# Past JAMS Seminars

The JAMS is a weekly seminar with talks given by junior researchers on a topic in all areas in Mathematics. For upcoming seminars check here

Past Seminars

(11/04/18)Supersonic Flow over a Film flow
Faezeh Khoshsepehr
Abstract

As a result of rainfall during the ascent or descent stages of the flight or de-icing operation, usually a thin layer of viscous liquid is formed or rain droplets are left above the aircraft wings. The presence of the droplet leads to significant changes in boundary-layer behaviour.

In this talk we are going to demonstrate a modified tripe-deck model which accounts for the presence of a droplet and its effects on the air flow. Analytical solutions are obtained for the linearised flow’s velocity field, pressure and skin friction.

(11/04/18)Reflection of Rossby Waves in the Stratosphere: The Role of Critical Layer Nonlinearity
Imogen Mhari Dell
Abstract

There exists a two-way coupling between the troposphere and the stratosphere; atmospheric waves are generated through a variety of mechanisms in the troposphere and they propagate upwards in to the otherwise stable stratosphere. Here, they may break down or they may be reflected downwards back in to the troposphere, thereby influencing tropospheric dynamics and forming a closed loop two- way coupling. Placing this coupling in an appropriate mathematical framework is crucial to improving climate and weather predictions, and this is investigated by first understanding the reflection of Rossby waves.
The mechanism by which Rossby waves are reflected is studied in terms of a nonlinear critical layer. Critical layers exist where the base flow velocity equals the phase speed, and it is here that nonlinear effects may become significant, and Rossby waves reflected. This work takes in to account the presence of all harmonics within the critical layer, and attempts to identify the reflected and transmitted waves as well as the consequential disturbances within the critical layer using multiple-scale techniques.

(24/01/18) Natural Language Processing with messy datasets
José Luis Ricón
[Slides] Abstract

In this talk, I will introduce basic Natural Language Processing ideas and the machine learning approaches that are generally used for dealing with the problem of text classification. After this I will highlight the particular challenges posed by this particular project.

(07/02/18) The problem with turbulence: how can air travel be made more sustainable?
Eleanor Johnstone
Abstract

“When I meet God, I am going to ask him two questions: Why relativity? And why turbulence? I really believe he will have an answer for the first.”- Werner Heisenberg.

It is thought that one transatlantic flight can add as much to your carbon footprint as a typical year’s worth of driving. Why are planes so inefficient? Well, part of the reason is turbulence, which causes drag on the aircraft. Therefore one of the most important questions to ask is how can we control the flow so turbulence is less prevalent? Not only could this save the aerospace industry millions of pounds in fuel, but it would also have a huge impact on the sustainability of air travel, and help reduce the dependence on oil which is expected to run out within the next century.

This talk will cover a brief introduction to the equations governing fluid flows, and the emergence and structure of turbulence in basic boundary-layer flows. Specifically, the talk will cover what structures emerge in the transition from laminar to turbulent flow and how these can be described mathematically. These structures can help to provide an understanding of the underlying ‘scaffold’ of turbulence. This knowledge could be implemented to reduce their prevalence and therefore reduce turbulence/drag on an aircraft.

Keywords: fluid dynamics, turbulence, boundary layer, laminar flow control

(21/02/18) Numerical tsunami simulation using goal-oriented mesh adaptivity
Joe Wallwork
Abstract

A mesh adaptive shallow water solver is created for solving tsunami modelling problems. This builds upon previous work at Imperial College in the Firedrake and Thetis projects. The former project provides efficient, finite element solving software and the latter specialises this for coastal, estuarine and ocean modelling on unstructured meshes. An anisotropic mesh adaptivity library is created which is capable of adaption both to fields related to the flow (such as free surface displacement) and as guided by adjoint solution data. Applying mesh adaptivity to shallow water problems, enables the efficient generation of numerical solutions to tsunami wave propagation problems. The case study of the tsunami which struck Fukushima, Japan, in 2011 is considered, wherein leading tsunami waves reached the coast in just ten minutes. Numerical results indicate computational cost can be reduced, whilst retaining a sufficiently high accuracy, under mesh adaptivity. Through establishing a highly efficient approach to numerical tsunami simulation, sufficient warning could be provided in future scenarios, allowing for evacuation and damage mitigation in those coastal areas determined to be most at risk.

(28/02/18) Mode Interactions in Spherical Rayleigh-Bénard Convection
Paul Mannix
Abstract

The critical Rayleigh number $Ra_c$ for thermally convective instability depends on the wave-length of the disturbance.

In an annular spherical domain with separation d, there are degenerate points $(Ra_c , d _c )$ at which instability to two different sets of thermal-rolls occurs simultaneously.
This study provides a weakly non-linear analysis of the multiple-bifurcation problem, demonstrating that four distinct coupled amplitude equations govern the non-linear evolution of these interactions. The choice of which can be predicted from the inherent symmetry of the interacting modes. Considering a variety of $\ell:m$ mode interactions at different values of the Prandtl number $\sigma$, it is found that mixed mode solutions can exist only within certain regions of the parameter space. While for special resonant mode interactions a stable-period solution is found at low Prandtl number $\sigma$. In each case the weakly non-linear prediction is verified using direct numerical simulation.

(14/03/18)Long time behaviour and phase transitions for the McKean-Vlasov equation on the torus.
Rishabh Gvalani
Abstract

We study the McKean-Vlasov equation on the flat torus which is obtained as the mean field limit of a system of interacting diffusion processes enclosed in a periodic box. The system acts as a model for several real world phenomena from statistical mechanics, opinion dynamics, collective behaviour, and stellar dynamics.

After commenting on the well-posedness of the equation, we study its long time behaviour and convergence to equilibrium. We then focus our attention on the stationary problem – under certain assumptions on the interaction potential, we show that the system exhibits multiple equilibria which arise from the uniform state through continuous bifurcations. This relates closely with previous work on phase transitions for the Mckean-Vlasov equation (cf. Chayes and Panferov, J. Stat. Phys., 2010). Finally, we attempt to classify continuous and discontinuous transitions for this system and show how this work, in conjunction with previous studies of the system, can be used to recover classical results on phase transitions for the noisy Kuramoto model. This is joint work with José Carrillo, Greg Pavliotis, and André Schlichting.

(14/03/18)Finite element methods as geometric integrators
James Jackaman
Abstract

For long time accurate numerical integration, it is of paramount importance to preserve certain geometric structures
associated to the underlying system. This has been a key area of research in numerical analysis, known as geometric numerical integration, over the past few decades. In this talk we investigate the continuous Galerkin method as a geometric numerical integrator. We will also introduce a new discontinuous finite element method and discuss what geometric properties this method possesses.

(21/03/18) Population dynamics in heterogeneous environments
Yong Sul Won
Abstract

We study a nonlinear reaction-diffusion system undergoing a logistic type competition. The diffusivity of species depends on the ratio of their population density and the resource distribution (nourishments). Such a feature provides us with not only a more realistic model of population dispersals but also interesting mathematical challenges. We will present some existence theorems together with a mollification strategy.

(21/03/18)The wonder of the Rado graph
Yibei Li
Abstract

The countable random graph, also referred to as the Rado graph, is a very interesting mathematical object as it appears in many different areas of Maths. It can be constructed using ideas from probability and number theory (more precisely the Dirichlet’s Theorem). It possesses nice properties in logic, group theory, the Ramsey theory and topological dynamics. We will touch upon all these areas to see why the Rado graph makes a fascinating structure.

(28/03/18) Mathematical and numerical modeling of swimming at the micrometer scale: the case of flagellated microswimmers.
Dr. Cecilia Rorai(QMUL)
Abstract

Abstract:
Many cells and microorganisms are motile: they are capable of navigating fluid environments to accomplish tasks required for survival or specialized functions. I will describe the physical principles obeyed by swimming at the micro scale and the mathematical models used to describe them. I will present numerical results for a sperm cell-like swimmer in an unbounded domain and compare several models of different fidelity based on the Stokes flow approximation. The models include a Regularised Stokeslet Method, a 3D Finite Element Method, the Resistive Force Theory versions of Lighthill and Gray and Hancock, as well as a simplified approximation based on computing the hydrodynamic forces exerted on the head and the flagellum separately. I will discuss applications that may arise from our ability of modeling and manipulating microswimmers and the challenges that lie ahead.

Biography:
Dr Cecilia Rorai earned her Ph.D. from the Doctoral School in Environmental and Industrial Fluid Mechanics, University of Trieste, Italy. She conducted her doctoral research at University of Maryland in collaboration with Professors D. P. Lathrop, R. M. Kerr and under the supervision of Professors K. R. Sreenivasan and Michael E. Fisher. She defended her thesis entitled “vortex reconnection in superfluid helium” in April 2012. Later, she moved as a visiting scientist at the National Center for Atmospheric Research (NCAR), Boulder, Colorado, where she worked on turbulence in stratified flows in collaboration with Professor Annick Pouquet, Dr D. Rosenberg and Professor P. D. Mininni. In 2013 she joined the Royal Institute of Technology (KTH) and the Nordic Institute of Theoretical Physics (NORDITA) Stockholm, Sweden, as the recipient of the postdoctoral fellowship from the Göran Gustafsson foundation. Her project dealt with the motion of elastic capsules in Stokes flows and involved Professor Luca Brandt, Dr Dhrubaditya Mitra and Dr Lailai Zhu. In 2016 she was awarded a Marie Skłodowska-Curie postdoctoral fellowship and she joined Queen Mary University where she works in collaboration with Dr Sergey Karabasov. Her current research focus is on biological fluid dynamics, more specifically, swimming at the micrometer scale. Her approach is theoretical and computational.

(28/03/18)A fluid-mechanical analogy for multi-resolution simulation of liquids at multiple scales.
Jingyi Hu (QMUL)
Abstract

Abstract:
A triple-scale simulation of a molecular liquid is presented where atomistic, coarse-grained, and hydrodynamic descriptions of a liquid matter are consistently coupled. In the previous dual-scale model in the literature, which we will briefly overview, the continuum and discrete representations of the same substance are coupled together in the framework of conservation laws for mass and momentum that are treated as effective phases of a nominally two-phase flow. The effective phase distribution in space defines the computational domain decomposition for the low-resolution and the high-resolution zones of the model as defined by the user. The continuum representation is based on the Navier-Stokes Fluctuating Hydrodynamics (NS-FH) equations, which use stochastic fluxes to model the effect of the Brownian motion at the small scale. Building on the dual-scale model, which used the classical molecular dynamic (MD) dynamics as a model for the discrete state of the fluid, the current triple-scale multiscale model replaces the pure MD model with an adaptive resolution scheme (AdResS). AdResS is a state-of-the art discrete particle method developed in the literature, which smoothly connects the atomistic and the mesoscopic scales. The new combined AdResS-NS-FH model is shown to perform more accurately and be less sensitive to the calibration parameters compared to the previous dual-scale model when the high-resolution zone is made to move arbitrarily for a stationary uniform medium test. We also present test results to show how the combined triple-scale model can correctly preserve the amplitude and the phase of a high-frequency acoustic wave that propagates across the continuum hydrodynamic, the mesoscopic particle, and the microscopic particle layers of the hybrid computational domain.

Biography:
Jingyi Hu currently is a third year PhD student of Dr Sergey Karabasov in the School of Engineering and Materials Science, Queen Mary University of London. Prior to becoming a PhD student, Jingyi got the bachelor degree in applied mathematics from Central South University, China in 2014. Focusing on the multiscale modelling in hydrodynamics, her PhD project aims at developing a new triple-level system that smoothly connects atomistic, mesoscale and continuum representation of liquids in application to shear effects in complex fluids.

Autumn 2017: [Archive]

Spring 2017: [Archive] Autumn 2016: [Archive]

Spring 2016: [Schedule] [Archive] Autumn 2015: [Schedule] [Archive]

Spring 2015: [Schedule] [Archive] Autumn 2014: [Schedule] [Archive]