arXiv subjects

Lothar Brendel

Publications and source records attributed to Lothar Brendel.

12 recordsLinked to original sources

arXiv2026

Resolving dense photodissociation regions: the structure of photochemical fronts in three-dimensional gas distributions

For decades, the Orion Bar has been the prototypical photodissociation region. Viewed nearly edge-on, it offers a unique window into the stratified chemical structure of the atomic-to-molecular transition of the interstellar medium. Understanding its photochemistry is essential to interpreting key observations originating from dense photo-dissociation regions. ALMA and JWST observations reveal that H2 photodissociation front overlaps with the C+ recombination front and exhibits a complex spatial morphology. Despite considerable theoretical effort, existing modeling approaches based on simplified geometrical assumptions have difficulties reproducing the spatial emission structure. Our aim is to investigate the response of photochemistry in realistic three-dimensional density distributions, using the Orion Bar as a representative application. We present the first fully three-dimensional high-resolution model of an Orion Bar analogue that resolves the relevant photochemical fronts using the upgraded steady-state 3D-PDR photo-dissociation region code, which allows for the treatment of plane-irradiatation and for the solution of the non-LTE H2 rovibrational levels. We find that the H2 dissociation front is characterized by a complex surface that overlaps with the C+ recombination front. Our 3D model can reproduce the complex morphology of H2 emission seen in observations, in particular the arc- and filament-like features, and provides a physical explanation of its chemistry. The overlapping H2 dissociation and C+ recombination fronts and the spatial emission morphology can be explained due to the three-dimensional gas distribution, resulting in shadowing and shielding by dense substructures. Our results mark a turning point for astrochemistry, where three-dimensional steady-state models can deliver fundamentally new insights into the chemistry of the interstellar medium. (Abridged)

astro-ph.GA↗

arXiv2025

Impact of accretor size on the morphology of supersonic Bondi-Hoyle-Lyttleton accretion flows

Fast-moving accretors are ubiquitous in astrophysics. Their interaction with surrounding gas leaves characteristic imprints, forming structures like bow shocks, Mach cones, and density trails. We study how various physical processes affect the flow structure around an accretor with a one-way surface, its accretion rate, and accretion anisotropy. These processes correspond to distinct length scales: the Bondi radius, the bow shock's stand-off distance, and the Hoyle-Lyttleton radius. We conducted adiabatic hydrodynamic simulations using a spherical coordinate grid centred on the accretor. By varying the accretor's (numerical) size across scales -- from much smaller than the stand-off distance to much larger than the Bondi radius -- we analyse how these spatial scales affect steady-state flow physics. All simulations reach a steady state. When the accretor is smaller than the stand-off distance, a bow shock forms ahead, and a nearly spherically symmetric atmosphere develops within. Accretors smaller than the Hoyle-Lyttleton radius produce a Mach cone, while larger ones exhibit a supersonic-to-subsonic flow transition on larger scales. Fully resolved simulations align with Hoyle-Lyttleton theory, showing slightly anisotropic accretion with enhanced inflow from behind. In contrast, larger accretors approach the geometrical limit, accreting mainly from the flow direction, with a low-density 'shadow' forming behind. The accretor's size strongly influences small- and large-scale morphologies. Resolving the Hoyle-Lyttleton radius is essential for capturing large-scale flow characteristics. Resolving the stand-off distance is needed only to study the bow shock: since it determines the shock's position, its non-resolution does not affect large-scale flow morphology.

astro-ph.EP↗

arXiv2021

Interacting jammed granular systems

More than 30 years ago Edwards and co-authors proposed a model to describe the statistics of granular packings by an ensemble of equiprobable jammed states. Experimental tests of this model remained scarce so far. We introduce a simple system to analyze statistical properties of jammed granular ensembles to test Edwards theory. Identical spheres packed in a nearly two-dimensional geometrical confinement were studied in experiments and numerical simulations. When tapped, the system evolves towards a ground state, but due to incompatible domain structures it gets trapped. Analytical calculations reproduce relatively well our simulation results, which allows us to test Edwards theory on a coupled system of two subsystems with different properties. We find that the joint system can only be described by the Edwards theory if considered as a single system due to the constraints in the stresses. The results show counterintuitive effects as in the coupled system the change in the order parameter is opposite to what is expected from the change in the compactivity.

cond-mat.stat-mech↗

arXiv2015

Trapping of interacting propelled colloidal particles in inhomogeneous media

A trapping mechanism for propelled colloidal particles based on an inhomogeneous drive is presented and studied by means of computer simulations. In experiments this method can be realized using photophoretic Janus particles driven by a light source, which shines through a shading mask and leads to an accumulation of the particles in the passive part. An equation for an accumulation parameter is derived using the effective inhomogeneous diffusion constant generated by the inhomogeneous drive. The impact of particle interaction on the trapping mechanism is studied, as well as the interplay between passivity-induced trapping and the emergent self-clustering of systems containing a high density of active particles. The combination of both effects makes the clusters more controllable for applications.

physics.comp-ph↗

arXiv2013

Self-learning kinetic Monte Carlo model for arbitrary surface orientations

While the self-learning kinetic Monte Carlo (SLKMC) method enables the calculation of transition rates from a realistic potential, implementations of it were usually limited to one specific surface orientation. An example is the fcc (111) surface in Latz et al. 2012, J. Phys.: Condens. Matter 24, 485005. This work provides an extension by means of detecting the local orientation, and thus allows for the accurate simulation of arbitrarily shaped surfaces. We applied the model to the diffusion of Ag monolayer islands and voids on a Ag(111) and Ag(001) surface, as well as the relaxation of a three-dimensional spherical particle.

cond-mat.mtrl-sci↗

arXiv2011

An adaptive hierarchical domain decomposition method for parallel contact dynamics simulations of granular materials

A fully parallel version of the contact dynamics (CD) method is presented in this paper. For large enough systems, 100% efficiency has been demonstrated for up to 256 processors using a hierarchical domain decomposition with dynamic load balancing. The iterative scheme to calculate the contact forces is left domain-wise sequential, with data exchange after each iteration step, which ensures its stability. The number of additional iterations required for convergence by the partially parallel updates at the domain boundaries becomes negligible with increasing number of particles, which allows for an effective parallelization. Compared to the sequential implementation, we found no influence of the parallelization on simulation results.

cond-mat.soft↗

arXiv2011

Spin waves cause non-linear friction

Energy dissipation is studied for a hard magnetic tip that scans a soft magnetic substrate. The dynamics of the atomic moments are simulated by solving the Landau-Lifshitz-Gilbert (LLG) equation numerically. The local energy currents are analysed for the case of a Heisenberg spin chain taken as substrate. This leads to an explanation for the velocity dependence of the friction force: The non-linear contribution for high velocities can be attributed to a spin wave front pushed by the tip along the substrate.

cond-mat.other↗

arXiv2011

Minimal dissipation theory and shear bands in biaxial tests

True biaxial tests of granular materials are investigated by applying the principle of minimal dissipation and comparing to two dimensional contact dynamics simulations. It is shown that the macroscopic steady state manifested by constant stress ratio and constant volume is the result of the ever changing microscopic structure which minimizes the dissipation rate. The shear band angle in the varying shear band structures is found to be constant. We also show that introducing friction on the walls reduces the degeneracy of the optimal shear band structures to one for a wide range of parameters which gives a non-constant stress ratio curve with varying aspect ratio that can be calculated.

cond-mat.stat-mech↗

arXiv2009

Fractal Substructures due to Fragmentation and Reagglomeration

Cohesive powders form agglomerates that can be very porous. Hence they are also very fragile. Consider a process of complete fragmentation on a characteristic length scale $\ell$, where the fragments are subsequently allowed to settle under gravity. If this fragmentation-reagglomeration cycle is repeated sufficiently often, the powder develops a fractal substructure with robust statistical properties. The structural evolution is discussed for two different models: The first one is an off-lattice model, in which a fragment does not stick to the surface of other fragments that have already settled, but rolls down until it finds a locally stable position. The second one is a simpler lattice model, in which a fragment sticks at first contact with the agglomerate of fragments that have already settled. Results for the fragment size distribution are shown as well. One can distinguish scale invariant dust and fragments of a characteristic size. Their role in the process of structure formation will be addressed.

cond-mat.stat-mech↗

arXiv2009

Spin excitations in a monolayer scanned by a magnetic tip

Energy dissipation via spin excitations is investigated for a hard ferromagnetic tip scanning a soft magnetic monolayer. We use the classical Heisenberg model with Landau-Lifshitz-Gilbert (LLG)-dynamics including a stochastic field representing finite temperatures. The friction force depends linearly on the velocity (provided it is small enough) for all temperatures. For low temperatures, the corresponding friction coefficient is proportional to the phenomenological damping constant of the LLG equation. This dependence is lost at high temperatures, where the friction coefficient decreases exponentially. These findings can be explained by properties of the spin polarization cloud dragged along with the tip.

cond-mat.other↗

arXiv2002

Pore Stabilization in Cohesive Granular Systems

Cohesive powders tend to form porous aggregates which can be compacted by applying an external pressure. This process is modelled using the Contact Dynamics method supplemented with a cohesion law and rolling friction. Starting with ballistic deposits of varying density, we investigate how the porosity of the compacted sample depends on the cohesion strength and the friction coefficients. This allows to explain different pore stabilization mechanisms. The final porosity depends on the cohesion force scaled by the external pressure and on the lateral distance between branches of the ballistic deposit r_capt. Even if cohesion is switched off, pores can be stabilized by Coulomb friction alone. This effect is weak for round particles, as long as the friction coefficient is smaller than 1. However, for nonspherical particles the effect is much stronger.

cond-mat↗

arXiv2002

Elastic behavior in Contact Dynamics of rigid particles

The systematic errors due to the practical implementation of the Contact Dynamics method for simulation of dense granular media are examined. It is shown that, using the usual iterative solver to simulate a chain of rigid particles, effective elasticity and sound propagation with a finite velocity occur. The characteristics of these phenomena are investigated analytically and numerically in order to assess the limits of applicability of this simulation method and to compare it with soft particle molecular dynamics.

cond-mat.soft↗