Seminars and Colloquia by Series

Non-­‐local models of anomalous transport

Series
CDSNS Colloquium
Time
Monday, February 20, 2012 - 11:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Diego Del Castillo-Negrete – Oak Ridge National Lab
The study of transport is an active area of applied mathematics of interest to fluid mechanics, plasma physics, geophysics, engineering, and biology among other areas. A considerable amount of work has been done in the context of diffusion models in which, according to the Fourier-­‐Fick’s prescription, the flux is assumed to depend on the instantaneous, local spatial gradient of the transported field. However, despiteits relative success, experimental, numerical, and theoretical results indicate that the diffusion paradigm fails to apply in the case of anomalous transport. Following an overview of anomalous transport we present an alternative(non-­‐diffusive) class of models in which the flux and the gradient are related non-­‐locally through integro-­differential operators, of which fractional Laplacians are a particularly important special case. We discuss the statistical foundations of these models in the context of generalized random walks with memory (modeling non-­‐locality in time) and jump statistics corresponding to general Levy processes (modeling non-­‐locality in space). We discuss several applications including: (i) Turbulent transport in the presence of coherent structures; (ii) chaotic transport in rapidly rotating fluids; (iii) non-­‐local fast heat transport in high temperature plasmas; (iv) front acceleration in the non-­‐local Fisher-­‐Kolmogorov equation, and (v) non-­‐Gaussian fluctuation-­‐driven transport in the non-­‐local Fokker-­‐Planck equation.

Parameterization of Invariant Manifolds for Lagrangian Systems with Long-range Interactions

Series
CDSNS Colloquium
Time
Monday, February 13, 2012 - 11:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Hector Lomeli – Univ. of Texas at Austin/ITAM
We generalize some notions that have played an important role in dynamics, namely invariant manifolds, to the more general context of difference equations. In particular, we study Lagrangian systems in discrete time. We define invariant manifolds, even if the corresponding difference equations can not be transformed in a dynamical system. The results apply to several examples in the Physics literature: the Frenkel-Kontorova model with long-range interactions and the Heisenberg model of spin chains with a perturbation. We use a modification of the parametrization method to show the existence of Lagrangian stable manifolds. This method also leads to efficient algorithms that we present with their implementations. (Joint work with Rafael de la Llave.)

Singularity Theory for KAM tori: A methodology

Series
CDSNS Colloquium
Time
Monday, January 30, 2012 - 11:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Alex Haro – Univ.. of Barcelona
We present a novel method to find KAM tori in degenerate (nontwist) cases. We also require that the tori thus constructed have a singular Birkhoff normal form. The method provides a natural classification of KAM tori which is based on Singularity Theory.The method also leads to effective algorithms of computation, and we present some preliminary numerical results. This work is in collaboration with R. de la Llave and A. Gonzalez.

A numerical algorithm for the computation of periodic orbits of the Kuramoto-Sivashinsky equation.

Series
CDSNS Colloquium
Time
Monday, January 23, 2012 - 11:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Jordi Lluis Figueras – Uppsala University
In this talk we will present a numerical algorithm for the computation of (hyperbolic) periodic orbits of the 1-D K-S equation u_t+v*u_xxxx+u_xx+u*u_x = 0, with v>0. This numerical algorithm consists on apply a suitable Newton scheme for a given approximate solution. In order to do this, we need to rewrite the invariance equation that must satisfy a periodic orbit in a form that its linearization around an approximate solution is a bounded operator. We will show also how this methodology can be used to compute rigorous estimates of the errors of the solutions computed.

An instability mechanism along the mean motion resonances in the restricted three body problem.

Series
CDSNS Colloquium
Time
Monday, January 9, 2012 - 11:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Marcel Guardia – Institute for Advanced Studies
We consider the restricted planar elliptic 3 body problem, which models the Sun, Jupiter and an Asteroid (which we assume that has negligible mass). We take a realistic value of the mass ratio between Jupiter and the Sun and their eccentricity arbitrarily small and we study the regime of the mean motion resonance 1:7, namely when the period of the Asteroid is approximately seven times the period of Jupiter. It is well known that if one neglects the influence of Jupiter on the Asteroid, the orbit of the latter is an ellipse. In this talk we will show how the influence of Jupiter may cause a substantial change on the shape of Asteriod's orbit. This instability mechanism may give an explanation of the existence of the Kirkwood gaps in the Asteroid belt. This is a joint work with J. Fejoz, V. Kaloshin and P. Roldan.

Tensor Products, Positive Operators, and Delay-Differential Equations

Series
CDSNS Colloquium
Time
Monday, December 5, 2011 - 11:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
John Mallet-Paret – Division of Applied Mathematics, Brown University –
We study a class of linear delay-differential equations, with a singledelay, of the form$$\dot x(t) = -a(t) x(t-1).\eqno(*)$$Such equations occur as linearizations of the nonlinear delay equation$\dot x(t) = -f(x(t-1))$ around certain solutions (often around periodicsolutions), and are key for understanding the stability of such solutions.Such nonlinear equations occur in a variety of scientific models, anddespite their simple appearance, can lead to a rather difficultmathematical analysis.We develop an associated linear theory to equation (*) by taking the$m$-fold wedge product (in the infinite dimensional sense of tensorproducts) of the dynamical system generated by (*). Remarkably, in the caseof a ``signed feedback'' where $(-1)^m a(t) > 0$ for some integer $m$, theassociated linear system is given by an operator which is positive withrespect to a certain cone in a Banach space. This leads to very detailedinformation about stability properties of (*), in particular, informationabout characteristic multipliers.

On the Hamilton-Jacobi variational formulation of the Vlasov equation

Series
CDSNS Colloquium
Time
Monday, November 7, 2011 - 11:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Phil Morrison – Univ. of Texas at Austin
The Vlasov-Poisson and Vlasov-Maxwell equations possess various variational formulations1 or action principles, as they are generally termed by physicists. I will discuss a particular variational principle that is based on a Hamiltonian-Jacobi formulation of Vlasov theory, a formulation that is not widely known. I will show how this formu- lation can be reduced for describing the Vlasov-Poisson system. The resulting system is of Hamilton-Jacobi form, but with nonlinear global coupling to the Poisson equation. A description of phase (function) space geometry will be given and comments about Hamilton-Jacobi pde methods and weak KAM will be made.Supported by the US Department of Energy Contract No. DE-FG03- 96ER-54346.H. Ye and P. J. Morrison Phys. Fluids 4B 771 (1992).D. Pfirsch, Z. Naturforsch. 39a, 1 (1984); D. Pfirsch and P. J. Morrison, Phys. Rev. 32A, 1714 (1985).

Generic properties of scalar parabolic equations

Series
CDSNS Colloquium
Time
Friday, April 1, 2011 - 11:00 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Genevieve Raugel – Universite Paris-Sud
In this talk, we generalize the classical Kupka-Smale theorem for ordinary differential equations on R^n to the case of scalar parabolic equations. More precisely, we show that, generically with respect to the non-linearity, the semi-flow of a reaction-diffusion equation defined on a bounded domain in R^n or on the torus T^n has the "Kupka-Smale" property, that is, all the critical elements (i.e. the equilibrium points and periodic orbits) are hyperbolic and the stable and unstable manifolds of the critical elements intersect transversally. In the particular case of T1, the semi-flow is generically Morse-Smale, that is, it has the Kupka-Smale property and, moreover, the non-wandering set is finite and is only composed of critical elements. This is an important property, since Morse-Smale semi-flows are structurally stable. (Joint work with P. Brunovsky and R. Joly).

Liquid-crystals are intermediate phases between solid and liquid states

Series
CDSNS Colloquium
Time
Monday, March 14, 2011 - 11:00 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Weishi Liu – University of Kansas –
They may flow like fluids but under constraints of mechanical energies from their crystal aspects. As a result, they exhibit very rich phenomena that grant them tremendous applications in modern technology. Based on works of Oseen, Z\"ocher, Frank and others, a continuum theory (not most general but satisfactory to a great extent) for liquid-crystals was formulated by Ericksen and Leslie in 1960s. We will first give a brief introduction to this classical theory and then focus on various important special settings in both static and dynamic cases. These special flows are rather simple for classical fluids but are quite nonlinear for liquid-crystals. We are able to apply abstract theory of nonlinear dynamical systems upon revealing specific structures of the problems at hands.

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