Biomedical subjects

Hans J Herrmann

Publications and source records attributed to Hans J Herrmann.

23 records · Page 2Linked to original sources

PubMed2002

Minimal model for aeolian sand dunes.

We present a minimal model for the formation and migration of aeolian sand dunes in unidirectional winds. It combines a perturbative description of the turbulent wind velocity field above the dune with a continuum saltation model that allows for saturation transients in the sand flux. The latter are shown to provide a characteristic length scale, called saturation length, which is distinct from the saltation length of the grains. The model admits two different classes of solutions for the steady-state profile along the wind direction: smooth heaps and dunes with slip face. We clarify the origin of the characteristic properties of these solutions and analyze their scaling behavior. We also investigate in some detail the dynamic evolution of heaps and dunes, including the steady-state migration velocity and transient shape relaxation. Although the minimal model employs nonlocal expressions for the wind shear stress as well as for the sand flux, it is simple enough to serve as a very efficient tool for analytical and numerical investigations and opens up the way to simulations of large scale desert topographies.

Journal Article↗

PubMed2002

Fracture model with variable range of interaction.

We introduce a fiber bundle model where the interaction among fibers is modeled by an adjustable stress-transfer function that can interpolate between the two limiting cases of load redistribution, i.e., the global and the local load sharing schemes. By varying the range of interaction, several features of the model are numerically studied and a crossover from mean-field to short-range behavior is obtained. The properties of the two regimes and the emergence of the crossover in between are explored by numerically studying the dependence of the ultimate strength of the material on the system size, the distribution of avalanches of breakings, and of the cluster sizes of broken fibers. Finally, we analyze the moments of the cluster size distributions to accurately determine the value at which the crossover is observed.

Journal Article↗

PubMed2002

Finite width of shear zones.

We present some experimental and numerical results based on a simple model designed to give an estimate for the width of a shear zone. We conclude that the observed finite size of the shear zone can be associated with the propagation of the force lines inside the medium. The model, based on a simple argument on the force distribution and dilatancy, predicts a width of about ten grain diameters.

Journal Article↗

PubMed2002

Creep rupture of viscoelastic fiber bundles.

We study the creep rupture of bundles of viscoelastic fibers occurring under uniaxial constant tensile loading. A fiber bundle model is introduced that combines the viscoelastic constitutive behavior and the strain controlled breaking of fibers. Analytical and numerical calculations showed that above a critical external load the deformation of the system monotonically increases in time resulting in global failure at a finite time t(f), while below the critical load the deformation tends to a constant value giving rise to an infinite lifetime. Our studies revealed that the nature of the transition between the two regimes, i.e., the behavior of t(f) at the critical load sigma(c), strongly depends on the range of load sharing: for global load sharing t(f) has a power law divergence at sigma(c) with a universal exponent of 0.5, however, for local load sharing the transition becomes abrupt: at the critical load t(f) jumps to a finite value, analogous to second- and first-order phase transitions, respectively. The acoustic response of the bundle during creep is also studied.

Journal Article↗

PubMed2002

Minimal model for sand dunes.

We propose a minimal model for aeolian sand dunes. It combines an analytical description of the turbulent wind velocity field above the dune with a continuum saltation model that allows for saturation transients in the sand flux. The model provides a qualitative understanding of important features of real dunes, such as their longitudinal shape and aspect ratio, the formation of a slip face, the breaking of scale invariance, and the existence of a minimum dune size.

Journal Article↗