Biomedical subjects

S Douady

Publications and source records attributed to S Douady.

11 recordsLinked to original sources

PubMed2006

Song of the dunes as a self-synchronized instrument.

Since Marco Polo it has been known that some sand dunes have the peculiar ability to emit a loud sound with a well-defined frequency, sometimes for several minutes. The origin of this sustained sound has remained mysterious, partly because of its rarity in nature. It has been recognized that the sound is not due to the air flow around the dunes but to the motion of an avalanche, and not to an acoustic excitation of the grains but to their relative motion. By comparing singing dunes around the world and two controlled experiments, in the laboratory and the field, we prove that the frequency of the sound is the frequency of the relative motion of the sand grains. Sound is produced because moving grains synchronize their motions. The laboratory experiment shows that the dune is not needed for sound emission. A velocity threshold for sound emission is found in both experiments, and an interpretation is proposed.

Colloids↗

PubMed2005

Four sided domains in hierarchical space dividing patterns.

The cracks observed in the glaze of ceramics form networks, which divide the 2D plane into domains. It is shown that, on the average, the number of sides of these domains is four. This contrasts with the usual 2D space divisions observed in Voronoi tessellation or 2D soap froths. In the latter networks, the number of sides of a domain coincides with the number of its neighbors, which, according to Euler's theorem, has to be six on average. The four sided property observed in cracks is the result of a formation process which can be understood as the successive divisions of domains with no later reorganization. It is generic for all networks having such hierarchical construction rules. We introduce a "geometrical charge," analogous to Euler's topological charge, as the difference from four of the number of sides of a domain. It is preserved during the pattern formation of the crack pattern.

Journal Article↗

PubMed2004

Corridors of barchan dunes: Stability and size selection.

Barchans are crescentic dunes propagating on a solid ground. They form dune fields in the shape of elongated corridors in which the size and spacing between dunes are rather well selected. We show that even very realistic models for solitary dunes do not reproduce these corridors. Instead, two instabilities take place. First, barchans receive a sand flux at their back proportional to their width while the sand escapes only from their horns. Large dunes proportionally capture more sand than they lose, while the situation is reversed for small ones: therefore, solitary dunes cannot remain in a steady state. Second, the propagation speed of dunes decreases with the size of the dune: this leads, through the collision process, to a coarsening of barchan fields. We show that these phenomena are not specific to the model, but result from general and robust mechanisms. The length scales needed for these instabilities to develop are derived and discussed. They turn out to be much smaller than the dune field length. As a conclusion, there should exist further, yet unknown, mechanisms regulating and selecting the size of dunes.

Journal Article↗

PubMed2002

Constitutive property of the local organization of leaf venation networks.

The leaf venation of dicotyledons forms complex patterns. In spite of their large variety of morphologies these patterns have common features. They are formed of a hierarchy of structures, which are connected to form a reticulum. Excellent images of these patterns can be obtained from leaves from which the soft tissues have been removed. A numerical image processing has been developed, specially designed for a quantitative analysis of this type of network. It provides a precise characterization of its geometry. The resulting data reveals a surprising property of reticula's nodes: the angles between vein segments are very well defined and it is shown that they are directly related by the radii of the segments. The relation between radii and angles can be expressed very simply using a phenomenological analogy to mechanics. This local organization principle is universal; all leaf venation patterns studied show the same behavior. The results are compared with physical networks such as fracture arrays or soap froth in terms of hierarchy and reorganization.

Biophysical Phenomena↗

PubMed2001

Selection of velocity profile and flow depth in granular flows.

The dynamics of a two-dimensional pile constituted by spherical grains organized in parallel layers is investigated theoretically. Only three effects are taken into account in the model: driving by gravity, nonlocal dissipation due to shocks, and trapping of grains by the bumps of the underneath layer. This is sufficient to recover the basic properties of granular avalanches: the transition between static and flowing state is hysteretic; the pile does not flow on the whole height but only in a layer at the surface; the velocity profile inside the flowing layer is approximately linear and is followed by a creep motion in the (quasi) static part. The flow height increases as a function of the pile angle and tends to infinity for a critical angle straight phi(infinity). The dependence of this critical angle with the static angle straight phi(s), the restitution coefficient rho, and the moment of inertia J, is investigated.

Journal Article↗

PubMed2000

Dynamics of a grain on a sandpile model

The dynamics of a macroscopic grain rolling on an inclined plane composed of fixed identical grains is investigated both experimentally and theoretically. As real sand, the system exhibits an hysteretic transition between static and dynamical states: for angles smaller than straight phi(d), the roller always stops, for angles larger than straight phi(s), it spontaneously starts rolling down. But for angles between straight phi(d) and straight phi(s), it can be either at rest or in motion with a constant velocity. It is shown that the limit velocity is given by the equilibrium between gravity driving and dissipation by the shocks. Moreover, the rough plane acts as a periodic potential trap whose width and depth decrease when the angle is increased: the static angle straight phi(s) corresponds to the angle for which the trap disappears; the dynamical angle straight phi(d) to that for which the limit velocity is sufficient to escape from the trap. Finally, a continuous description of the force globally acting on the grain is proposed, which preserves this hysteretic behavior.

Journal Article↗

PubMed1993