminution

Materials
 

Molecular Dynamics of comminution in ball mills

European Physical Journal, 16, 169-182

(2000)

Research Projects
Abstract

We investigate autogenous fragmentation of dry granular material in rotating cylinders using two-dimensional molecular dynamics. By evaluation of spatial force distributions achieved numerically for various rotation velocities we argue that comminution occurs mainly due to the existence of force chains. A statistical analysis of these force chains explains the spatial distribution of comminution efficiency in ball mills as measured experimentally by Rothkegel [1] and Rolf [2]. For animated sequences of our simulations see url http://www.mss.cbi.uni-erlangen.de/php/research/granular/RotatingCylinder/Comminution/

Materials
 

Interaction of a granular stream with an obstacle

Granular Matter, 1, 33-41

(1998)

Research Projects
Abstract

We investigate numerically the interaction of a stream of granular particles with a resting obstacle in two dimensions. For the case of high stream velocity we find that the force acting on the obstacle is proportional to the square of the stream velocity, the density and the obstacle size. This behaviour is equivalent to that of non-interacting hard spheres. For low stream velocity a gap between the obstacle and the incoming stream particles appears which is filled with granular gas of high temperature and low density. As soon as the gap appears the force does not depend on the square of velocity of the stream but the dependency obeys another law.

Materials
 

Adaptive evolutionary optimization of team work

Int. J. Bifurc. Chaos, 7, 751-757

(1997)

Research Projects
Abstract

A complex optimisation problem is studied using an evolution game. Each individual which undergoes evolution is a set of points in the plane. During the evolution process the positions of the points in the plane and the number of points which belong to each individual are optimised. It is shown that in certain cases it might be more effective to solve a sequence of problems which degree of complexity is increased stepwise than to solve the original difficult problem at once.

Materials
 

Force Distribution and Comminution in Ball Mills

Friction, Arching and Contact Dynamics (Wolf, D. E. and Grassberger, P.), 265-273, World Scientific, Singapore

(1997)

Research Projects
Abstract

The motion of granular material in a ball mill is investigated using molecular dynamics simulations in two dimensions. In agreement with experimental observations by Rothkegel [1] we find that local stresses – and hence the comminution efficiency – are maximal close to the bottom of the container. This effect will be explained using analysis of statistics of force chains in the material.

Materials
 

Swirling granular matter: from rotation to reptation

Physical Review E, 54, R4560-R4563

(1996)

Research Projects
Abstract

Experiments and numerical simulations of granular material under swirling motion of the container are presented. At low packing densities the material rotates in the same direction as the swirling motion of the container (rotation). At higher densities the cluster of granular material rotates in the opposite direction (reptation). The change of the direction of the motion of the cluster takes place at a critical packing density while the diffusion coefficient changes significantly. The measured critical density of the packing is in good agreement with results obtained by molecular-dynamics simulation.

Materials
 

Avalanche statistics of sand heaps

Journal of Statistical Physics, 84, 1373-1378

(1996)

Research Projects
Abstract

Large scale computer simulations are presented to investigate the avalanche statistics of sand piles using molecular dynamics. We could show that different methods of measurement lead to contradicting conclusions, presumably due to avalanches not reaching the end of the experimental table.

Materials
 

Complex flow of granular material in a rotating cylinder

Chaos, Solitons and Fractals, 5, 1901-1912

(1995)

Research Projects
Abstract

The flow of granular material in a rotating cylinder was simulated by molecular dynamics in two dimensions using spherical as well as nonspherical grains. At very low but constant angular velocity we found that the flow varies irregularly with time. The particles move stick-slip like i.e. there are avalanches of different size at the surface of the granular material. Observing the traces of the particles we found that there are unstable convection cells. Our results agree with recent experiments by Rajchenbach and Rolf.

Materials
 

Simulation of rotating drum experiments using non-circular particles

Physica A, 216, 199-212

(1995)

Research Projects
Abstract

We investigate the flow of granular material in a rotating cylinder numerically using molecular dynamics in two dimensions. The particles are described by a new model which allows to simulate geometrically complicated shaped grains. The results of the simulation agree significantly better with experiments than the results which are based on circular particles.

Materials
 

Molecular dynamics of arbitrarily shaped granular particles

J. Phys. I. France, 5, 1431-1455

(1995)

Research Projects
Abstract

We propose a new model for the description of complex granular particles and their interaction in molecular dynamics simulations of granular material in two dimensions. The grains are composed of triangles which are connected by deformable beams. Particles are allowed to be convex or concave. We present first results of simulations using this particle model.

Materials
 

Numerical investigation of the evolution of sandpiles

Physica A, 202, 390-401

(1994)

Research Projects
Abstract

The evolution of a pile of granular material is investigated by molecular dynamics using a new model including nonsphericity of the particles instead of introducing static friction terms. The angle of repose of the piles as well as the avalanche statistics gathered by the simulation agree with experimental results. The angle of repose of the pile is determined by the shape of the grains. Our results are compared with simulations using spherical grains and static friction.

Materials
 

A vectorized algorithm for molecular dynamics of short range interacting particles

Int. J. Mod. Phys. C, 4, 1049-1057

(1993)

Research Projects
Abstract

We report on a lattice based algorithm, completely vectorized for molecular dynamics simulations. Its algorithmic complexity is of the order O(N), where N is the number of particles. The algorithm works very effectively when the particles have short range interaction, but it is applicable to each kind of interaction. The code was tested on a CRAY YMP EL in a simulation of flowing granular material.

Materials
 

Static friction phenomena in granular materials: Coulomb law versus particle geometry

Physical Review Letters, 71, 3963-3966

(1993)

Research Projects
Abstract

The static as well as the dynamic behavior of granular material are determined by dynamic and static friction. There are well known methods to inlcude static friction in molecular dynamics simulations using scarcely understood forces. We propose an ansatz based on the geometrical shape of nonspherical particles which does not involve an explicit expression for static friction. It is shown that the simulations based on this model are close to experimental results.

Materials