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Series: Math Physics Seminar

We study a gas of N hard disks in a box with semi-periodic boundary
conditions. The unperturbed gas is hyperbolic and ergodic (these facts are
proved for N=2 and expected to be true for all N>2). We study
various perturbations by "twisting" the outgoing velocity at collisions with
the walls. We show that the dynamics tends to collapse to various stable
regimes, however we define the perturbations and however small they are.

Series: Math Physics Seminar

In this talk I will discuss a family of lower bounds on the indirect Coulomb energy for atomic and molecular systems in two dimensions in terms of a functional of the single particle density with gradient correction terms

Series: Math Physics Seminar

Abstract: In this talk we will survey some recent developments in the scattering theory of complete, infinite-volume manifolds with ends modeled on quotients of hyperbolic space. The theory of scattering resonances for such spaces is in many ways parallel to the classical case of eigenvalues on a compact Riemann surface. However, it is only relatively recently that progress has been made in understanding the distribution of these resonances. We will give some introduction to the theory of resonances in this context and try to sketch this recent progress. We will also discuss some interesting outstanding conjectures and present numerical evidence related to these.

Series: Math Physics Seminar

I'll discuss two methods for finding bounds on sums of graph eigenvalues (variously for the Laplacian, the renormalized Laplacian, or the adjacency matrix). One of these relies on a Chebyshev-type estimate of the statistics of a subsample of an ordered sequence, and the other is an adaptation of a variational argument used by P. Kröger for Neumann Laplacians. Some of the inequalities are sharp in suitable senses. This is ongoing work with J. Stubbe of EPFL

Series: Math Physics Seminar

In this talk we will discuss the definition of chaoticity and entropic chaoticity, as well as the background that led us to define these quantities, mainly Kac's model and the Boltzmann equation. We will then proceed to investigate the fine balance required for entropic chaoticity by exploring situations where chaoticity is valid, but not entropic chaoticity. We will give a general method to construct such states as well as two explicit example, one of which is quite surprising.

Series: Math Physics Seminar

Host: Predrag Cvitanovic

More than 125 years ago Osborne Reynolds launched the quantitative
study of turbulent transition as he sought to understand the conditions
under
which fluid flowing through a pipe would be laminar or turbulent. Since
laminar and turbulent flow have vastly different drag laws, this question is
as important now as it was in Reynolds' day. Reynolds understood how one
should define "the real critical value'' for the fluid velocity beyond
which
turbulence can persist indefinitely. He also appreciated the difficulty in
obtaining this value. For years this critical Reynolds number, as we now
call
it, has been the subject of study, controversy, and uncertainty. Now, more
than a century after Reynolds pioneering work, we know that the onset of
turbulence in shear flows is properly understood as a statistical phase
transition. How turbulence first develops in these flows is more closely
related to the onset of an infectious disease than to, for example, the
onset
of oscillation in the flow past a body or the onset of motion in a fluid
layer
heated from below. Through the statistical analysis of large samples of
individual decay and proliferation events, we at last have an accurate
estimate of the real critical Reynolds number for the onset of turbulence in
pipe flow, and with it, an understanding of the nature of transitional
turbulence.
This work is joint with: K. Avila, D. Moxey, M. Avila, A. de Lozar, and B.
Hof.

Series: Math Physics Seminar

The relative isoperimetric inequality inside an open, convex cone C states that under a volume constraint, the ball intersected the cone minimizes the perimeter inside C. In this talk, we will show how one can use optimal transport theory to obtain this inequality, and we will prove a corresponding sharp stability result. This is joint work with Alessio Figalli.

Series: Math Physics Seminar

The phenomenon of wave run-up has the capital importance for the beach erosion, coastal protection and flood hazard estimation. In the present talk we will discuss two particular aspects of the wave run-up problem. In this talk we focus on the wave run-up phenomena on a sloping beach. In the first part of the talk we present a simple stochastic model of the bottom roughness. Then, we quantify the roughness effect onto the maximal run-up height using Monte-Carlo simulations. A critical comparison with more conventional approaches is also performed.In the second part of the talk we study the run-up of simple wave groups on beaches of various geometries. Some resonant amplification phenomena are unveiled. The maximal run-up height in resonant cases can be 20 times higher than in regular situations. Thus, this work can provide a possible mechanism of extreme tsunami run-up conventionally ascribed to "local site effects".References:Dutykh, D., Labart, C., & Mitsotakis, D. (2011). Long wave run-up on random beaches. Phys. Rev. Lett, 107, 184504.Stefanakis, T., Dias, F., & Dutykh, D. (2011). Local Runup Amplification by Resonant Wave Interactions. Phys. Rev. Lett., 107, 124502.

Series: Math Physics Seminar

In this seminar we will show that the nonlinear mechanics of solids with distributed dislocations can be formulated as a nonlinear elasticity problem provided that the material manifold – where the body is stress-free − is chosen appropriately. Choosing a Weitzenböck manifold (a manifold with a flat and metric-compatible affine connection that has torsion) with torsion tensor identified with the given dislocation density tensor the body would be stress-free in the material manifold by construction. For classical nonlinear elastic solids in order to calculate stresses one needs to know the changes of the relative distances, i.e. a metric in the material manifold is needed. For distributed dislocations this metric is the metric compatible with the Weitzenböck connection. We will present exact solutions for the residual stress field of several distributed dislocation problems in incompressible nonlinear elastic solids using Cartan's method of moving frames. We will also discuss zero-stress dislocation distributions in nonlinear dislocation mechanics.

Series: Math Physics Seminar

An
implicit method [1, 2], TARDIS (Transient Advection Reaction
Diffusion Implicit Simulations), has been developed that successfully
couples the compressible flow to the comprehensive chemistry and
multi-component transport properties. TARDIS has been demonstrated in
application to two fundamental combustion problems of great interest.
First,
TARDIS was used to investigate stretched laminar flame velocities in
eight flame configurations: outwardly and inwardly propagating
H2/air and CH4/air in cylindrical and spherical geometries.
Fractional power laws are observed between the velocity deficit and
the flame curvature Second,
the response of transient outwardly propagating premixed H2/air and
CH4/air flames subjected to joint pressure and equivalence ratio
oscillations were investigated. A fuller version of the abstract can be obtained from http://www.math.gatech.edu/~rll6/malik_abstract-Apr-2012.docx
[1]
Malik, N.A. and Lindstedt, R.P. The response of transient
inhomogeneous flames to pressure fluctuations and stretch: planar
and outwardly propagating hydrogen/air flames.
Combust.
Sci. Tech. 82(9), 2010.
[2]
Malik,
N. A. “Fractional
powers laws in stretched flame velocities in finite thickness flames:
a numerical study using realistic chemistry”.
Under
review, (2012).
[3]
Markstein, G.H. Non-steady Flame Propagation. Pergamon Press, 1964.
[4]
Weis,M., Zarzalis, N., and Suntz, R. Experimental study of markstein
number effects on laminar flamelet velocity in turbulent premixed
flames. Combust. Flame,
154:671--691, 2008.