The UCL PG Fluid Dynamics Seminars run on a bi-weekly basis in term time. The meetings usually take place on Wednesdays, 4-5pm in room 707 in the Maths Department. The speakers were originally UCL PhD students only. Starting in 2018, now we occasionally also invite PhD students from other UK universities (highlighted in red) to present their research. The seminar series is also very happy to welcome postdoctoral researchers to either attend or to give a talk (highlighted in blue).

I no longer organise these seminars and this website is no longer maintained! Information is now updated on the UCL website .

Autumn 2019

• 23 Oct 2019, Jurriaan Gillissen, Research Associate, UCL: Shear Thickening in Dense Suspensions
  Abstract:
  I will present a tensorial constitutive model for the microstructure and stress in shear thickening suspensions. The model is applied to time varying flows (oscillatory shear and abrupt shear reversal).


• 13 Nov 2019 (room 505), Lois Baker, Imperial College London: Superharmonics of Internal Tides in Non-Uniform Stratification
  Abstract:
  Internal tides are internal gravity waves that are generated in the stratified ocean when the tide sloshes over rough seafloor topography. The mechanism by which energy cascades from the large scale internal tides to smaller scales where mixing and dissipation can act is of great importance in understanding energy pathways in the ocean and correctly parametrising climate models, but remains an open question. Here, the excitation of near-resonant superharmonics are investigated for their possible role in extracting energy from the mode-1 internal tide.


• 20 Nov 2019 (room 505), Elle McLean, UCL: Using series truncation and collocation to investigate supercritical spillway flows
  Abstract:
  Assuming a fluid is inviscid and incompressible, and the flow is steady, two-dimensional and irrotational, it follows that through potential flow theory we can use conformal mappings to investigate the behaviour of the flow. This can lead to analytic solutions. For this to be the case, gravity is ignored, leading to what are known as free-streamline solutions. However, if we include the effects of gravity, we can use conformal mappings, supplemented by a numerical method, to obtain a numerical solution. In this talk, I will present a method used by Vanden-Broeck and Keller to find the free surface of a supercritical spillway flow. Results will also be discussed for similar problems for which the same method can be employed.


• 27 Nov 2019 (room 505), Edward Goldsmith, UCL: The Shallow Water Equations over large amplitude topography
  Abstract:
  In this talk we consider the rotating shallow water equations over small-scale topography. By using multi-scale methods to derive a "homogenized" form of the RSWE, we are able to capture the interaction of a large-scale, propagating wave with a range of topographic profiles, including periodically spaced sea-mounts, randomly spaced seamounts, and non-uniformly spaced hills with randomly distributed radii and height. This builds on previous work by fluid-dynamicists who have done similar analysis in the quasi-geostrophic limit, as it allows us to consider large-amplitude topography (including islands), the topographic response of Rossby AND Gravity waves, and also permits analysis over all regions of Earth's oceans, rather than just the mid-latitudes.

Winter 2019

• 20 Feb 2019 (Cruciform Building B1.03), Alexander Doak, UCL: Free-streamline flow
  Abstract:
  A broad set of problems in fluid dynamics come under the category ‘free-boundary problems’. They are characterised as problems where one or more boundaries of the flow domain are unknown, and must be found as part of the solution. These problems require two boundary conditions on the unknown boundary, usually given by a kinematic boundary condition, and continuity of pressure (dynamic boundary condition). When considering two-dimensional potential flow, there was interest in the 18th and early 19th century (Helmholtz, Kirchoff, Zhukovsky, Hoptkinson, Love, Rayleigh… to name a few) on what is known as free-streamline problems. These are problems where gravity and all other forces are ignored, such that the constant pressure condition on a free-surface is given by |velocity|=constant.
  
  The interest stemmed partly from the fact that these fully nonlinear problems had known analytic solutions. These solutions were found using a variety of powerful theorems from complex analysis. In this talk, I will present a method assossiated to Love (1891), and later improved by Hoptkinson (1898), which, in the most generalised form, can be used to solve a two-dimensional potential flow, bounded solely by straight walls and free streamlines, even with singularities within the flow domain.

Autumn 2018

• 24 Oct 2018, Jurriaan Gillissen, Research Associate, UCL: Contact Forces and Normal Stresses in Particle Suspensions
  Abstract:
  A model is presented for particle suspension microstructure and stress, that includes hydrodynamic and contact interaction forces. The model provides an explanation for experimental observations of the first normal stress difference in shear thickening suspensions.


• 07 Nov 2018, Tom Grylls, Imperial College London: Towards an integrated large-eddy simulation model for urban pollution dispersion
  Abstract:
  Air quality is the largest single environmental health risk globally. Computational modelling presents an integral tool in understanding and facing the challenge of urban air quality as cities continue to grow and become more dense. The unsteady and turbulent nature of the urban flow field alongside the range of temporal and spatial scales associated with the urban environment result in a complex and computationally demanding modelling problem.
  
  The large-eddy simulation model, DALES, has been adapted and simulation capabilities have been developed to investigate the dominant factors determining local air quality within a city (e.g. urban morphologies, atmospheric stability, chemistry and trees) at the highest possible resolutions. Progress to date is presented including 1) a methodology to produce steady-state non-neutral planetary boundary layers, 2) a minimal tree model with which to investigate the role of vegetation (e.g. drag, evapotranspiration, deposition) within the urban canopy layer and 3) an evaluative case study over South Kensington, London against the operational street network model SIRANE.
  


• 21 Nov 2018, Sean Jamshidi, UCL: Steady states in geostrophic adjustment
  Abstract:
  One of the classical questions in ocean and atmospheric dynamics is; given an initially unbalanced pressure distribution (due to a gradient in density, height, temperature etc) how does fluid adjust to a balanced state under the influence of the Earth’s rotation? In this talk, I will present some simple models that have been used to answer this question in an ocean setting. I will outline the basic principles and philosophy that direct the approach (balance of forces, conserved quantities, reduction to an algebraic system), and then discuss steady states for a number of particular cases including: the creation of capped eddies, the structure of the thermocline in a lake, and how topography and coastal geometry can affect adjustment.


• 28 Nov 2018, Liam Escott, UCL: Particle migration induced by quadratic flow in non-Newtonian fluid
  Abstract:
  It seems self evident to say that upon placing a solid particle in a stream with parallel walls, under uni-directional flow, the particle will simply flow downstream. Its position relative to both walls will remain constant. However, all I have to do is state that the fluid is no longer water, and such assurances become dubious at best. In this talk, I shall show that a particle in Second Order fluid (non-Newtonian) does not follow the streamlines exactly, that is to say migrates in a direction other than that of the flow. In the proceedings, I will outline the tensorial method used, and display results in a particular quadratic background flow, which may be found in flows past a corner, and in plate-plate rheometers.


• 05 Dec 2018, Laura Cope, University of Cambridge: Variability of Stochastically Forced Zonal Jets
  Abstract:
  Turbulent flows on a beta-plane lead to the spontaneous formation and equilibration of persistent zonal jets. However, the equilibrated jets are not steady and the nature of the time variability in the equilibrated phase is of interest both because of its relevance to the behaviour of naturally occurring jet systems and for the insights it provides into the dynamical mechanisms operating in these systems. Variability is studied within a barotropic model, damped by linear friction, in which stochastic exogenous forcing generates a kind of turbulence that in more complicated systems would be generated by internal dynamical instabilities such as baroclinic instability. This nonlinear (NL) system is used to investigate the variability of zonal jets across a broad range of parameters. Comparisons are made with a reduced quasilinear (QL) system, where eddy-eddy interactions are neglected, permitting only nonlocal interactions between eddies and the zonal mean flow. Both systems reveal a rich variety of jet variability. In particular, the NL model is found to admit the formation of systematically migrating jets, a phenomenon that is observed to be robust in subsets of parameter space. Jets migrate north or south with equal probability, occasionally changing their direction of migration.

Spring 2018

• 2 May 2018 (room 505), Dane Grundy, University of East Anglia: The Effect of Surface Stress on Interfacial Solitary Waves
  Abstract:
  The theory of solitary waves on the surface of a fluid is well developed when a purely normal stress is applied at the surface (for example due to uniform surface tension), which leads to the KdV equation. However when a liquid is subjected to an electric field [1], or surfactant [2], is present at the surface leading to non-uniform surface tension, then a tangential stress arises at the free surface. In this case, taking the large Reynolds' number limit leads to a boundary layer at the free surface and a significant increase in the complexity of the problem. Preliminary results suggest a wide range of possible solitary wave behaviour and I will focus on solutions possible when electric fields and surfactants are present. In addition, comparisons will be drawn with other approaches with numerical methods using boundary integral methods.
  
  [1] Hammerton, P. W., Existence of solitary travelling waves in interfacial electrohydrodynamics. Wave Motion, 50 (4). pp. 676-686 (2013).
  [2] P. W. Hammerton and A. P. Bassom, The effect of surface stress condition on interfacial solitary wave propagation, Q. J. Mech. Appl.Math. 66, 395-416 (2013).


• 16 May 2018 (room 500), Sean Jamshidi, UCL: Dispersive and diffusive shock regularisation
  Abstract:
  A common problem in inviscid fluid mechanics is: how does one select the correct solution when multiple possibilities exist after the flow has developed a shock or discontinuity? One way to do this is to ‘regularise’ the problem by adding a small higher order term (viscosity or dispersion) and then take the limit of the (now unique) solution as the small parameter tends to zero. Another method is known as Lax admissibility. In many cases these two methods produce the same result, but in those cases where they do not the regularisation approach can lead to new types of solution and different physical interpretations. The study of these ‘non-convex’ problems is an active and modern area of research, and has been used in all areas of applied mathematics including, fairly recently, shallow-water theory.
  
  In this talk I will explore how this method is applied to Burger’s equation, a simple nonlinear conservation law that has been used to model one-dimensional gas flow. The talk will be based on two papers:
  Travelling-wave solutions to the modified KdV-Burgers equation (Jacobs et al, 1995)
  Dispersive and diffusive-dispersive shock waves for nonconvex conservation laws (El et al, 2015)



• 23 May 2018 (room 500), Alexander Doak, UCL: Internal waves: a heuristic overview of the dead water effect, the mill-pond effect, and other phenomena
  Abstract:
  The existence of surface water waves is of no surprise to anyone. We see them when paddling in the sea or staring contemplatively into a shaken cup of coffee. The existence of internal waves, that is waves propagating inside stratified fluids, is less obvious. From the Rayleigh-Taylor and Kelvin-Helmholtz instability, to a look at internal solitary waves, I will cover a variety of interesting phenomena occurring in stratified flows, focusing mainly on multi-layer problems (as opposed to continuously stratified fluids).



• 07 June 2018 (Gordon Square (16-18), Room B09), Chi Zhang, Department of Aeronautics, Imperial College London: Quasi-steady quasi-homogeneous description of near-wall turbulence
  Abstract:
  A formal definition to the two hypotheses of the quasi-steady and quasi-homogeneous (QSQH) theory is proposed. This theory is supposed to explain the phenomenon of the large-scale structures influencing the small-scale strucures in the near-wall part of the turbulent boundary layer. A multi-objective optimisation was performed to find the optimal cut-off parameters for a new large-scale filter. The new filter was proved to obtain much clearer large-scale structures than the filter suggested by the previous studies. Within the quasi-steady quasi-homogeneous theory expansions for various quantities were found up to the second order of magnitude with respect to the amplitude of the large-scale fluctuations. Including the nonlinear effects improved the agreement with numerical data. Two extrapolation methods based on QSQH theory were developed, with the advantages and disadvantages of them being discussed. The accuracy of the predictions based on the QSQH theory was observed improving when the Reynolds number increases. The results of the present work demonstrated the relevance of the quasi-steady quasy-homogeneous theory for near-wall turbulent flows.

Winter 2018

• 24 Jan 2018, Edward Goldsmith, UCL: Buoyancy Driven Turbulent Plumes
  Abstract:
  Buoyant plumes occur on many different spatial and temporal scales, and in a number of industrial and natural settings. In the natural environment, they play a huge role in meteorology, oceanography, and volcanology, whilst industrial examples include ventilation, chimney plumes, and waste-water disposal. In this talk, we will discuss the history of turbulent plume theory, beginning with the steady plume model of Morton, Taylor and Turner (1956), and how their results have been used in more recent unsteady models. As we move towards the unsteady case, we find that many of the models break down under certain circumstances. It has recently been demonstrated that this arises from an ill-posed or spatially unstable system of governing equations, and therefore we will finish by discussing some of the more recent work into how we can rectify these issues.


• 31 Jan 2018, Alexander Chamolly, University of Cambridge: Stochastic dynamics of active microswimmers
  Abstract:
  The design and behaviour of artificial microswimmers has garnered significant research interest in recent years. Potential applications such as controlled drug-delivery make them a very promising tool once perfected. However, the vast majority of current designs suffers from a common problem: the swimmers remain in the fluid indefinitely, posing risks of clogging and damage. Inspired by recent design proposals, we investigate the dynamics of a degradable microswimmer. In this seminar, we will give an introduction to the stochastic calculus necessary for the description of a colloid with slowly varying diffusivity, and use this to present and compare three models of the decay of a swimmer, taking into account the nature of the chemical reaction at its surface. From this we can draw valuable conclusions about the practicability of a wide range of materials for the design of such a colloid.


• 07 Feb 2018, Jurriaan Gillissen, Research Associate, UCL: Sphere Suspension Rheology
  Abstract:
  We conduct numerical simulations, to gain insight into the stress in a suspension of spheres, which interact via short-ranged lubrication forces. From the simulations, we extract the so-called fabric tensor A, which describes the orientations of the interacting particle pairs. It is observed, that, for moderate volume fractions (c < 30%), A is isotropic, while, for c > 30%, A aligns with the compressive part of the macroscopic rate of strain tensor. Based on these observations, a dynamical model is proposed for the suspension stress. The model is used to compute the velocity and density profiles in laminar channel flow, which are compared to experimental data from the literature.


• 14 Feb 2018, Natasha Senior, University of East Anglia: Eddy-mean-flow interactions in geostrophic turbulence
  Abstract:
  Owing to its similarity with the 2D vorticity equation, the quasi-geostrophic equation permits the existence of a dual cascade in energy and enstrophy. This is accompanied by the formation of large scale structures as a consequence of energy being transported to successively larger scales through interactions between eddies of similar wavenumber. Spontaneously forming zonal jets in the oceans could be explained as intermediate or final states in the evolution of an inverse energy cascade. Jets have been known to be supported by a negative viscosity, where the divergence of the Reynolds' stress pattern reveals eddies tilting in a manner to flux momentum into the mean flow. What is unclear is how these eddy-mean-flow interactions fit in to the picture of geostrophic turbulence. Using a channel model forced at intermediate scales and dissipated through Rayleigh friction, the relationship between eddy-tilts and the formation of turbulence zonal jets is investigated.


• 21 Feb 2018, Hugo Castillo Sanchez, UCL: A brief introduction to Transport Phenomena
  Abstract:
  Conservation laws are fundamental to our understanding of the physical world. Each conservation law states that the total value of the quantity governed by that law, (e.g., energy) remains unchanged during physical processes. Although momentum, energy and mass transfer were developed independently as branches of classical physics long ago, Byron Bird (1960) unified their study through the publication of the textbook called Transport Phenomena, which gives an integrated view of the transport of three physical quantities (momentum, energy and mass). During the first part of the talk, I will mention some historical aspects about transport phenomena and explain basic concepts of the laws of conservation, but most importantly, I will focus on showing the mathematical similarity among the three transport processes (diffusion and governing equations, dimensionless numbers, etc). The other half of the talk will be full of industrial applications related to fluids.

Autumn 2017

• 01 Nov 2017, Marton Mester, UCL: Frontogenesis
  Abstract:
  The occurrence of fronts is important both in the oceans and in the atmosphere. By using a relatively simple model, the talk will focus on the derivation of the omega-equation and the role of the Q-vector and the frontogenesis function. At the end of the talk, the theory will be applied to one or two weather maps.


• 08 Nov 2017, Hugo Castillo Sanchez, UCL: The rheology of worm-like micellar solutions
  Abstract:
  In this talk, I will make a brief introduction to a non-Newtonian model, which is called the BMP model, that is used to describe complex fluids that present structural memory and flow-induced changes in their internal structure such as polymer-like micellar solutions and liquid crystal polymers. The simple shear-flow of worm-like micellar solutions is studied. Chaos and bifurcation theory, chemical kinetics, thermodynamic and mechanical potentials are going to be useful to me to explain and analyze non-linear phenomena which were observed on this research, such as multiple-steady states, phase-coexistence, phase-transition, critical point and shear-banding


• 15 Nov 2017, Sean Jamshidi, UCL: The problem with moving contact lines
  Abstract:
  Many problems of interest, particularly in industry, involve both fluids and solids. The behaviour at the fluid-solid interface is simple to discuss for a steady problem, and results in an equation that allows for the shape of the meniscus to be determined. However, there are additional complications for the time-dependent problem, and the question of how to approach a ‘moving contact line’ (where the fluid advances or recedes across a solid surface) is still unsolved. In this talk I will discuss why the problem is so difficult, and examine a few methods that have been tried and in what situations they are appropriate.


• 22 Nov 2017, Liam Escott, UCL: Deriving the Oldroyd-B model
  Abstract:
  The problem of how best to mathematically describe a non-Newtonian fluid has been at the forefront of research for at least a good half century now, mainly for their unique and interesting properties. There have been many attempts over the years to properly model these fluids, the Oldroyd-B model being one of the most noted, earliest and widest accepted. In this talk I will outline the methodology and inspiration behind the model, including the use of a microscopic dumbbell approach and more.


• 06 Dec 2017, Eleanor Doman, UCL: Deriving Darcy's Law from Microstructure
  Abstract:
  Darcy's Law describes the flow through porous media and is of interest to a large range of different industries. From modelling the ground water flow or examining the extraction of tertiary oil reserves to predicting the drug penetration into a tumor, these situations may be reduced to the question how fluid travels through a porous medium. Darcy's Law, although originally obtained from experimental data, has since been derived from the Navier-Stokes equations. We will consider one such derivation from Whitaker (1986) where averaging over multiple scales is used to derive Darcy's Law from Stokes flow for two phases in 3D.


• 13 Dec 2017, Alexander Doak, UCL: Basics of gravity-capillary waves
  Abstract:
  This talk will be something of an introduction to the classical problem of gravity-capillary waves. Beginning with the Stoke’s expansion for gravity waves, we will move onto discussions of capillary waves (Crapper’s exact solution), higher-mode resonance in the gravity-capillary problem, and if time allows, a discussion of internal waves. This will act as a nice springboard for future talks, where numerical attempts at computing these solutions in fully non-linear regimes will be considered, as well as a possibility to discuss a variety of destabilizing (and stabilizing) mechanisms present due to effects of both gravity and surface tension.