arXiv subjects

Stefan Schwarzer

Publications and source records attributed to Stefan Schwarzer.

4 recordsLinked to original sources

arXiv1996

Fluid Induced Particle Size Segregation in Sheared Granular Assemblies

We perform a two-dimensional molecular-dynamics study of a model for sheared bidisperse granular systems under conditions of simple shear and Poiseuille flow. We propose a mechanism for particle-size segregation based on the observation that segregation occurs if the viscous length scale introduced by a liquid in the system is smaller than of the order of the particle size. We show that the ratio of shear rate to viscosity must be small if one wants to find size segregation. In this case the particles in the system arrange themselves in bands of big and small particles oriented along the direction of the flow. Similarly, in Poiseuille flow we find the formation of particle bands. Here, in addition, the variety of time scales in the flow leads to an aggregation of particles in the zones of low shear rate and can suppress size segregation in these regions. The results have been verified against simulations using a full Navier-Stokes description for the liquid.

cond-mat↗

arXiv1995

Number of branches in diffusion-limited aggregates: The skeleton

We develop the skeleton algorithm to define the number of main branches $N_b$ of diffusion-limited aggregation (DLA) clusters. The skeleton algorithm provides a systematic way to remove dangling side branches of the DLA cluster and has successfully been applied to study the ramification properties of percolation. We study the skeleton of comparatively large ($\approx 10^6$ sites) off-lattice DLA clusters in two, three and four spatial dimensions. We find that initially with increasing distance from the cluster seed the number of branches increases in all dimensions. In two dimensions, the increase in the number of branches levels off at larger distances, indicating a fixed number of $N_b = 7.5\pm 1.5$ main branches of DLA. In contrast, in three and four dimensions, the skeleton continues to ramify strongly as one proceeds from the cluster center outward, and we find no indication of a constant number of main branches. Likewise, we find no indication for a fixed $N_b$ in a study of DLA on the Cayley tree. In two dimensions, we find strong corrections to scaling of logarithmic character, which can help to explain recently reported deviations from self-similar behavior.

cond-mat↗

arXiv1995

Sedimentation and Flow Through Porous Media: Simulating Dynamically Coupled Discrete and Continuum Phases

We describe a method to address efficiently problems of two-phase flow in the regime of low particle Reynolds number and negligible Brownian motion. One of the phases is an incompressible continuous fluid and the other a discrete particulate phase which we simulate by following the motion of single particles. Interactions between the phases are taken into account using locally defined drag forces. We apply our method to the problem of flow through random media at high porosity where we find good agreement to theoretical expectations for the functional dependence of the pressure drop on the solid volume fraction. We undertake further validations on systems undergoing gravity induced sedimentation.

cond-mat↗

arXiv1993

Localization of Growth Sites in DLA Clusters: Multifractality and Multiscaling

The growth of a diffusion limited aggregation (DLA) cluster with mass $M$ and radius of gyration $R$ is described by a set of growth probabilities $\{ p_i\}$, where $p_i$ is the probability that the perimeter site $i$ will be the next to grow. We introduce the joint distribution $N(α, x, M)$, where $N(α,x,M)dαdx$ is the number of perimeter sites with $α$-values in the range $α\le α_i \le α+dα$ (``$α$-sites'') and located in the annulus [x, x+dx] around the cluster seed. Here, $α_i \equiv -\ln p_i / \ln R$ if $p_i>0$, $x\equiv r_i/R$, and $r_i$ is the distance of site $i$ from the seed of the DLA cluster. We use $N(α,x,M)$ to relate multifractal and multiscaling properties of DLA. In particular, we find that for large $M$ the location of the $α$-sites is peaked around a fixed value $\bar x(α)$; in contrast, the perimeter sites with $p_i=0$ are uniformly distributed over the DLA cluster.

cond-mat↗