Fri, 14 Jun 2013

14:30 - 15:30
DH 3rd floor SR

Freezing colloidal suspensions: ice segregation and pattern formation

Dr. Anthony Anderson
(University of Cambridge)
Abstract

Colloidal suspensions do not freeze uniformly; rather, the frozen phase (e.g. ice) becomes segregated, trapping bulk regions of the colloid within, which leads to a fascinating variety of patterns that impact both nature and technology. Yet, despite the central importance of ice segregation in several applications, the physics are poorly understood in concentrated systems and continuum models are available only in restricted cases. I will discuss a particular set of steady-state ice segregation patterns that were obtained during a series of directional solidification experiments on concentrated suspensions. As a case study, I will focus of one of these patterns, which is very reminiscent of ice lenses observed in freezing soils and rocks; a form of ice segregation which underlies frost heave and frost weathering. I will compare these observations against an extended version of a 'rigid-ice' model used in previous frost heave studies. The comparison between theory and experiment is qualitatively correct, but fails to quantitatively predict the ice-lensing pattern. This leaves open questions about the validity of the assumptions in 'rigid-ice' models. Moreover, 'rigid-ice' models are inapplicable to the study of other ice segregation patterns. I conclude this talk with some possibilities for a more general model of freezing colloidal suspensions.

Fri, 03 May 2013

14:30 - 15:30
DH 3rd floor SR

Vigorous convection in porous media, with application to CO${}_2$ sequestration

Duncan Hewitt
(University of Cambridge)
Abstract

Convection in a porous medium plays an important role in many geophysical and industrial processes, and is of particular current interest due to its implications for the long-term security of geologically sequestered CO_2. I will discuss two different convective systems in porous media, with the aid of 2D direct numerical simulations: first, a Rayleigh-Benard cell at high Rayleigh number, which gives an accurate characterization both of the convective flux and of the remarkable dynamical structure of the flow; and second, the evolution and eventual `shut-down' of convection in a sealed porous domain with a source of buoyancy along only one boundary. The latter case is also studied using simple box models and laboratory experiments, and can be extended to consider convection across an interface that can move and deform, rather than across a rigid boundary. The relevance of these results in the context of CO_2 sequestration will be discussed.

Thu, 25 Apr 2013

14:00 - 15:00
Rutherford Appleton Laboratory, nr Didcot

Scalable Data Analytics

Dr Tobias Berka
(University of Cambridge)
Abstract

Very-large scale data analytics are an alleged golden goose for efforts in parallel and distributed computing, and yet contemporary statistics remain somewhat of a dark art for the uninitiated. In this presentation, we are going to take a mathematical and algorithmic look beyond the veil of Big Data by studying the structure of the algorithms and data, and by analyzing the fit to existing and proposed computer systems and programming models. Towards highly scalable kernels, we will also discuss some of the promises and challenges of approximation algorithms using randomization, sampling, and decoupled processing, touching some contemporary topics in parallel numerics.

Wed, 13 Mar 2013

14:00 - 15:00
OCCAM Common Room (RI2.28)

Exact solutions to the total generalised variation minimisation problem

Konstantinos Papafitsoros
(University of Cambridge)
Abstract

********** PLEASE NOTE THE SPECIAL TIME **********

Total generalised variation (TGV) was introduced by Bredies et al. as a high quality regulariser for variational problems arising in mathematical image processing like denoising and deblurring. The main advantage over the classical total variation regularisation is the elimination of the undesirable stairscasing effect. In this talk we will give a small introduction to TGV and provide some properties of the exact solutions to the L^{2}-TGV model in the one dimensional case.

Mon, 04 Mar 2013

14:15 - 15:15
Oxford-Man Institute

Bond Percolation on Isoradial Graphs

IOAN MANOLESCU
(University of Cambridge)
Abstract

The star-triangle transformation is used to obtain an equivalence extending over a set bond percolation models on isoradial graphs. Amongst the consequences are box-crossing (RSW) inequalities and the universality of alternating arms exponents (assuming they exist) for such models, under some conditions. In particular this implies criticality for these models.

(joint with Geoffrey Grimmett)

Mon, 28 May 2012

12:00 - 13:00
L3

Instanton - a window into physics of M5-branes

Sungjay Lee
(University of Cambridge)
Abstract

Instantons and W-bosons in 5d N=2 Yang-Mills theory arise from a circle

compactification of the 6d (2,0) theory as Kaluza-Klein modes and winding

self-dual strings, respectively. We study an index which counts BPS

instantons with electric charges in Coulomb and symmetric phases. We first

prove the existence of unique threshold bound state of U(1) instantons for

any instanton number. By studying SU(N) self-dual strings in the Coulomb

phase, we find novel momentum-carrying degrees on the worldsheet. The total

number of these degrees equals the anomaly coefficient of SU(N) (2,0) theory.

We finally propose that our index can be used to study the symmetric phase of

this theory, and provide an interpretation as the superconformal index of the

sigma model on instanton moduli space. 

Tue, 29 May 2012
12:00

Solitons from geometry.

Dr M Dunajski
(University of Cambridge)
Abstract

Solitons are localised non-singular lumps of energy which describe particles non perturbatively. Finding the solitons usually involves solving nonlinear differential equations, but I shall show that in some cases the solitons emerge directly from the underlying space-time geometry: certain abelian vortices arise from surfaces of constant mean curvature in Minkowski space, and skyrmions can be constructed from the holonomy of gravitational instantons.

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