arXiv subjects

Alex Hansen

Publications and source records attributed to Alex Hansen.

At least 145 records · Page 8Linked to original sources

arXiv2000

Viscous stabilization of 2D drainage displacements with trapping

We investigate the stabilization mechanisms due to viscous forces in the invasion front during drainage displacement in two-dimensional porous media using a network simulator. We find that in horizontal displacement the capillary pressure difference between two different points along the front varies almost linearly as function of height separation in the direction of the displacement. The numerical result supports arguments taking into account the loopless displacement pattern where nonwetting fluid flow in separate strands (paths). As a consequence, we show that existing theories developed for viscous stabilization, are not compatible with drainage when loopless strands dominate the displacement process.

physics.flu-dyn↗

arXiv2000

Modelling an Imperfect Market

We propose a simple market model where agents trade different types of products with each other by using money, relying only on local information. Value fluctuations of single products, combined with the condition of maximum profit in transactions, readily lead to persistent fluctuations in the wealth of individual agents.

cond-mat↗

arXiv2000

Diamagnetic Susceptibility and Current Distributions in Granular Superconductors at Percolation

Comments: 4 pages RevTeX, 4 Postscript figures. References added. We study a two-dimensional granular superconducting network at the percolation threshold under the influence of an external perpendicular magnetic field. By numerical simulations on the full nonlinear problem, we determine the scaling exponent for the magnetic susceptibility. Further, we report on the scaling properties of the current distribution. The scaling of the current is found to be independent of the value of the magnetic field. Our results are in contradiction with previous numerical results based on linearized equations. We find a value for the susceptibility exponent which does not agree with existing theoretical suggestions, but agrees perfectly with renormalization group calculations.

cond-mat↗

arXiv1999

Pathways in Two-State Protein Folding

The thermodynamics of proteins indicate that folding/unfolding takes place either through stable intermediates or through a two-state process without intermediates. The rather short folding times of the two-state process indicate that folding is guided. We reconcile these two seemingly contradictory observations quantitatively in a schematic model of protein folding. We propose a new dynamical transition temperature which is lower than the thermodynamic one, in qualitative agreement with in vivo measurement of protein stability using E.coli. Finally we demonstrate that our framework is easily generalized to encompass cold unfolding, and make predictions that relate the sharpness of the cold and hot unfolding transitions.

cond-mat↗

arXiv1999

Asymptotic Distribution of Eigenvalues for a Self-Affine String

We consider a string with fixed endpoints where the mass density and/or the elastic coefficient vary in a self-affine way as function of position. It is demonstrated how the eigenvalues in the asymptotic limit are distributed. Scaling laws for the Weyl term of the asymptotic integrated density of states is established and confirmed numerically.

cond-mat.dis-nn↗

arXiv1999

A Fast Algorithm for Generating Long Self-Affine Profiles

We introduce a fast algorithm for generating long self-affine profiles. The algorithm, which is based on the fast wavelet transform, is faster than the conventional Fourier filtering algorithm. In addition to increased performance for large systems, the algorithm, named the wavelet filtering algorithm, a priori gives rise to profiles for which the long-range correlation extends throughout the entire system independently of the length scale.

cond-mat.dis-nn↗

arXiv1999

A Model for the Thermodynamics of Globular Proteins

Comments: 6 pages RevTeX, 6 Postscript figures. We review a statistical mechanics treatment of the stability of globular proteins based on a simple model Hamiltonian taking into account protein self interactions and protein-water interactions. The model contains both hot and cold folding transitions. In addition it predicts a critical point at a given temperature and chemical potential of the surrounding water. The universality class of this critical point is new.

cond-mat↗

arXiv1999

From the Liouville Equation to the Generalized Boltzmann Equation for Magnetotransport in the 2D Lorentz Model

We consider a system of non-interacting charged particles moving in two dimensions among fixed hard scatterers, and acted upon by a perpendicular magnetic field. Recollisions between charged particles and scatterers are unavoidable in this case. We derive from the Liouville equation for this system a generalized Boltzmann equation with infinitely long memory, but which still is analytically solvable. This kinetic equation has been earlier written down from intuitive arguments.

cond-mat↗

arXiv1999

Modelling Molecular Motors as Folding-Unfolding Cycles

We propose a model for motor proteins based on a hierarchical Hamiltonian that we have previously introduced to describe protein folding. The proposed motor model has high efficiency and is consistent with a linear load-velocity response. The main improvement with respect to previous models is that this description suggests a connection between folding and function of allosteric proteins.

cond-mat↗

arXiv1998

Determination of the Hurst Exponent by Use of Wavelet Transforms

We propose a new method for (global) Hurst exponent determination based on wavelets. Using this method, we analyze synthetic data with predefined Hurst exponents, fracture surfaces and data from economy. The results are compared with those obtained from Fourier spectral analysis. When many samples are available, the wavelet and Fourier methods are comparable in accuracy. However, when one or only a few samples are available, the wavelet method outperforms the Fourier method by a large margin.

cond-mat.mtrl-sci↗

arXiv1998

Simulating temporal evolution of pressure in two-phase flow in porous media

We have simulated the temporal evolution of pressure due to capillary and viscous forces in two-phase drainage in porous media. We analyze our result in light of macroscopic flow equations for two-phase flow. We also investigate the effect of the trapped clusters on the pressure evolution and on the effective permeability of the system. We find that the capillary forces play an important role during the displacements for both fast and slow injection rates and both when the invading fluid is more or less viscous than the defending fluid. The simulations are based on a network simulator modeling two-phase drainage displacements on a two-dimensional lattice of tubes.

physics.flu-dyn↗

arXiv1998

A hierarchical scheme for cooperativity and folding in proteins

We propose a protein model based on a hierarchy of constraints that force the protein to follow certain pathways when changing conformation. The model exhibits a first order phase transition, cooperativity and is exactly solvable. It also shows an unexpected symmetry between folding and unfolding pathways as suggested by a recent experiment.

cond-mat↗

arXiv1998

Critical aspects of hierarchical protein folding

We argue that the first order folding transitions of proteins observed at physiological chemical conditions end in a critical point for a given temperature and chemical potential of the surrounding water. We investigate this critical point using a hierarchical Hamiltonian and determine its universality class. This class differs qualitatively from those of other known models.

cond-mat.soft↗

arXiv1997

Statistical mechanics of warm and cold unfolding in proteins

We present a statistical mechanics treatment of the stability of globular proteins which takes explicitly into account the coupling between the protein and water degrees of freedom. This allows us to describe both the cold and the warm unfolding, thus qualitatively reproducing the known thermodynamics of proteins.

cond-mat↗

arXiv1997

Fracture in Three-Dimensional Fuse Networks

We report on large scale numerical simulations of fracture surfaces using random fuse networks for two very different disorders. There are some properties and exponents that are different for the two distributions, but others, notably the roughness exponents, seem universal. For the universal roughness exponent we found a value of zeta = 0.62 +/- 0.05. In contrast to what is observed in two dimensions, this value is lower than that reported in experimental studies of brittle fractures, and rules out the minimal energy surface exponent, 0.41 +/- 0.01.

cond-mat.soft↗

arXiv1997

Tunneling and Universality in the Integer Quantum Hall Effect

We show analytically and numerically that omission of quantum interference from the Chalker-Coddington model of the integer quantum Hall effect gives a localization length exponent nu=4/3 as in ordinary two-dimensional percolation. Thus, contrary to semi-classical scaling arguments, tunneling alone does not lead to nu=7/3.

cond-mat.mes-hall↗

arXiv1997

A two-dimensional network simulator for two-phase flow in porous media

We investigate a two-dimensional network simulator capable of modeling different time dependencies in two-phase drainage displacements. In particular, we focus on the temporal evolution of the pressure due to capillary and viscous forces and the time dependence of the interface between the two liquids. The dynamics of the capillary effect are taken into account and we report on high accuracy pressure measurements. Moreover, the simulator includes important features in drainage, like burst dynamics of the invading fluid and simultaneous flow of two liquids in a section of a tube. The validity of the model is checked by comparing simulation results with well known experimental properties in drainage displacement.

physics.flu-dyn↗

arXiv1997

Criticality in the Integer Quantum Hall Effect

We review some elementary aspects of the critical properties of the series of metal-insulator transitions that constitute the integer quantum Hall effect. Numerical work has proven essential in charting out this phenomenon. Without being complete, we review network models that seem to capture the essentials of this critical phenomenon.

cond-mat.mes-hall↗