arXiv subjects

Juergen Vollmer

Publications and source records attributed to Juergen Vollmer.

8 recordsLinked to original sources

arXiv2019

Equivalence of position-position auto-correlations in the Slicer Map and the Lévy-Lorentz gas

The Slicer Map is a one-dimensional non-chaotic dynamical system that shows sub-, super-, and normal diffusion as a function of its control parameter. In a recent paper [Salari et al., CHAOS 25, 073113 (2015)] it was found that the moments of the position distributions as the Slicer Map have the same asymptotic behaviour as the Lévy-Lorentz gas, a random walk on the line in which the scatterers are randomly distributed according to a Lévy-stable probability distribution. Here we derive analytic expressions for the position-position correlations of the Slicer Map and, on the ground of this result, we formulate some conjectures about the asymptotic behaviour of position-position correlations of the Lévy-Lorentz gas, for which the information in the literature is minimal. The numerically estimated position-position correlations of the Lévy-Lorentz show a remarkable agreement with the conjectured asymptotic scaling.

cond-mat.stat-mech↗

arXiv2008

Phase Separation under Ultra-Slow Cooling: Onset of Nucleation

We discuss the interplay between a slow continuous drift of temperature, which induces continuous phase separation, and the non-linear diffusion term in the $ϕ^4$-model for phase separation of a binary mixture. This leads to a bound for the stability of diffusive demixing. It is argued that these findings are not specific to the $ϕ^4$ model, but that they always apply up to slight modifications of the bound. In practice stable diffusive demixing can only be achieved when special precautions are taken in experiments on real mixtures. Therefore, the recent observations on complex dynamical behavior in such systems should be considered as a new challenge for understanding generic features of phase-separating systems.

cond-mat.stat-mech↗

arXiv2006

Statistical analysis of coherent structures in transitional pipe flow

Numerical and experimental studies of transitional pipe flow have shown the prevalence of coherent flow structures that are dominated by downstream vortices. They attract special attention because they contribute predominantly to the increase of the Reynolds stresses in turbulent flow. In the present study we introduce a convenient detector for these coherent states, calculate the fraction of time the structures appear in the flow, and present a Markov model for the transition between the structures. The fraction of states that show vortical structures exceeds 24% for a Reynolds number of about Re=2200, and it decreases to about 20% for Re=2500. The Markov model for the transition between these states is in good agreement with the observed fraction of states, and in reasonable agreement with the prediction for their persistence. It provides insight into dominant qualitative changes of the flow when increasing the Reynolds number.

physics.flu-dyn↗

arXiv2002

Reply to E.G.D. Cohen, L. Rondoni, Physica A 306 (2002) 117

In their paper Cohen and Rondoni severely question the physical relevance of studies of transport by means of multibaker maps and other classes of dynamics considering the motion of independen particles - like e.g. the Lorentz gas. We argue that this is to a large extent due to inappropriate interpretation of the models. In particular, the arguments concerning the lack of local thermodynamic equilibrium are inconsistent for the models previously worked out by ourselves.

nlin.CD↗

arXiv2001

Escape-rate formalism, decay to steady states, and divergences in the entropy-production rate

In summer 1997 we were sitting with Bob Dorfman and a few other friends interested in chaotic systems and transport theory on a terrace close to Oktogon in Budapest. While taking our (decaf) coffee after a very nice Italian meal, we discussed about logarithmic divergences in the entropy production of systems with absorbing boundary conditions and their consequences for the escape-rate formalism. It was guessed at that time that the problem could be resolved by a careful discussion of the physical content of the absorbing boundary conditions. To our knowledge a thorough analysis of this long-standing question is still missing. We dedicate it hereby to Bob on occasion of his 65th birthday.

nlin.CD↗

arXiv2001

A multibaker map for shear flow and viscous heating

A consistent description of shear flow and the accompanied viscous heating as well the associated entropy balance is given in the framework of a deterministic dynamical system. A laminar shear flow is modeled by a Hamiltonian multibaker map which drives velocity and temperature fields. In an appropriate macroscopic limit one recovers the Navier-Stokes and heat conduction equations along with the associated entropy balance. This indicates that results of nonequilibrium thermodynamics can be described by means of an abstract, sufficiently chaotic and mixing dynamics. A thermostating algorithm can also be incorporated into this framework.

nlin.CD↗

arXiv2000

Shear flow, viscous heating, and entropy balance from dynamical systems

A consistent description of a shear flow, the accompanied viscous heating, and the associated entropy balance is given in the framework of a deterministic dynamical system, where a multibaker dynamics drives two fields: the velocity and the temperature distributions. In an appropriate macroscopic limit their transport equations go over into the Navier-Stokes and the heat conduction equation of viscous flows. The inclusion of an artificial heat sink can stabilize steady states with constant temperatures. It mimics a thermostating algorithm used in non-equilibrium molecular-dynamics simulations.

nlin.CD↗

arXiv2000

Modelling thermostatting, entropy currents and cross effects by dynamical systems

A generalized multibaker map with periodic boundary conditions is shown to model boundary-driven transport, when the driving is applied by a ``perturbation'' of the dynamics localised in a macroscopically small region. In this case there are sustained density gradients in the steady state. A non-uniform stationary temperature profile can be maintained by incorporating a heat source into the dynamics, which deviates from the one of a bulk system only in a (macroscopically small) localized region such that a heat (or entropy) flux can enter an attached thermostat only in that region. For these settings the relation between the average phase-space contraction, the entropy flux to the thermostat and irreversible entropy production is clarified for stationary and non-stationary states. In addition, thermoelectric cross-effects are described by a multibaker chain consisting of two parts with different transport properties, modelling a junction between two metals.

nlin.CD↗