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The present understanding of supernova explosion of massive stars as a two-step process, with an initial gravitational collapse toward the center of the star followed by an expansion of matter after a bouncing on the core, meets several difficulties. We show that it is not the only possible one: a simple model based on fluid mechanics, catastrophe theory, and stability properties of the equilibrium state shows that one can have also a simultaneous inward/outward motion in the early stage of the instability of the supernova described by a dynamical saddle-center bifurcation. The existence of this simultaneous inward/outward motion is sensitive to the model in such systems with long-range interactions. If a constant temperature is assumed (canonical ensemble), an overall inward motion occurs, but if one imposes with the same equation of state the constraint of energy conservation (microcanonical ensemble) there is an inward velocity field near the center of the star together with an outward velocity field in the rest of the star. We discuss the expansion stage of the remnants away from the collapsed core, and propose a new explanation for the formation of shock waves in the ejecta which differs from the usual Sedov-Taylor self-similar description.

Supernovae explosions of massive stars are nowadays believed to result from a two-step process, with an initial gravitational core collapse followed by an expansion of matter after a bouncing on the core. This scenario meets several difficulties. We show that it is not the only possible one: a simple model based on fluid mechanics and stability properties of the equilibrium state shows that one can have also a simultaneous inward/outward motion in the early stage of the instability of the supernova. This shows up in the slow sweeping across a saddle-center bifurcation found when considering equilibrium states associated to the constraint of energy conservation. We first discuss the weakly nonlinear regime in terms of a Painlev\'e I equation. We then show that the strongly nonlinear regime displays a self-similar behavior of the core collapse. Finally, the expansion of the remnants is revisited as an isentropic process leading to shocks formation., Comment: arXiv admin note: substantial text overlap with arXiv:1909.03474

Supernovae explosions of massive stars are nowadays believed to result from a two-step process, with an initial gravitational core collapse followed by an expansion of matter after a bouncing on the core. This scenario meets several difficulties. We show that it is not the only possible one: a simple model based on fluid mechanics and stability properties of the equilibrium state shows that one can have also a simultaneous inward/outward motion in the early stage of the instability of the supernova. This shows up in the slow sweeping across a saddle-center bifurcation found when considering equilibrium states associated to the constraint of energy conservation. We first discuss the weakly nonlinear regime in terms of a Painlev\'e I equation. We then show that the strongly nonlinear regime displays a self-similar behavior of the core collapse. Finally, the expansion of the remnants is revisited as an isentropic process leading to shocks formation., Comment: arXiv admin note: substantial text overlap with arXiv:1909.03474

The present understanding of supernova explosion of massive stars as a two-step process, with an initial gravitational collapse toward the center of the star followed by an expansion of matter after a bouncing on the core, meets several difficulties. We show that it is not the only possible one: a simple model based on fluid mechanics, catastrophe theory, and stability properties of the equilibrium state shows that one can have also a simultaneous inward/outward motion in the early stage of the instability of the supernova described by a dynamical saddle-center bifurcation. The existence of this simultaneous inward/outward motion is sensitive to the model in such systems with long-range interactions. If a constant temperature is assumed (canonical ensemble), an overall inward motion occurs, but if one imposes with the same equation of state the constraint of energy conservation (microcanonical ensemble) there is an inward velocity field near the center of the star together with an outward velocity field in the rest of the star. We discuss the expansion stage of the remnants away from the collapsed core, and propose a new explanation for the formation of shock waves in the ejecta which differs from the usual Sedov-Taylor self-similar description.

In this contribution to the volume in memoriam of Michel Henon, we thought appropriate to look at his early scientific work devoted to the dynamics of large assemblies of interacting masses. He predicted in his PhD thesis that, in such a system, first a collapse of mass occurs at the center and that later binaries stars are formed there. Henceforth, the negative energy of binding of pairs becomes a source of positive energy for the rest of the cluster which evaporate because of that. We examine under what conditions such a singularity can occur, and what could happen afterwards. We hope to show that this fascinating problem of evolution of self-gravitating clusters keeps its interest after the many years passed since Henon thesis, and is still worth discussing now., Comment: Invited talk at the "hommage a Michel Henon" conference held at the "Institut Henri Poincare" Paris, GRAVASCO trimestre, autumn 2013. Proceeding by Hermann Press, editors: Jean-Michel Alimi, Roya Mohayaee, Jerome Perez. Videos of all talks are available at: https://www.youtube.com/playlist?list=PL9kd4mpdvWcBLrN8u04IZW6-akNgLG7-_

In this contribution to the volume in memoriam of Michel Henon, we thought appropriate to look at his early scientific work devoted to the dynamics of large assemblies of interacting masses. He predicted in his PhD thesis that, in such a system, first a collapse of mass occurs at the center and that later binaries stars are formed there. Henceforth, the negative energy of binding of pairs becomes a source of positive energy for the rest of the cluster which evaporate because of that. We examine under what conditions such a singularity can occur, and what could happen afterwards. We hope to show that this fascinating problem of evolution of self-gravitating clusters keeps its interest after the many years passed since Henon thesis, and is still worth discussing now., Comment: Invited talk at the "hommage a Michel Henon" conference held at the "Institut Henri Poincare" Paris, GRAVASCO trimestre, autumn 2013. Proceeding by Hermann Press, editors: Jean-Michel Alimi, Roya Mohayaee, Jerome Perez. Videos of all talks are available at: https://www.youtube.com/playlist?list=PL9kd4mpdvWcBLrN8u04IZW6-akNgLG7-_

The supernovae are typical complex phenomena in fluid mechanics with very different time scales. We describe them in the light of catastrophe theory, assuming that successive equilibria between pressure and gravity present a saddle-node bifurcation. In the early stage we show that the loss of equilibrium may be described by a generic equation of the Painlev\'e I form. In the final stage of the collapse, just before the divergence of the central density, we show that the existence of a self-similar collapsing solution compatible with the numerical observations imposes that the gravity forces are stronger than the pressure ones. This situation differs drastically in its principle from the one generally admitted where pressure and gravity forces are assumed to be of the same order. Our new self-similar solution (based on the hypothesis of dominant gravity forces) which matches the smooth solution of the outer core solution, agrees globally well with our numerical results. Whereas some differences with the earlier self-similar solutions are minor, others are very important. For example, we find that the velocity field becomes singular at the collapse time, diverging at the center, and decreasing slowly outside the core, whereas previous works described a finite velocity field in the core which tends to a supersonic constant value at large distances. This discrepancy should be important for explaining the emission of remnants in the post-collapse regime. Finally we describe the post-collapse dynamics, when mass begins to accumulate in the center, also within the hypothesis that gravity forces are dominant., Comment: Workshop in Honor of Paul Manneville, Paris (2013)

The supernovae are typical complex phenomena in fluid mechanics with very different time scales. We describe them in the light of catastrophe theory, assuming that successive equilibria between pressure and gravity present a saddle-node bifurcation. In the early stage we show that the loss of equilibrium may be described by a generic equation of the Painlev\'e I form. In the final stage of the collapse, just before the divergence of the central density, we show that the existence of a self-similar collapsing solution compatible with the numerical observations imposes that the gravity forces are stronger than the pressure ones. This situation differs drastically in its principle from the one generally admitted where pressure and gravity forces are assumed to be of the same order. Our new self-similar solution (based on the hypothesis of dominant gravity forces) which matches the smooth solution of the outer core solution, agrees globally well with our numerical results. Whereas some differences with the earlier self-similar solutions are minor, others are very important. For example, we find that the velocity field becomes singular at the collapse time, diverging at the center, and decreasing slowly outside the core, whereas previous works described a finite velocity field in the core which tends to a supersonic constant value at large distances. This discrepancy should be important for explaining the emission of remnants in the post-collapse regime. Finally we describe the post-collapse dynamics, when mass begins to accumulate in the center, also within the hypothesis that gravity forces are dominant., Comment: Workshop in Honor of Paul Manneville, Paris (2013)

In these notes we consider a semilinear heat equation in a bounded domain of IRd , with control on a subdomain and homogeneous Dirichlet boundary conditions. We consider nonlinearities for which, in the absence of control, blow up arises. We prove that when the nonlinearity grows at infinity fast enough, due to the local (in space) nature of the blow up phenomena, the control may not avoid the blow up to occur for suitable initial data. This is done by means of localized energy estimates. However, we also show that when the nonlinearity is weak enough, and provided the system admits a globally defined solution (for some initial data and control), the choice of a suitable control guarantees the global existence of solutions and moreover that the solution may be driven in any finite time to the globally defined solution. In order for this to be true we require the nonlinearity f to satisfy at infinity the growth condition f(s) |s| log3/2 (1 + |s|) → 0 as |s| → ∞. This is done by means of a fixed point argument and a careful analysis of the control of linearized heat equations relying on global Carleman estimates. The problem of controlling the blow up in this sense remains open for nonlinearities growing at infinity like f(s) ∼ |s|logp (1 + |s|) with 3/2 ≤ p ≤ 2.

In these notes we consider a semilinear heat equation in a bounded domain of IRd , with control on a subdomain and homogeneous Dirichlet boundary conditions. We consider nonlinearities for which, in the absence of control, blow up arises. We prove that when the nonlinearity grows at infinity fast enough, due to the local (in space) nature of the blow up phenomena, the control may not avoid the blow up to occur for suitable initial data. This is done by means of localized energy estimates. However, we also show that when the nonlinearity is weak enough, and provided the system admits a globally defined solution (for some initial data and control), the choice of a suitable control guarantees the global existence of solutions and moreover that the solution may be driven in any finite time to the globally defined solution. In order for this to be true we require the nonlinearity f to satisfy at infinity the growth condition f(s) |s| log3/2 (1 + |s|) → 0 as |s| → ∞. This is done by means of a fixed point argument and a careful analysis of the control of linearized heat equations relying on global Carleman estimates. The problem of controlling the blow up in this sense remains open for nonlinearities growing at infinity like f(s) ∼ |s|logp (1 + |s|) with 3/2 ≤ p ≤ 2.

In these notes we consider a semilinear heat equation in a bounded domain of IRd , with control on a subdomain and homogeneous Dirichlet boundary conditions. We consider nonlinearities for which, in the absence of control, blow up arises. We prove that when the nonlinearity grows at infinity fast enough, due to the local (in space) nature of the blow up phenomena, the control may not avoid the blow up to occur for suitable initial data. This is done by means of localized energy estimates. However, we also show that when the nonlinearity is weak enough, and provided the system admits a globally defined solution (for some initial data and control), the choice of a suitable control guarantees the global existence of solutions and moreover that the solution may be driven in any finite time to the globally defined solution. In order for this to be true we require the nonlinearity f to satisfy at infinity the growth condition f(s) |s| log3/2 (1 + |s|) → 0 as |s| → ∞. This is done by means of a fixed point argument and a careful analysis of the control of linearized heat equations relying on global Carleman estimates. The problem of controlling the blow up in this sense remains open for nonlinearities growing at infinity like f(s) ∼ |s|logp (1 + |s|) with 3/2 ≤ p ≤ 2.

In these notes we consider a semilinear heat equation in a bounded domain of IRd , with control on a subdomain and homogeneous Dirichlet boundary conditions. We consider nonlinearities for which, in the absence of control, blow up arises. We prove that when the nonlinearity grows at infinity fast enough, due to the local (in space) nature of the blow up phenomena, the control may not avoid the blow up to occur for suitable initial data. This is done by means of localized energy estimates. However, we also show that when the nonlinearity is weak enough, and provided the system admits a globally defined solution (for some initial data and control), the choice of a suitable control guarantees the global existence of solutions and moreover that the solution may be driven in any finite time to the globally defined solution. In order for this to be true we require the nonlinearity f to satisfy at infinity the growth condition f(s) |s| log3/2 (1 + |s|) → 0 as |s| → ∞. This is done by means of a fixed point argument and a careful analysis of the control of linearized heat equations relying on global Carleman estimates. The problem of controlling the blow up in this sense remains open for nonlinearities growing at infinity like f(s) ∼ |s|logp (1 + |s|) with 3/2 ≤ p ≤ 2.

In these notes we consider a semilinear heat equation in a bounded domain of IRd , with control on a subdomain and homogeneous Dirichlet boundary conditions. We consider nonlinearities for which, in the absence of control, blow up arises. We prove that when the nonlinearity grows at infinity fast enough, due to the local (in space) nature of the blow up phenomena, the control may not avoid the blow up to occur for suitable initial data. This is done by means of localized energy estimates. However, we also show that when the nonlinearity is weak enough, and provided the system admits a globally defined solution (for some initial data and control), the choice of a suitable control guarantees the global existence of solutions and moreover that the solution may be driven in any finite time to the globally defined solution. In order for this to be true we require the nonlinearity f to satisfy at infinity the growth condition f(s) |s| log3/2 (1 + |s|) → 0 as |s| → ∞. This is done by means of a fixed point argument and a careful analysis of the control of linearized heat equations relying on global Carleman estimates. The problem of controlling the blow up in this sense remains open for nonlinearities growing at infinity like f(s) ∼ |s|logp (1 + |s|) with 3/2 ≤ p ≤ 2.

We have calculated the first non-vanishing correction (∝γ2) to the self-diffusion coefficient D of a Van der Waals fluid, where the attractive forces are characterized by the inverse range γ, taken as a smallness parameter. © 1975., SCOPUS: ar.j, info:eu-repo/semantics/published

Préface de David Ruelle. « De rédaction excellente, toujours très claire sans jamais offenser la rigueur, les auteurs ont réussi (superbement) ce tour de force de présenter une science toute récente sous une forme abordable à un étudiant muni en mathématique d'un bage qui n'excède pas le contenu de l'ex-propédeutique. Tout l'ouvrage respire une revigorante atmosphère d'optimisme. » René Thom.

Préface de David Ruelle. « De rédaction excellente, toujours très claire sans jamais offenser la rigueur, les auteurs ont réussi (superbement) ce tour de force de présenter une science toute récente sous une forme abordable à un étudiant muni en mathématique d'un bage qui n'excède pas le contenu de l'ex-propédeutique. Tout l'ouvrage respire une revigorante atmosphère d'optimisme. » René Thom.

We critically discuss the various tools and methods which are used to describe the role of long wave length hydrodynamical processes in the analysis of time-dependent correlation functions. We also review the various physical problems (long time behavior of Green-Kubo integrands, 2 dimensional hydrodynamics, transport properties of the Van der Waals fluid, critical phenomena .) where these methods have received fruitful applications. © 1975., SCOPUS: re.j, info:eu-repo/semantics/published

In plasmas the 3-body collision operator diverges even after its most diverging part has been included in the Balescu Lénard collision operator. The virial expansion of transport coefficient after the Boltzmann order includes fractional powers and logarithms. © 1976., SCOPUS: ar.j, info:eu-repo/semantics/published

Préface de David Ruelle. « De rédaction excellente, toujours très claire sans jamais offenser la rigueur, les auteurs ont réussi (superbement) ce tour de force de présenter une science toute récente sous une forme abordable à un étudiant muni en mathématique d'un bage qui n'excède pas le contenu de l'ex-propédeutique. Tout l'ouvrage respire une revigorante atmosphère d'optimisme. » René Thom.

The molecular diffusivity of a tracer species in a given medium is a measure of the average spread of the tracer due to fluctuations in its velocity on the molecular scale. In a medium that is heterogeneous on a macroscopic scale, a tracer will experience an additional spreading due to variations in its velocity as it moves through the heterogeneous medium. This spreading due to macroscale velocity fluctuations is known as dispersion and is the subject of this article.

The general topic of this chapter is molecular mixing and chemical reaction in turbulent shear flow, in particular, in two-dimensional mixing layers and axi-symmetric jets. The discussion is centered around recent experiments in these two flows and upon proposed new interpretations of the observations. The central issue is the nature of the pathway from the freestream to the molecularly mixed state. The experimental results suggest that the path does not resemble that given by turbulent diffusion concepts. They imply, as well, that molecular transport coefficients influence molecular mixing rates even in what are conventionally called fully-developed turbulent flows. In the proposed model, this influence is felt at the Taylor scale.

We critically discuss the various tools and methods which are used to describe the role of long wave length hydrodynamical processes in the analysis of time-dependent correlation functions. We also review the various physical problems (long time behavior of Green-Kubo integrands, 2 dimensional hydrodynamics, transport properties of the Van der Waals fluid, critical phenomena .) where these methods have received fruitful applications. © 1975., SCOPUS: re.j, info:eu-repo/semantics/published

In plasmas the 3-body collision operator diverges even after its most diverging part has been included in the Balescu Lénard collision operator. The virial expansion of transport coefficient after the Boltzmann order includes fractional powers and logarithms. © 1976., SCOPUS: ar.j, info:eu-repo/semantics/published

Starting from the formal expansion of an arbitrary transport coefficient X in series of the inverse range γ of a van der Waals potential, we establish the explicit form of the first γ correction to X; we show that it can be expressed solely in terms of the Fourier transform Vy L of the long-range interaction and in terms of the equilibrium and transport properties of the short-range reference system. A comparison with previous work on related problems is also given. Copyright © 1974 American Institute of Physics., SCOPUS: ar.j, info:eu-repo/semantics/published

We have obtained a kinetic justification to the t- 3 2 behavior of the Green-Kubo integrands in three-dimensional dense fluids. Our calculation, which is of a self-consistent nature, also supports the recent conjectures on the higher order correction to this behavior. © 1973., SCOPUS: ar.j, info:eu-repo/semantics/published

We exploit here the previously established formal inequality for the time-dependent collision operator and we get an estimate of the long-time behaviour of the various contributions to this operator. We also derive microscopically the well-known asymptotic formulas for the Green-Kubo integrands. © 1974., SCOPUS: ar.j, info:eu-repo/semantics/published

We exploit here the previously established formal inequality for the time-dependent collision operator and we get an estimate of the long-time behaviour of the various contributions to this operator. We also derive microscopically the well-known asymptotic formulas for the Green-Kubo integrands. © 1974., SCOPUS: ar.j, info:eu-repo/semantics/published

We have obtained a kinetic justification to the t- 3 2 behavior of the Green-Kubo integrands in three-dimensional dense fluids. Our calculation, which is of a self-consistent nature, also supports the recent conjectures on the higher order correction to this behavior. © 1973., SCOPUS: ar.j, info:eu-repo/semantics/published