Universität Stuttgart
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Item Open Access Untersuchung der Pfropfendynamik und -stabilität bei der vertikalen und horizontalen Pfropfenförderung(2005) Strauß, Martin; Herrman, Hans J. (Prof. Dr.)Die Pfropfenförderung stellt ein produktschonendes Transportverfahren für granulare Medien dar, dass in der Industrie breite Verwendung findet. Eine zuverlässige Voraussage des Massentransportes bei der Auslegung industrieller Anlagen ist beim derzeitigen Wissensstand jedoch nicht möglich, da Grundkenntnisse über diesen Fördertyp fehlen. Ursache ist, dass sich die Untersuchung der bei der Pfropfenförderung mittels Druckluft durch Transportröhren getriebenen Granulatpfropfen sich weitestgehend experimentellen Messmethoden entzieht. Um Zugang zu den Prozessen während des Transportes zu erhalten, wurde für diese Arbeit die Förderung mittels einer Kombination aus Molekulardynamiksimulation und einem Löser für den Druckverlust am System auf dem Computer nachgebildet. Die Gültigkeit des Ansatzes wurde mittels experimenteller Messdaten an einer Anlage im Maßstab 1 zu 1 nachgewiesen. Es wurden sowohl Parameterstudien für die vertikale als auch die horizontale Förderung durchgeführt. Die Ergebnisse zeigen unter anderem, dass die Stabilität der Pfropfen durch entgegengesetzt auf den Pfropfen wirkende Kräfte bewirkt wird. Maßgeblich sind dabei die vorantreibende Druckluft und die auf den Pfropfen treffenden bremsenden Granulatteilchen zwischen den Pfropfen. Der Effekt ist unabhängig von der Transportrichtung. Unterschiede im Längenwachstum und der inneren Dynamik der Pfropfen zwischen vertikaler und horizontaler Förderung zeigen, dass eine einheitliche Abhandlung der Förderungsrichtungen bei der Auslegung von industriellen Anlagen nicht sinnvoll ist.Item Open Access Simulations on evolutionary phenomena with ageing models(2006) Schwämmle, Veit; Herrmann, Hans (Prof.)The idea of reducing the characteristics of highly complex systems to some basic features without losing the essential informations has been successful for the comprehension of not only physical systems. The experience in treating systems which consist of many elements that interact over different time scales as well as over wide distances, has opened many interdisciplinary fields to statistical physicists. Although animals and human beings behave in a much more complex way than for instance atoms or molecules, simple statistical models are able to reproduce many biological phenomena. The goal is to reproduce the collective behaviour of a large number of organisms: their individual properties are not of crucial importance. Due to the strongly increasing computer power along the last fifty years, computational models have gained more and more importance for research in many areas. The limitation of analytical models to describe accurately non-linear systems with critical behavior makes computational models nearly irreplaceable.Item Open Access Sand dunes on Mars and on Earth(2007) Ribeiro Parteli, Eric Josef; Herrmann, Hans Jürgen (Prof. Dr.)In this work the dune model introduced by Sauermann et al. (2001) is extended and applied to investigate the formation of different dune shapes on Mars and on Earth as function of wind directionality and sand availability. The formation of sand dunes on Mars under the present atmospheric conditions of the red planet is studied and conclusions about wind speed, migration velocity of dunes and changing wind regimes on Mars are presented. Field measurements of the shape of coastal transverse dunes are presented and the formation of coastal dune fields is explained. Finally, the formation of linear dunes by bimodal wind regimes is calculated. The simulations explain the appearance of exotic bimodal dune shapes in areas of low sand availability on Mars and on Earth.Item Open Access Simulation of peloids(2007) Hecht, Martin; Herrmann, Hans J. (Prof. Dr.)In this work we investigate dense colloidal suspensions of alumina particles, which we regard as a model system for clay-like soils (peloids). Beyond soil mechanics these suspensions are important for wet processing of work-pieces in ceramics. In the present work we investigate the rheological properties of the suspensions and connect them to the underlying microstructure. Experimental findings depending on the pH value and the salt concentration in the sample (expressed as ionic strength) are reviewed (viscosity versus shear rate, shear thinning, oedometer data, sedimentation experiments, cyclic loading...) and computer simulations are performed to carry out further research work. For this purposes a coupled Molecular Dynamics (MD) and Stochastic Rotation Dynamics (SRD) code has been developed and the simulation results are compared to the experimental data. To describe the surface charge of the particles a charge regulation model, which describes adsorption and desorption of the charge determining ions on the particle surface, has been developed within Debye Huckel theory. This model has been calibrated to measurements of the zeta potential. Starting from the known values, the model allows us to extrapolate to different experimental conditions. Based on characteristic time scales and dimensionless numbers like the Reynolds number and the Peclet number, we apply a scaling scheme to determine the simulation parameters, so that we can achieve a quantitative comparability of simulation and experiment. Using the shear viscosity, shear force, pair correlation function, density fluctuations and structure factor we can identify three different regimes: a clustered regime, a stable suspension, a repulsive structure similar to the structure known from glassy systems. The microstructures are plotted in a stability diagram depending on pH value an ionic strength. The microstructures found in the simulations provide a possibility to explain the relations found in the experiments.Item Open Access Fibre models for shear failure and plasticity(2007) Raischel, Frank; Herrmann, Hans J. (Prof. Dr.)The fracture and failure of fiber reinforced materials such as wood, reinforced concrete, GFRP (glass fiber reinforced polymers) or CFRP (carbon fiber reinforced polymers) in considerably determined by the disordered properties of these materials and their constituents. A comprehensive understanding of their degradation and failure therefore requires the inclusion of statistical methods and models. This thesis describes the use of fibre bundle models, a well established model class used in statistical physics, to investigate the failure of fiber reinforced materials under shear, and the appearance of plasticity. We also consider the appearance of statistical precursors of imminent failure, and we improve already existing fibre bundle models for continuous damage to account for the failure of materials with a pronounced hierarchical setup.Item Open Access Vegetated dunes and barchan dune fields(2007) Durán, Orencio; Herrmann, Hans J. (Prof. Dr.)Desertification is closely related to aeolian sediment transport, including sand dunes formation, evolution and migration. Sand dunes propagate in huge clusters of thousand of dunes with an internal complex dynamics that determines their size and spatial distribution. Furthermore, it is known that the vegetation growing on sand dunes is an active agent that modifies dune mobility and sand distribution in a desert area, ultimately leading to an inactive sand landscape. This work addresses three crucial problems related to aeolian sand transport, barchan dune fields and vegetated dunes. First, what are the phenomenological parameters behind our sand transport model, and how they are related with the physical properties of the system. Second, how are barchan dunes distributed over desert? Which factors determine the size and spatial distribution of dune fields? And, third, how can vegetation deactivate a barchan dune? Which conditions are behind this inactivation process? As a result, we were able to extend the phenomenological parameters of our sand transport model to different physical conditions, such as under water or in the Mars atmosphere. We found that the size distribution of barchan dune fields is log-normal and dunes are regularly spaced. Therefore, barchan dune fields are fully described by the mean dune size, the standard deviation and the inter-dune spacing. Furthermore, these properties are not independent but are related by a simple constitutive equation and are dynamically selected by the upwind boundary conditions in the dune field. Finally, we found that active barchan dunes are transformed into inactive parabolic dunes under the effect of vegetation growth. This inactivation process occurs if the fixation index, a parameter fully determined by initial conditions, is below a critical value. This is a simple condition that leads to the first quantitative solution of the crucial dune inactivation problem.Item Open Access Polydisperse granular packings and bearings(2005) Mahmoodi Baram, Reza; Herrmann, Hans (Prof. Dr.)In this thesis we developed an algorithm for constructing three dimensional self-similar space-filling packings of spheres. Among the constructed configurations is the classical three dimensional Apollonian packing which was the only previously-known configuration. The difference between the calculated fractal dimensions indicates the topological difference between these configurations. Among the obtained packings there is one for which only two colors are necessary for coloring the spheres such that no two spheres having the same color touch each other. This is equivalent to saying that the packing consists of only even loops of touching spheres. In two dimensions this is the necessary and sufficient condition for a packing to act like a bearing. In three dimensions, however, it is not obvious that this is the case, since the particles have more rotational degrees of freedom. We showed that having only even loops is the necessary and sufficient condition for a packing to act like a bearing in three dimensions as well. We have given an explicit expression for the angular velocity of every sphere of the entire packing, in terms of the angular velocity of an arbitrarilily chosen sphere, and a factor $c\geq 0$.Item Open Access Fast Algorithms to simulate extremely polydisperse media(2006) Wackenhut, Martin; Herrmann, Hans J. (Prof. Dr.)In this work different algorithms to simulate extremely polydisperse media, which follow a truncated power law, were developed. The basic difficulty is the nonlinear dependence of the computing time on the polydispersity. It is possible to decouple the computing time from the polydispersity through a self-consistent and macroscopical discription of the so called local packings. These are the many small particles inbetween the large ones.Item Open Access Sedimentation of oblate ellipsoids(2004) Fonseca Fonseca, Frank Rodolfo; Herrmann, Hans (Prof. Dr.)The tree leaves flutter to the ground in autumn, exhibiting a complex motion and refusing to follow the shortest path. The way in which objects fall to the ground has been studied since antiquity. Objects were thought to return to their natural'' places by the ancient Greeks. During Renaissance, Galileo Galilei dropped two metal balls from the leaning Tower of Pisa and showed that they fall at the same rate despite having different masses. Newton showed, that the bodies fall on earth driven by a constant acceleration and he also observed the complex motion of objects falling in both air and water. But despite gravity's undeniable attraction, not all falling objects travel downwards in straight trajectories. The consideration of fluid surrounding the objects, introduces a very complicated and nonlinear interaction between the object and the fluid. The first pioneering effort was made by Maxwell, who was the first to consider the fluid-object interaction and proposed a odel for a falling paper strip. In the beginning, theoreticians made few assumptions a) constrained motion in 2-d was taken into account b) vortices in fluid were ignored c) considered a fluid with zero viscosity Based on these assumptions, Gustav Kirchhof showed that the problem reduces to a simplified set of equations that can be solved for simple particle geometries. This method also appears in the Horace lamb's classic treatise on hydrodynamics, (Lamb). A deeper understanding of the motion of falling objects in a fluid is of great technical importance, and has been investigated in a variety of contexts, including meteorology (Kajikawa), aircraft stability (Mises), power generation (Lugt), chemical engineering (Marchildon), and also in the study of stability of submarines and the centrifugation of cells by biological techniques.