arXiv subjects

Dominik Krengel

Publications and source records attributed to Dominik Krengel.

8 recordsLinked to original sources

arXiv2026

Self-organisation in hard-soft granular mixtures

Self-organisation of granular systems is a key determinant of their macroscopic behaviour and has been studied extensively in assemblies of hard particles. We use numerical simulations to test this understanding in mixtures of hard and soft particles, focusing on cells, the smallest irreducible loops of the contact network, as structural descriptors. We show that, while the cell statistics display robust qualitative features under isotropic compaction, they depend on inter-particle friction, $μ$, and soft-particle fraction, $κ$. Specifically, (i) the quadron area distributions retain a $Γ$ form, albeit with parameters that vary systematically with $μ$ and $κ$. (ii) Predictions of the cell order distribution (COD) by maximising the entropy, without taking mechanical stability into consideration, become increasingly inaccurate at large cell orders. (iii) Irrespective of $μ$, the normalised cell stress distributions collapse onto one master Weibull form, whose only shape parameter depends weakly on $κ$. This suggests a quasi-universal form that may deteriorate slightly at very high fractions of soft particles. (iv) Cells align preferentially along the local major principal stress direction, showing the same coordinated stress--structure self-organisation as in hard particles. The relative robustness of cell statistics to the addition of soft particles suggests that hard and hard--soft granular mixtures can be described by one model.

cond-mat.soft↗

arXiv2026

Nonuniqueness of Tangential and Kinematic Restitution in Frictional Oblique Particle Impacts

We systematically investigate the different coefficients of restitution of a dissipative, frictional disc as a function of the impact angle $θ$, the friction coefficient $μ$ and initial rotation $ω_{\mathrm{in}}$. We observe a non-monotonic, non-linear behaviour of $e_{\mathrm{T}}$ and $e_{\mathrm{kin}}$ with a clear minimum at $μ$-dependent values of $θ$ and a convergence for all $μ$ at large $θ$, bounded by the cases of pure rolling and pure sliding. Changing the dissipative normal interaction affects the $μ$ convergence at large $θ$ and leads to a convergence at low $θ$. The presence of initial angular velocity $ω_{\mathrm{in}}$ can significantly alter the functional behaviour of $e_{\mathrm{T}}$ at steep impacts and slightly change the magnitude of $e_{\mathrm{kin}}$, with friction playing only a minor role. Overall, our results indicate that any specific value of $e_{\mathrm{T}}$ or $e_{\mathrm{kin}}$ is highly situational and can simultaneously describe entirely different contact configurations.

cond-mat.soft↗

arXiv2026

Tree codes and sort-and-sweep algorithms for neighborhood computation: A cache-conscious comparison

Neighborhood algorithms may take a considerable percentage of computer time in discrete element methods (DEM). While the sort-and-sweep algorithm is ideal in some ways, as it only deal with particles whose relative positions change in one coordinate direction, the other directions must be processed too, for all particles. In contrast, tree-codes deal only with adjacent particles. We compare sort-and-sweep and tree-code neighborhood algorithms for two-dimensional DEM simulations of polygonal particles in a rotating drum with up to 12000 particles. We discuss the effects of system size and inlining on the performance with respect to the cache memory. For the tree code, the performance is slightly better, at the cost of significantly increased cyclomatic complexity. In particular, one benefit is improved possibilities for shared memory parallelization.

physics.comp-ph↗

arXiv2025

Vertical pullout of a non-spherical intruder from a granular medium: From system-wide response to avalanching around the intruder

Intruder mechanics in a granular aggregate is a common subject in engineering and geotechnical applications. However, most studies are limited to spherical intruders or small displacement regimes up to the point of failure. In this work we investigate the vertical pullout of a plate-like intruder buried within a granular aggregate well past the point of failure. While we find the maximum resistance force to depend on the material properties, in the post failure regime the resistance force converges onto the same curve for all friction coefficients. Likewise, the effective geometry of the intruder will always develop the same conical shape on top of the plate, independent of the magnitude of friction, that remains unchanged during the pullout process once established. Further, between the intruder and the aggregate a natural hopper flow develops in which material is transported into the void below the intruder by discrete flow events.

cond-mat.soft↗

arXiv2025

Effects of particle angularity on granular self-organization

Recent studies of two-dimensional poly-disperse disc systems revealed a coordinated self-organisation of cell stresses and shapes, with certain distributions collapsing onto a master form for many processes, size distributions, friction coefficients, and cell orders. Here we examine the effects of grain angularity on the indicators of self-organisation, using simulations of bi-disperse regular $N$-polygons and varying $N$ systematically. We find that: the strong correlation between local cell stresses and orientations, as well as the collapses of the conditional distributions of scaled cell stress ratios to a master Weibull form for all cell orders $k$, are independent of angularity and friction coefficient. In contrast, increasing angularity makes the collapses of the conditional distributions sensitive to changes in the friction coefficient.

cond-mat.soft↗

arXiv2025

Implementing van der Waals forces for polytope particles in DEM simulations of clay

Clay minerals are non-spherical nano-scale particles that usually form flocculated, house-of-card like structures under the influence of inter-molecular forces. Numerical modeling of clays is still in its infancy as the required inter-particle forces are available only for spherical particles. A polytope approach would allow shape-accurate forces and torques while simultaneously being more performant. The Anandarajah solution provides an analytical formulation for van der Waals forces for cuboid particles but in its original form is not suitable for implementation in DEM simulations. In this work, we discuss the necessary changes for a functional implementation of the Anandarajah solution in a DEM simulation of rectangular particles and their extension to cuboid particles.

cond-mat.soft↗

arXiv2025

On the planar impact of a disc and a square particle considering the impact angle and particle orientation

Despite the simple impact of a rigid particle being a centuries old problem, a conclusive treatment of the general case is still outstanding. The influences of particle shape as well as elastic and plastic deformation of the particle upon contact significantly complicate the problem. Experiments have shown the possibilities of backward movement as well as forward movement with backward rotation after an impact. This paper investigates the planar impact of a disc and square particle under systematic consideration of the impact angle and initial particle orientation. We investigate the kinematic variables of the particle both post-impact as well as their evolution during contact. For impacts with friction, we find oscillatory behaviour of the tangential contact force in both discs and squares depending on the impact angle and particle orientation. We further demonstrate the possibility of backwards deflection for square particles with backward orientation under steep impact angles, which is not possible for round particles under the same conditions. In contrast, high deflection angles are achieved with a kind of "double-impact" where the centroid of the particle moves away from the plane so that the surface detaches first, but reconnects again due to induced rotation, which also is not possible for round particles. Lastly, we present different coefficients of restitution for the square particle with respect to the combined influence of impact angle and particle orientation upon impact.

cond-mat.soft↗

arXiv2024

Tree Code Based Neighborhood Algorithms for Discrete Element Methods

We report our experiences for the development of a neighborhood algorithm implemented via tree-codes to optimize the performance of a discrete element method (DEM) for convex polytopes. Our implementation of the two-dimensional tree code needs $N\log N$, as does the sort and sweep approach. For our choice of boundary conditions (a rotating drum) and system sizes (up to several thousand particles), the performance of the tree-code is slightly better, but the algorithm is considerably more complicated than the sort and sweep approach.

physics.comp-ph↗