Conferences, conferences & more conferences!

Well, it's that time of year again, no not Christmas, conference time!

Recently members from the MNF group have been at a number of large conferences, with Prof. David Emerson attending both SuperComputing 2017 and then, with other members of the group, the APS conference in Denver in America. 

I recently found myself at the UK's version of SuperComputing, the STFC run Computing Insight UK, although a smaller event than SuperComputing, this year still saw around 400 people come together in Manchester in the UK to see the latest computing technologies, discuss how to join up the UK's e-Infrastructure (i.e. how can we all get better access to the nations HPC resources) and, the reasons I was there, a day long session on emerging computing technology, which I ran! This was an exciting event for me as we didn't just have speakers, instead we also ran a 3 hour practical work-shop on hands-on Quantum Computing in collaboration with IBM Research. This went down fantastically and we hope to run something similar in the future.

The next exciting event is the annual MNF Christmas conference and workshop on the 18th and 19th of December! This is behing held over 2 days in Cheshire, with the first day being devoted to engaging with our industrial partners in a steering and impact committee day and the second for the group to come together and update each other one what we have all been doing! Events like this are essential with research groups as large as this one, we are spread over a number of institutions and not all working together so this event is a really great opportunity.

In the meantime, here are a few photos from the EMiT@CIUK 2017 workshop showing me looking awkward in front of a camera (watch the STFC media feeds for the full interview if you want something to laugh at) and Dr Stefan Filipp from IBM Research Zurich teaching us all about the state of quantum computing, how we can learn it now and what it can be applied to in the future. Fascinating stuff, especially for the future of molecular modelling!








Finally, if you want to have a play with quantum computing yourself, I enourage you to go to the IBM Quantum Experience website, where you can run on an actual quantum machine hosted in the IBM York Town research facility. More importantly though it offers a great set of tutorials to help you find out the important basics such as "what is a qubit?", "how do i teleport data between them", "who or what is a Hadamard gate?" and many others! Have a look here:



My experience at the 2017 Division of Fluid Dynamics APS meeting

Recently, from the 19th to the 21st November of 2017, I was part of a group of researchers from the University of Warwick that attended the 70th annual meeting of the Division of Fluid Dynamics (DFD) of the American Physical Society (APS) in Denver, Colorado, USA.  This is a well-established international meeting with a large turnout.  Besides having invited talks, posters, and the well-regarded Gallery of Fluid Motion with videos, the main body of the conference consists of usually more than thirty-five parallel sessions in all topics related to fluid mechanics, from theoretical aspects and mathematical foundations to experimental techniques, passing through high-performance computational fluid dynamics, and from nano- and micro-fluidics to flow in the Earth’s atmosphere, oceans, and even in the outer space.

Mykyta Chubynsky giving his talk at the 2017 APS DFD.

Mykyta Chubynsky giving his talk at the 2017 APS DFD.‚Äč

Because I attended many talks in those three days and it would be impractical to refer to each of them, I will briefly summarize here just a few of them that I consider as highlights.  They are all related with phenomena typical of the flow at the nano- and micro-meter scales.

A rotating impeller in an open container:  Highly resolved numerical simulations with code BLUE by Kahouadji, Chergui, Juric, Shin, Craster, and Matar @MatarLab.

First, in the third session on evaporation and heat transfer on drops, Takeru Yano discussed the solution of the Boltzmann equation in the half-space for the non-equilibrium gas flow having a planar interface undergoing evaporation or condensation.  He studied the flow on the basis of the numerical solution of the Boltzmann–Krook–Welander (BKW) equation.  In the journal “Fluid Dynamics Research”, this author presented an article that explains in detail part of his talk at the 2017 APS DFD meeting.

Secondly, within the focus session on “Modeling, Computations and Applications of Wetting/Dewetting Problems III”, Vladimir Ajaev talked about modeling the motion of contact lines over substrates with spatially-periodic non-uniform patterns.  The mathematical analysis was carried out using the lubrication approximation, and including evaporation, capillary effects due to interfacial-tension variation with temperature, and disjoining pressure (DP).  It is worth noting that the last factor was modeled as the superposition of the van der Waals DP and the electrostatic DP.  These two-component DP model is recommended when dealing with fluids, such as aqueous solutions, where electrical charges, in the form of ions, are present in the body of the liquid. In the first part of this focus session, a member of our Micro- and Nano-Flows Group at Warwick, James Sprittles, presented his talk on kinetic effects in dynamic wetting.

In the session on instability, break-up and splashing of drops, a presentation that caught my attention was that co-authored by Riboux and Gordillo.  In their talk, they extended previous work on modeling the fluid dynamics of the thin liquid sheet ejected after drop contact with a solid surface in the splashing process to include the effects of the boundary layer.  They showed good agreement between theory and experiments.  More details of their work can be found in this article.

Dave Emerson, from STFC Daresbury Laboratory, during his presentation at the 2017 APS DFD.

In the session on Modeling of Microscale Flows, Dave Emerson, another member of our group affiliated with STFC Daresbury Laboratory also in the UK, presented a talk discussing the challenges with modeling thermal flows in the slip-flow regime.  He commented on recent results that suggest that using the Navier-Stokes-Fourier equations with modified boundary conditions to account for rarefaction effects for thermal problems in the slip flow regime can produce erroneous answers.  Emerson presented alternative modeling approaches based on macroscale field variables capable of overcoming these limitations.

There were also several invited talks. For instance, Jen Eggers talked about the role of singularities in hydrodynamics and Detlef Lohse discussed the characteristics of turbulent flow arising from Rayleigh-Benard or Taylor-Couette instabilities.  In particular, I would like to refer to the invited talk given by Sungyon Lee on two-phase flow in a Hele-Shaw cell.  Flow in a Hele-Shaw refers to fluid motion in the narrow space between two wide parallel plates, with a gap in the order of a few millimeters or smaller.  It is a useful and practical set-up that serves as a surrogate to study the inaccessible flow of one or more phases in porous media.  In her talk, by means of videos of experiments conducted by her research team, she showed that a more viscous fluid displacing a less viscous one in a Hele-Shaw cell --- a stable configuration in the sense that an initially smooth interface remains smooth throughout its motion --- can lead to an unstable moving front when solid particles are injected and transported near the interface.  More details can be found here.

Sungyon Lee talking about particle-induced instability when a more viscous fluid displaces a less viscous one at the 2017 APS DFD.

The participation of J.C. Padrino in the APS-DFD annual meeting of 2017 in Denver was possible thanks to a research fellowship funded in the United Kingdom by the Engineering and Physical Sciences Research Council (EP/N016602/1).

How small insects fly?

It is widely acknowledged that the flight principle of biological flapping wings (birds, bats and insects) is different from the one of the fixed wings (flight vehicles). For a cruising aeroplane, gases around the wings can be simplified to the steady flow. Lifts come from pressure differences caused by airfoil shapes. However, unsteady characteristics can never be neglected when studying the flapping wings. The flapping motions usually generate vortexes over the upper surfaces of wings. Moreover, the flexibility of biological material makes the problem more difficult because fluid-structure interaction also has to be taken into consideration.


The video above shows that honey bees flap their wings up to 250 times a second. It is quite amazing for their small short wings to get the 'fat' body off the ground.


An interesting idea is that such low Reynolds number flow may exhibit rarefied phenomena at very small scales. Some researchers found that low Reynolds number flows are viscous and compressible, and rarefied effects increase when the Reynolds number decreases (Sun Q. and Boyd I.D., 2004). They also concluded that a flat plate having a thickness ratio of 5% has better aerodynamic performance than conventional streamlined airfoils in rarefied gases. However, it seems that few studies of the topic were carried out after the paper. In my opinion, it is interesting and important to study the rarefied gas effects around the flapping wings to get a deeper understanding of the flight principles of small insects.

Beautiful fluids

This is the time of year when I’m reminded of the beauty of fluids. That’s because we’re coming up to the fluid dynamics meeting of the American Physical Society (APS), which organises a headline grabbing “Gallery of Fluid Motion” photo and video competition. If you need convincing of the beauty of fluid dynamics, take a few moments to browse through past winners and entrants, here

An image of some mini vortex rings taken from S. Morris & C.H.K Williamson’s winning poster entry (Cornell)

But why is this competition so popular? Why are fluid flows so easy on the eye? Personally, I think it’s because there is a balance between order and surprise that we see in flow patterns. They are at once intuitive and unpredictable, familiar and bizarre. Like a good film or piece of music – it satisfies us by playing by the rules that we understand, only to surprise us when it breaks them.  Ben Collyer (a past PhD student of the group) would probably say I was complicating things. I recall him saying that flows are pretty, “simply because the fields are two-times continuously differentiable”. He had an ironic sense of humour.

Anyway, with all this in mind, I decided to see if I could make my own ‘fluids art’. I thought it would be fun to be able to create viable fluid fields from photographs (of anything!). The basic technical idea is to convert the intensity of an image (how light or dark it is) into a stream function. A stream function is a simple way to define a 2D velocity field that satisfies the (incompressible) continuity equation (which most fluid flows must). So, this ensures that the flows I generate from an image look physically feasible. I then use a mathematical programming language (Matlab) to create streamlines in the velocity field.

So, like many great artists (!), I decided first to embark on a self portrait:

Hmmm... Well, undeterred by the results, I wanted to see if I could simulate the dissipation of these flow patterns in time (as you would see in the bath, say). I did this be using the velocity field as an initial condition to a numerical solution to the basic fluid equations of motion (here, the 2D unsteady incompressible laminar Navier-Stokes equations). This is what you get (note, the video below is played in reverse, then forwards, then looped).

It was stupid of me to expect that reversing the time would give the impression that my face emerges from the flow dynamics…but it’s sort of interesting to watch. You just can’t beat the real thing, though (watch from 1:30):

So, the conclusion? I’m not going to submit anything to this year’s Gallery of Fluid Motion. But you never know what might be produced in the Micro Nano Flows group for next year…



Droplets and optics

I thought I will mention something about perhaps, the less talked about aspect of droplets, which is its optical properties. Ever since the modern understanding of light and a sound explanation of  the rainbow from an optics point of view (both credited to Sir Isaac Newton in the sevententh century), droplet optics has been studied widely and is today exploited in modern applications. 

An interesting application of this is in lenses and microscopes. In what is called a droplet lens, the natural shape of the droplets is tuned to create a microscope lens. In one of the ways, apparently... the droplet is placed on a glass plate covered with Teflon, to keep the drop round and prevent spreading. An electric current is then applied to the plate to change the droplet shape and by changing the amount of current, one can focus an image through the drop. More recently, researchers at MIT have devised tiny micro-lenses from 'beads of oil mixed in water', which is comparable in size to the width of a human hair. They reconfigure the properties of each droplet to adjust the way they filter and scatter light, similar to adjusting the focus on a microscope. A combination of chemistry and light is used to precisely shape the curvature of the interface between the internal bead and the surrounding droplet. 

More info on this here ......

A very busy week in China

As I write this blog I'm sitting in my hotel room on the 22nd floor having just had lunch Chengdu style, which is a hotpot where you have your personal pot boiling away. The base stock for me was mushroom and you can then add in various ingredients from meat, fish or vegetable. There are also side ingredients and I went for a healthy portion of garlic, chopped spring onions, and some chilli as we are in one of the spicey areas of China.

Before arriving in Chengdu it was Beijing where I gave a talk in Beihang University. Having arrived just four hours earlier it seemed to go ok! The talk was followed by a Beijing-style hotpot dinner, which is a shared hotpot so quite different from the Chengdu meal. The next day was a morning visit to the Chinese Academy of Aerospace Aerodynamics (CAAA) to discuss work on rarefied flows, code coupling, HPC and CFD. In a talk by the CAAA they explained their code had a variety of slip models which included not only the standard Maxwell model but an implementation of the Lockerby model. The person who had implemented this model was not available to explain which particular model it was referring to but still nice to know our work is being used. Naturally the talks were followed by lunch and one of the CAAA's skills is brewing beer and we "had" to sample a very nice home-brewed wheat beer with our meal.

The Thursday afternoon was a talk to EDF China about our work before flying to Chengdu for a meeting with the Nuclear Power Institute of China (NPIC). Saturday morning was a visit to Sichuan University where my colleagues and their students gave presentations. Sunday I will travel to Wuxi for a Monday visit to the world famous Sunway TaihuLight Supercomputer, currently the most powerful computer in the world. The plan is to install Code_Saturne and try to run a simulation on the whole machine.

Magic (?) Ball

A few nights ago Ernest (my two year old son) and I made a discovery that could have potentially revolutionary impact on the field of fluid dynamics; at bath time.  One of his balls had wandered under the running tap... and, as if by magic, got stuck there!

"Wow, that's incredible, why doesn't the jet push the ball away?", I hear you all cry.  "What black magic is at play in your bath? Where is the invisible string?"

Our initial thought was that this looks like the reverse of the well-known effect where a air jet can stably levitate a ball (e.g. a hair dryer levitating a table tennis ball or, for Ernest, the airjet in the ballpit at softplay).  This is all due to the Bernoulli effect - the suspended ball remains stable as a small deviation to one side slows the stream on that side, increases the pressure there and hence restores the ball back to the centre.

But then we found a YouTube video:

Which says it's not the same effect when a ball is levitated by a water jet.  But our ball doesn't seem to spin as violently as theirs, but rather bobs from side to side.

So now we're confused again.



Viscoelasticity in fluid droplets

Upon collision with a solid wall, a solid sphere will bounce, while a liquid drop will tend to splash. But what happens in between these two extremes? A few years ago, researchers from the National Autonomous University of Mexico investigated this 'intermediate' regime by observing the collision process of viscoelastic gelatin droplets on a glass wall using a high-speed camera. By varying the aqueous mixture of gelatin and the drop speed, a range of Weissenberg numbers were tested. The Weissenberg number (We) is a dimensionless number which compares the relative strength of the viscous and elastic forces; a low value of We represents a more solid-like collision, while a high value of We yields more liquid-like behaviour. The results show the evolution from solid-like to liquid-like collisions: most interestingly, at very high We values, the drop splashes on impact, spreads into a thin sheet, then recoils and reforms into a (highly distorted) droplet.

Nucleation: from water vapor to droplet

Homogeneous vapor-to-liquid nucleation takes place when the water molecules in the gaseous phase undergoes condensation, resulting in the formation of nuclei of water droplets in the liquid phase.

This happens spontaneously and without interactions with other materials. Homogeneous nucleation takes place at random points that are randomly distributed in the volume.

This kind of nucleation does not occur often, because almost always there are possible points where water molecules interact with another material or surface.

The video shows how homogeneous vapor-to-liquid nucleation occurs, based on molecular dynamic simulation.

2nd meeting of the "Multiscale and Non-continuum flows" - Special Interest Group (SIG)

On September 27th, the second Special Interest Group (SIG) meeting was hosted at the University of Warwick.

This series of meetings aims at bringing together theoreticians with experimental groups who share interest in the area of multiscale and non-continuum flows.
Beside the pleasure of seeing again many members of the Micro & Nano Flows group, the meeting was a good opportunity to meet other researchers across the United Kingdom.

After a welcome and 2-minute introductions by all the participants, four main lectures were given.
Dr. Kislon Voitchovsky (Durham University) discussed the liquid behaviour at nanoscale interfaces both from the numerical and, even more interestingly, experimental standpoints.
Dr. James Sprittles (University of Warwick) showed how non-equilibrium effects of the vapor flows can (unexpectedly) play a major role on the dynamics of the collisions between microdrops and  on their impact/spreading on solid surfaces.
Dr. Sergey Karabasov and Dr. Ivan Korotkin  (Queen Mary University of London) presented an interesting hybrid model which smoothly combines molecular dynamics with the Landau-Lifshitz fluctuating hydrodynamics.

I did appreciate the stimulating and constructive environment the meeting has created, as well as the main message of encouraging researchers to reflect on the impact that their studies may have beyond the academic community and to engage with industrial partners in collaborative research.

Latest News

Recent Publications

R Pillai, JD Berry, DJE Harvie, MR Davidson (2017) Electrophoretically mediated partial coalescence of a charged microdropChemical Engineering Science, 169: 273-283. (access here)

JF Xie, BY Cao (2017) Fast nanofluidics by travelling surface wavesMicrofluidics and Nanofluidics, 21: 111 (access here)

AP Gaylard, A Kabanovs, J Jilesen, K Kirwan, DA Lockerby (2017) Simulation of rear surface contamination for a simple bluff bodyJournal of Wind Engineering and Industrial Aerodynamics, 165: 13-22. (full paper here)