arXiv subjects

N. Vandewalle

Publications and source records attributed to N. Vandewalle.

50 records · Page 3Linked to original sources

arXiv2002

Ripple and kink dynamics

We propose a relevant modification of the Nishimori-Ouchi model [{\em Phys. Rev. Lett.} {\bf 71}, 197 (1993)] for granular landscape erosion. We explicitly introduce a new parameter: the angle of repose $θ_r$, and a new process: avalanches. We show that the $θ_r$ parameter leads to an asymmetry of the ripples, as observed in natural patterns. The temporal evolution of the maximum ripple height $h_{max}$ is limited and not linear, according to recent observations. The ripple symmetry and the kink dynamics are studied and discussed.

cond-mat.stat-mech↗

arXiv2002

Labyrinthic granular landscapes

We have numerically studied a model of granular landscape eroded by wind. We show the appearance of labyrinthic patterns when the wind orientation turns by $90^\circ$. The occurence of such structures are discussed. Morever, we introduce the density $n_k$ of ``defects'' as the dynamic parameter governing the landscape evolution. A power law behavior of $n_k$ is found as a function of time. In the case of wind variations, the exponent (drastically) shifts from 2 to 1. The presence of two asymptotic values of $n_k$ implies the irreversibility of the labyrinthic formation process.

cond-mat.stat-mech↗

arXiv2002

Granular spirals on erodible sand bed submitted to a circular fluid motion

An experimental study of a granular surface submitted to a circular fluid motion is presented. The appearance of an instability along the sand-water interface is observed beyond a critical radius $r_c$. This creates ripples with a spiral shape on the granular surface. A phase diagram of such patterns is constructed and discussed as a function of the rotation speed $ω$ of the flow and as a function of the height of water $h$ above the surface. The study of $r_c$ as a function of $h$, $ω$ and $r$ parameters is reported. Thereafter, $r_c$ is shown to depend on the rotation speed according to a power law. The ripple wavelength is found to decrease when the rotation speed increases and is proportional to the radial distance $r$. The azimuthal angle \az of the spiral arms is studied. It is found that \az scales with $hωr$. This lead to the conclusion that \az depends on the fluid momentum. Comparison with experiments performed with fluids allows us to state that the spiral patterns are not the signature of an instability of the boundary layer.

cond-mat.soft↗

arXiv2002

Patterns in hydraulic ripples with binary granular mixtures

An experimental study of a binary granular mixture submitted to a transient shear flow in a cylindrical container is reported. The formation of ripples with a spiral shape is observed. The appearance of phase segregation in those spiral patterns is shown. The relative grain size bewteen sand species is found to be a relevant parameter leading to phase segregation. However, the relative repose angle is an irrelevant parameter. The formation of sedimentary structures is also presented. They result from a ripple climbing process. The ``sub-critical'' or ``super-critical'' character of the lamination patterns is shown to depend on the rotation speed of the container.

cond-mat.stat-mech↗

arXiv2001

Diffusive foam wetting process in microgravity

We report the experimental study of aqueous foam wetting in microgravity. The liquid fraction $\ell$ along the bubble edges is measured and is found to be a relevant dynamical parameter during the capillary process. The penetration of the liquid in the foam, the foam inflation, and the rigidity loss are shown all to obey strict diffusion processes.

cond-mat.stat-mech↗

arXiv2001

Crashes : symptoms, diagnoses and remedies

A brief historical perspective is first given concerning financial crashes, - from the 17th till the 20th century. In modern times, it seems that log periodic oscillations are found before crashes in several financial indices. The same is found in sand pile avalanches on Sierpinski gaskets. A discussion pertains to the after shock period with illustrations from the DAX index. The factual financial observations and the laboratory ones allow us some conjecture on symptoms and remedies for discussing financial crashes along econophysics lines.

cond-mat.stat-mech↗

arXiv2000

Self-organized critical topology of stock markets

We have analyzed the cross-correlations of daily fluctuations for N=6 358 US stock prices during the year 1999. From those $N(N-1)/2$ correlations coefficients, the Minimum Spanning Tree (MST) has been built. We have investigated the topology exhibited by the MST. Eventhough the average topological number is $ \approx 2$, the variance $σ$ of the topological distribution f(n) diverges. More precisely, we have found that $f(n) \sim n^{-2.2}$ holding over two decades. We have studied the topological correlations for neighbouring nodes: an extremely broad set of local configurations exists, confirming the divergence of $σ$.

cond-mat.stat-mech↗

arXiv2000

Stripes ordering in self-stratification experiments of binary and ternary granular mixtures

The self-stratification of binary and ternary granular mixtures has been experimentally investigated. Ternary mixtures lead to a particular ordering of the strates which was not accounted for in former explanations. Bouncing grains are found to have an important effect on strate formation. A complementary mechanism for self-stratification of binary and ternary granular mixtures is proposed.

cond-mat.soft↗

arXiv2000

A New Universality for Random Sequential Deposition of Needles

Percolation and jamming phenomena are investigated for random sequential deposition of rectangular needles on $d=2$ square lattices. Associated thresholds $p_c^{perc}$ and $p_c^{jam}$ are determined for various needle sizes. Their ratios $p_c^{perc} / p_c^{jam}$ are found to be a constant $0.62 \pm 0.01$ for all sizes. In addition the ratio of jamming thresholds for respectively square blocks and needles is also found to be a constant $0.79 \pm 0.01$. These constants exhibit some universal connexion in the geometry of jamming and percolation for both anisotropic shapes (needles versus square lattices) and isotropic shapes (square blocks on square lattices). A universal empirical law is proposed for all three thresholds as a function of $a$.

cond-mat.dis-nn↗

arXiv2000

Domino effect for world market fluctuations

In order to emphasize cross-correlations for fluctuations in major market places, series of up and down spins are built from financial data. Patterns frequencies are measured, and statistical tests performed. Strong cross-correlations are emphasized, proving that market moves are collective behaviors.

cond-mat.stat-mech↗

arXiv2000

Avalanches of popping bubbles in collapsing foams

We report acoustic experiments on foam systems. We have recorded the sound emitted by crackling cells during the collapsing of foams. The sound pattern is then analyzed using classical methods of statistical physics. Fundamental processes at the surface of the collapsing foam are found. In particular, size is not a relevant parameter for exploding bubbles.

cond-mat.stat-mech↗

arXiv2000

Non-gaussian electrical fluctuations in a quasi-2d packing of metallic beads

The electrical properties of a two-dimensional packing of metallic beads are studied. Small mechanical perturbations of the packing leads to giant electrical fluctuations. Fluctuations are found to be non-gaussian and seem to belong to Levy stable distributions. Anticorrelations have been also found for the sign of these fluctuations.

cond-mat.stat-mech↗

arXiv1999

Sandpiles on fractal bases: pile shape and phase segregation

Sandpiles have become paradigmatic systems for granular flow studies in statistical physics. New directions of investigations are discussed here. Rather than varying the nature of the pile (sand, salt, rice,..) we have investigated changes in the boundary conditions. We have investigated experimentally and numerically sandpile structures on bases having a fractal perimeter. This type of perimeter induces the formation of a quite complex set of ridges and valleys. A screening effect of the valleys is observed and depends on the angle of repose. Binary granular systems have also been investigated on such bases: a spectacular demixing is obtained along the valleys. This type of phase segregation is discussed with respect to numerical studies.

cond-mat.soft↗

arXiv1998

The Kronig-Penney-Ising picture of the colossal magnetoresistance

From general arguments, it is shown that a magnetic Kronig-Penney model based on the thermodynamics of an Ising model can be used for describing the Colossal Magnetoresistance (CMR) phenomenon. The model considers a tunneling-like transmission process of hopping electrons through a dynamic lattice characterized by evolving magnetic clusters. In this model, correlations between the magnetic states are considered to be more relevant than the lattice strain effects for obtaining the CMR features. Physical arguments lead to the theoretical description of the intrinsic temperature and field dependences of the CMR observed in typical manganite materials.

cond-mat.stat-mech↗