Symmetry breaking of sheets to vorticity filaments by a finite perturbation. Hot suspension of temperature-sensitive phosphor particles was dripped into a cuvette half-filled with cold water. The cuvette was illuminate by a UV light sheet. The particles were cooling down due to the mixing process, and the emission spectrum shift from blue towards green. This emission property sheds light on a novel technique that can implement simultaneous temperature and velocity imaging in fluids. (Original photo; no enhancement.) Heat from lamps above the experiment is absorbed by dyed, salty fluid, heating it up and making it less dense. This warm fluid rises, but the heat diffuses away, leaving dense, salty fluid in narrow vertical plumes. (Minor adjustments were made to brightness and contrast.) The oblique splash of a 3.3 mm diameter water droplet on a thin flowing liquid film. The droplet's fall height is 0.25 m from the impact surface while the film is flowing at 3 L/min, on a glass substrate inclined 15 degrees to the horizontal. Image captured 7.2 ms after impact. This image is a composite of five vorticity time series from simulations of stratified shear layers with increasing stratification. Each pixel column corresponds to a single time step from each simulation. (The image was created using a cmocean colormap, and was further edited for saturation and contrast using RawTherapee.) The image shows the flow topology for a flat plate wing rotated 180 degrees about the mid chord. The experiment was conducted in a quiescent water tank, and the shear layer feeding vortices behind each plate edge is illuminated using a milk based dye and twin laser light sheets. This is a picture from a high speed video showing jetting behaviour of a 20uL water droplet induced by ZnO surface acoustic waves (from left side). The jetting is induced by sub-nanometre amplitudes of surface acoustic waves interacting with water droplet, causing the droplet jetting along Rayleigh angle. However, due to a combination of acoustic pressure, internal flow, gravity force, surface tension and capillary, the droplet has been deformed significantly before fully jetted from substrates. (The photo is not a composite and has not been enhanced.) Jet noise is the dominant source when the aircraft takes off. It can be even higher when the engine is installed closed to the wing. The additional noise can be caused by nonlinear hydrodynamic jet-wing interactions and linear TE acoustics scatterings. Large-eddy simulation is performed to understand noise sources, and noise reduction technology is explored using serrated nozzles. A computer generated image of model sperm cells suspended in a Stokesian fluid, including the fluid velocity field. All sperm cells swim in the same plane and interact hydrodynamically, leading to visible cluster formation and locally enhanced flows. <p>Slice from a 3D two-way coupled DNS homogeneous isotropic turbulent flow containing 20 million solid particles. The background represents the Okubo-Weiss parameter, with particles avoiding red zones (vorticity overwhelms strain) and concentrating in the blue regions (the opposite). Taylor Reynolds is 35.4, Stokes 0.4 and mass loading 0.5.</p> This video shows sediment being fluidised due to basal overpressure. Water is pumped through a bed comprising two particle diameters (36 micron and 689 micron in 2:3 ratio; 20% of coarse particles dyed blue to aid visualisation). This models processes forming a sediment extrudite (e.g. sand volcanoes, blows or boils). This was the first video I took of the capabilities of Flowave taken shortly after it opened. It has generated over 900,000 hits on YouTube. The principles of 3D wave generation were researched back in the early 80's by Ian Bryden and myself [Bryden, I.G. and C.A. Greated (1984) ‘Generation of 3-dimensional random waves’ Journal of Physics D – Applied Physics 17(12), 2351-2366] A large amplitude internal solitary wave (ISW) in the laboratory. The ISW is subject to a shear force across its centre. If the shear is strong enough, the ISW breaks and Kelvin-Helmholtz billows are induced as shown. Visualisation is of neutrally buoyant (light-reflecting) tracer particles suspended in brine solution. The video shows the formation of a 'beads on the tail' of a viscoelastic droplet as it moves on an inclined superhydrophobic surface. The interaction of the surface features and the viscoelasticity of the drop results in a significant slowing of the drop – in comparison with an equivalent-viscosity Newtonian drop – and an instability on this fluid tail gives rise to the bead-like structure. Our projects are concerned with the study of turbulent buoyant flows structure and dynamics. This video aims at presenting artistically the beauty of turbulent buoyant flows evolution in time and space. We will be using visualization methods to present plumes, gravity currents and vortex rings, sometimes making them interact with each other. Through intelligent processing of flame images, the weak chemiluminescence can be visualised under direct high-speed imaging. Meanwhile, the flame infrared emission features have been captured for the first time. As a result, multiple flame light emission from visible to infrared spectrum can be visualised simultaneously in the flame ignition process. This is a high speed video showing jetting behaviour of a 20μL water droplet induced by ZnO surface acoustic waves (from left side). The jetting is induced by sub-nanometre amplitudes of surface acoustic waves interacting with water droplet, causing the droplet jetting along Rayleigh angle. However, due to a combination of acoustic pressure, internal flow, gravity force, surface tension and capillary, the droplet has been deformed significantly before fully jetted from substrates. The video is not a composite and has not been enhanced. The video is a combination of data from simulations and experiments on tandem flapping flippers. Some enhancements (adjustments of levels and brightness) have been done. Slice from a 3D two-way coupled DNS homogeneous isotropic turbulent flow containing 35 million solid particles. The background represents the Okubo-Weiss parameter, with particles avoiding red zones (vorticity overwhelms strain) and concentrating in the blue regions (the opposite). Taylor Reynolds is 35.4, Stokes 0.2, mass loading 0.5 and total time 1.1s. The flow driven by a rotating impeller inside an open fixed cylindrical cavity is studied numerically using the code BLUE, a solver for massively parallel simulations of fully three-dimensional multiphase flows. The impeller is composed of four blades at a 45 degree inclination all attached to a central hub and tube stem. In BLUE, solid forms are constructed by means of a module for the definition of immersed objects via a distance function that takes into account the object's interaction with the flow for both single and two-phase flows. The fluid interface solver is based on a parallel implementation of a hybrid Front Tracking/Level Set method designed to handle highly deforming interfaces with complex topology changes. Parallel GMRES and multigrid iterative solvers are applied to the linear systems arising from the implicit solution for the fluid velocities and pressure in the presence of strong density and viscosity discontinuities across fluid phases. The images show the evolution of the structure of a shock-wave formed on a flexible aluminium plate as the supersonic flow interacts with the changing geometry of the deforming surface. Each pop art-inspired image is a composite of eight individual frames acquired using Schlieren photography in a supersonic wind tunnel. A fluid-driven fracture propagates radially in a transparent hydrogel. The fracture surface post experiment consists of beautiful fractal patterns called step-lines. These are small changes in height along the surface of the gel, which follow a logarithmic spiral shape. (No adjustments were made to the image.) The image comprises a mixture of two liquid crystal mesogens cooling under polarizing optical microscopy. One of the mesogens forms an intertwining polymer network, which can be seen nucleating from individual sites. The solid polymer matrix propagating through the sample develops a complex interface with the fluid liquid crystal. A fluid flow drives the migration of a sand dune. Particles are entrained on the upstream slope and roll/jump to the crest where they avalanche or are ejected into the flow. The internal part of the dune behaves like a solid whereas the moving layer of particles is fluid-like. A solid sphere wrapped up in a thin polymer sheet and hanging from an air-liquid interface. The sphere had impacted a floating sheet but didn't submerge it, and now surface tension acting on the film boundary balances the negative buoyancy of the sphere, so it's 'hanging out' in the liquid. 'We should show life neither as it is, nor as it should be, but as we see it in our dreams' (The Seagull, by Anton Chekhov). Liquid crystal droplets in water, (accidentally) forming a pattern resembling a seagull's landing, viewed through a cross-polarising microscope. (Minor brightness and contrast adjustments). An experimental investigation is carried out to study particle entrainment processes (including rolling, hopping and saltation) under well controlled conditions, for a number of particle densities (rho_s>1100kg/m^3), flow regimes (Re>10^6) and bed roughness arrangements. When a solid sphere hits an elastic sheet floating on the surface of water, wrinkles form and a capillary-like ripple propagates. The ripple propagates and the wrinkles grow with time due to an interplay between the elasticity of the sheet and the inertia of the fluid. This is a video showing a liquid droplet rolling up on a flexible microfluidic device, in which droplet flows along the slope formed by simply bending-deforming a 600 micron thick aluminium (Al) plate. The droplet was remotely pushed up by the acoustic wave propagating along the Al plate surface, which is generated by a thin layer of ZnO film (with electrode onto it). An 18mm-diameter, 1mm cross-section Latex ring floats on the surface of the water in a funnel. The water is slowly removed, the surface area decreases and the ring is compressed and buckles. A solid sphere impacts a thin polymer sheet floating at an air-liquid interface. The video is slowed down 200 times. This video shows the transfer of mechanical energy and radiative pressure from Surface Acoustic Wave (SAW) travelling onto a solid surface into a droplet of water, dropped in the propagation path of SAWs, at 100k fps acquisition (slowed down 500 and 1500 times). The behaviour resembles an eruption and is used in medicine nebulisers. This is a video of a novel experiment where photoelastic discs are rolled down a narrow, inclined chute. Here the forces exerted when discs interact can be measured from the intensities of the light they transmit with the aim of investigating how stresses are distributed within avalanches. Composite binary images showing the spatio-temporal patterns formed when air displaces oil in a Hele-Shaw channel with an elastic upper boundary. By manipulating the initial level of collapse of the upper boundary, as well as the flow rate, we may observe a range of patterns, some reminiscent of feathers. A stream of ambient air flows uniformly through the bottom of a shallow granular layer. The flow oscillates and the periodic perturbation to the energy input into the granules transforms into a spatiotemporal pattern due to the energy dissipated through inelastic collisions. (Minor adjustments made to enhance brightness and contrast). This image from a 3D immersed-boundary lattice-Boltzmann simulation vividly depicts a suspension of deformable red blood cells travelling through a bifurcating blood vessel. The RBCs are immersed in a Newtonian plasma. The main branch of the blood vessel is 96-micrometer wide, and the child branches are 50-micrometer wide. This is an image captured from a high speed video showing jetting behaviour of a 5uL water droplet induced by ZnO surface acoustic waves (two waves from both sides). The vertical jetting is induced by sub-nanometre amplitudes of surface acoustic waves propagating on a ZnO coated silicon device, interacting with water droplet from both sides. Before the liquid beam breaks up, it forms a shape of crystal snake-like structure. A column is formed in a rotating glass of water as fluorescent green dye is introduced. At low Rossby number, the Coriolis force dominates, controlling the dye motion. This unfiltered UV-illuminated image shows spiral filaments, mirroring the structure of a large-scale natural storm, also governed by the same invisible force. The colour-enhanced high-speed image illustrates a high velocity water jet breaking a stream of kerosene by inducing waves, which can cause oil ligament breakup and drop detachment. The mirrored images of these generated waves can have forms, which to the pattern-seeking human eye can transform into familiar yet unexpected figures. A composite image comparing the visible and infrared, showing the aftermath of a cold droplet impacting on a hot film. Linear features due to the uneven mixing are visible in the infrared image. A fluid-driven fracture propagates radially in a transparent hydrogel. The fracture surface that is revealed post-experiment consists of beautiful fractal patterns called step-lines. These are small changes in height along the surface of the gel, which follow a logarithmic spiral shape. (Raw image) A snapshot from a three-dimensional direct numerical simulation of a falling liquid film with inlet forcing is shown. The wave structure shows a large solitary hump preceded by series of front-running capillary ripples and succeeded by a long flat tail. The film Reynolds number is 100. The simulation has been carried out with 56 cores. Density gradient magnitude reproduced by large eddy simulation of an under expanded air jet issued from a circular nozzle with an exit diameter of 12.7 mm and a pressure ratio of 5. Prandtl-Meyer expansion fans, Mach reflection, annular shear layers, propagating pressure waves, shocklets-shear interactions and turbulent mixing are visualised. Microparticles forming a network of chains at the interface of a bubble. The microstructures formed by colloids at fluid interfaces are usually due to electrostatics and capillarity. This unique pattern was obtained by deforming a bubble decorated with colloidal particles with ultrasonic waves, resulting in complex interactions between the particles. (Left) A dandelion seed in a wind tunnel, with added clouds for effect. (Right) The same dandelion with the flow around it visualised using a laser sheet (contrast enhanced), showing the formation of a drag-enhancing vortex unlike any other observed before in nature, which we term a 'halo' vortex. Vortices with a maximum Reynolds number of 5000 were randomly distributed throughout a square 2D domain and allowed to evolve numerically. Grey contours show the resulting fluid displacement and the red and blue lines show the Lagrangian Coherent Structures, as indicated by the peaks in the Hessian of the Lyapunov exponent. Composite image showing the times lapsed images of thermodynamic cycle of a trampolining elastic gel on a hot surface via release of water/gas molecules. By manipulating the gel/surface interface, we observe extreme Leidenfrost effect that provides a sheer platform for overall shape morphing of the gel. Fluid motion often happens in the blink of an eye. Without special equipment, it is hard to observe its shape. Using elastomeric mixtures that are initially liquid and cure in finite time, the periodic development of large terminal drops at the end of a thin jet of viscoelastic fluid is beautifully preserved. The video submission shows the fingering phenomenon that arises when two parallel plates are normally separated. The video also shows the experimental rig that was created to study the fingering and the effective viscosity of the fluids. The footage has not been enhanced. This raw video shows wake pattern behind an oscillating cylinder. The forcing amplitude keeps constant; the frequency linearly ramps down. It is part of a systematic study where ramping up and down at various rates. Compared to the fixed frequency cases, ramping results in wake mode transition, skipping and hysteresis. A composite video of visible and invisible Infrared showing that when a droplet at 19.7°C impacts on a hot film at 45.8°C, the droplet liquid only partially mixes. The infrared provides us with important information about the mixing processes needed to understand this phenomenon. This is a video of a novel experiment where photoelastic discs are rolled down a narrow, inclined chute. Here the forces exerted when discs interact can be measured from the intensities of the light they transmit with the aim of investigating how stresses are distributed within avalanches. Fingering patterns form when air is injected into dyed glycerol in a Hele-Shaw cell with a soft upper boundary. As a result, the soft layer deforms. The change in the colour intensity of the dyed fluid reveals both global and local deformations of the soft layer. Gentle evaporation of dichloromethane from monodispersed droplets composed of a mixture of poly(vinyl acetate) and polystyrene dissolved in dichloromethane caused thermodynamic instability and phase separation into hydrophobic polystyrene and hydrophilic poly(vinyl acetate) halves. Poly(vinyl acetate) regions are more porous due to higher instability of this polymer in dichloromethane. Visualization of the trapping of finite Stokes number particles in a laminar flow. Three particles encounter a flow instability in a chaotic flow region and eventually settle into a Kolmogorov-Arnold-Moser (KAM) tube where they move in helical orbits. Particles used in this case are few LED capsules. Wildfires associated with slash and burn agriculture on peatlands have a massive impact on climate change and regional transboundary air pollution. This August, my research group conducted the world's first field-scale peatland fire experiments. The experimental field campaign aimed to understand the behaviour of peat fires. One of the most important objectives is to understand the gas emission, which can cause deadly haze crisis. In this photo, after surface soil burning to black, there is still smouldering underground, giving haze emission continuously. This kind of emission is different to the smoke emission from flaming fires, which can go up to atmosphere. The gas emission from peat fire has low buoyancy, which makes it spread close to land and transport to cities. The state of the gas transportation in this photo shows the instability of the gas flow. As a liquid bath is perturbed, at a particular acceleration a standing wave, called Faraday waves, will appear on the whole surface of the bath. Regular square patterns are observed for single-frequency forcing and the wavelength depends on the liquid properties and exciting frequency. The regular pattern becomes more complex by adding a second frequency component into the forcing signal. A slight presence of the second frequency does not affect the instability as triggered by the first. However when both acceleration are large, the more complex quasi patterns are produced. (Liquid bath: Silicone oil 20 Cst, light with pink and white diffuse, Nikon camera with macro lens.) The sequence in false colours shows the impact of a water droplet onto a suspended stainless steel mesh. After the impact, the droplet penetrates completely under the porous mesh, generating a spray cone and breaking up in a higher number of smaller droplets. Multi-physics system consisting of a heavy fluid sinking into lighter fluid, externally controlled using electric fields. Central three images (not enhanced, left to right): vorticity field, fluid phases, underlying computational grid. Side images: dedicated watercolour paintings of the instability by illustrator and collaborator Anca Pora (https://ancapora.com/). What if a picolitre ethanol drop hits a thin water film of a few microns? High speed camera caught the moment of drop impact: water film is contracted into a drop and repelled, whereas ethanol drop spreads with the development of the fingering instability at the nearside of water droplet. Dye visualisation of the formation of a synthetic jet in water (top left to bottom right), whose non-dimensional stroke length exceeds the circulation limit of the primary vortex. Consequently, excess circulation is shed resulting in an unstable shear column, followed by the roll-up of secondary (and tertiary) vortices. Composite image; not enhanced. A glycerol-in-oil emulsion has decomposed into a single coalesced glycerol drop (right), but the incompressible particle-laden interface reacts by buckling into ripples and droplets. The image was produced via fluorescence confocal microscopy, with nanoparticles shown as green and glycerol as red; the optical transmission channel is overlaid in blue. When freezing unidirectionally at low velocity, particles pile up at the ice-water interface. After sufficient accumulation of the particles, it destabilises the interface and bulk ice differentiates into smaller ice-fingers. The image was coloured with the orientation of particles, the kaleidoscopic pattern suggests the particles forms clusters with different orientations. Flow visualization for a circular cylinder section in laminar flow conditions within a wind tunnel. Non-composite photograph, standard touch-up with (contrast, highlights, shadows etc.) This video relates our journey to look for order in the chaos of stratified turbulent flows. We tell the story of how we found three-dimensional coherent structures using cutting-edge experiments, and how we are just beginning to understand their origin using a modern stability analysis. A vortex breakdown occurs as fluid flows from a rotating pipe into a stationary pipe at a constant rate. The decaying swirl creates an adverse pressure gradient on the pipe axis, which reverses the flow and forms a recirculation "bubble". The bubble's size is controlled by the speed of rotation. As a liquid bath is perturbed, at a particular acceleration standing waves, called Faraday waves, will appear on the whole surface of the bath. As acceleration increases beyond the Faraday threshold acceleration, the amplitude of the Faraday waves becomes sufficiently large to form cylindrical structures. The amplitude of the Faraday waves grows with increasing forcing acceleration. When the amplitude becomes comparable to the Faraday wavelength, the waves spike and break up into droplets. Droplet ejection is a continuous process that occurs in waves due to the restoring forces, either surface tension effects or gravitation effects. This behaviour, similar to the Rayleigh-Taylor instability, was reported and rationalised by Goodridge et al. What if a picolitre ethanol drop hits a thin water film of a few microns? High speed camera caught the moment of drop impact: water film is contracted into a drop and repelled, whereas ethanol drop spreads with the development of the fingering instability at the nearside of water droplet. Direct numerical simulations around Dassault Aviation's V2C airfoil were carried out at a Reynolds number Re=500,000 and a Mach number M=0.7. The movie shows vorticity contours in red and blue, and strong pressure gradients in the background. A wide range of instabilities is observed. This video shows water droplet jetted by Rayleigh wave generated from a surface acoustic wave (SAW) device based on ZnO/Al bimorph structure. The droplet movement is along the Rayleigh angle. The video was taken using high speed camera with 50000 fps. The play speed was lowered to 5 fps. Video of Jet formation on an axisymmetric standing wave created in the FloWave circular wave tank at the University of Edinburgh. A jet emanates from the crest of the wave, which is produced by the collapse of the preceding trough, the jet then undergoes free fall where Plateau-Rayleigh instability is observed. The video has been slowed down from a frame rate of 125 fps to 30 fps. This video shows finite element simulations of the air-fluid interface in a lifting Hele-Shaw cell. These demonstrate the existence of several different self-similar modes of propagation for the same value of the aspect ratio, and also the disordered front propagation readily observed experimentally, caused by application of several random perturbations. Evaporation of a picolitre droplet of ethanol and water with suspended particles that track Marangoni driven flows. A radial instability in the flow leads to the appearance of a propeller before the droplet finishes drying in just over a second. +50% contrast. Every year the Gallery of Fluid Motion video contest features the newest and most beautiful research in fluid dynamics. Get an inside look at some of this year's entries in these interviews. This video is part of a collaboration between FYFD and the Journal of Fluid Mechanics featuring a series of interviews with researchers from the APS DFD 2017 conference. With cold weather and the Winter Olympics currently upon us, we bring you a special winter-themed video from JFM and FYFD. Featuring interviews with researchers from the APS DFD 2017 conference discussing their work on snow friction, granular flows and freezing bubbles. Oil jets from citrus fruits, balls that bounce on water, and self-propelled levitating plates - step inside some of the latest fluid dynamics research! This video is part of a collaboration between FYFD and the Journal of Fluid Mechanics featuring a series of interviews with researchers from the APS DFD 2017 conference. Spring has almost sprung which means we're taking a look at the fluid dynamics of bees, how to build squid-inspired robots and modelling oil plumes eight years on from the Deepwater Horizon oil spill. Featuring interviews with researchers from the APS-DFD 2017 conference discussing their work on particle-laden viscous fluids, self-propelling underwater vehicles and rotating buoyant plumes. Dandelion seeds that outperform manmade parachutes, designing the flow over forest canopies, and bee-inspired micro-robots for exploring Mars - catch up on the latest in fluid dynamics research. Listening to tornadoes to increase warning times and save lives, studying the effect of ice on the combustion of oil spills, and investigating how sea ice affects our climate - discover the latest research in Fluid Dynamics. <p>Get your summer sports kick with a peek at some of the latest research on sailing aerodynamics, cycling in groups, and getting that pesky water unstuck from your ear after swimming!</p> Tracking the invasive Rhinoceros Beetle in Hawaii, studying how stinging nettles inject their toxin, and investigating the mechanism used by jellyfish to attack their prey - step inside the latest in Fluid Mechanics research. Learn how termites are inspiring new building designs, how the star-nosed mole can sniff underwater, and what goes into making your toothpaste! A robot that swims like a dinosaur, simulations of industrial mixing, and how to hide an object by controlling its wake - take a sneak peek at the latest in fluid dynamics research. Featuring interviews with researchers from the APS-DFD 2017 conference discussing their work on bio-inspired underwater vehicles, numerical simulations of boundary flow and controlling the wake produced by a flexible tail. Learn how snakes catch prey underwater, what birds do when flying through gusts, and how sperm swim. Citrus fruits contain small pockets of liquid which burst upon contact releasing a jet of strong smelling oil into the air. The strong smell is designed to attract animals to the site to help to spread the seeds of the fruit as far as possible. Andrew Dickerson at the University of Central Florida has recorded the squirting motion using high speed cameras to try to understand the exact process of these 'micro-jets' of citrus oil. Female musicians from the northern islands of Vanuatu use the water surface as an instrument to create a variety of unique sounds - slap, plunge, plow - which they accompany with singing. Each interaction with the water surface produces a different acoustic response corresponding to the air-water-hand interaction, each of which has been studied by Randy Hurd and Tadd Truscott of Utah State University. Vortex ring collisions are incredibly beautiful and also incredibly complex. Ryan McKeown of Harvard University explains his amazing experiments visualising colliding vortex rings and their transition to turbulence. By bouncing elastic spheres across the surface of Bear Lake in Utah researchers have discovered the physics behind stone skipping. The mechanism of 'water walking' occurs when a deformed sphere rotates continuously across the surface of the water giving the appearance that the sphere is literally walking on water. Cooking oil in a frying pan can be dangerous when the 'explosive' droplets touch your skin, but new research shows that they also increase the risk of indoor air pollution if not properly ventilated. Plesiosaurs ruled the oceans during the time of the dinosaurs with specially adapted flippers that enabled them to swim faster and with greater efficiency than any other animal. Luke Muscutt studied the 4-flipper arrangement by conducting experiments at the University of Southampton to investigate exactly how it all worked... By improving our understanding of turbulent flow over canopies we can design better cities to improve air quality. This is just one of the applications of the work of Alfredo Pinelli, a professor at City University of London working on Large Eddy Simulations (LES) of turbulence. How are dandelion seeds able to travel distances of over 150km across oceans, with only small feathery bristles and the power of the wind? According to research by Cathal Cummins at the University of Edinburgh, the answer can be found in the fluid dynamics of the air flow around the plants 'micro-parachute', and in the future it could lead to improved flight for miniaturised vehicles such as drones. Air-tables create a thin film of air capable of supporting objects and causing them to levitate. By adding grooves to the table or the object, Professor John Hinch at the University of Cambridge was able to control the objects motion and describe the resultant acceleration in terms of a simple scaling relationship involving gravity and the aspect ratio. Using the surface tension of water and a hydrophobic coating on their legs, many insects are able to walk on water. The surface tension acts like an invisible blanket across the top of the water, while the hydrophobic coating on the insects legs means that they are repelled from water molecules, much like the repulsion of two magnets with the same pole. Freezing bubbles are not only beautiful, but also demonstrate incredibly complex physics. Here, Professor Jonathan Boreyko explains how bubbles freeze with examples of slow motion videos filmed in his laboratory at Virginia Tech. Measuring the forces present in an avalanche using light. Amalia Thomas from the University of Cambridge explains how to measure the forces between colliding particles in an avalanche based on their photo-elastic response and refractive index. The brazil nut effect describes the movement of large particles to the top of a container after shaking. The same effect also occurs in avalanches where large blocks of ice and rocks are seen on the surface, and in a box of cereal where the large pieces migrate to the top and the smaller dusty particles remain at the bottom. In this video, Nathalie Vriend and Jonny Tsang from the University of Cambridge explain how the granular fingering instability causes granular convection and particle segregation, with examples of experiments and numerical simulations from their research. Five-time Olympic Biathlon Skiing Champion Martin Fourcade enlisted the help of two scientists - Caroline Cohen and Christophe Clanet at Ecole Polytechnique - to help to decide the best type of wax to use on his skis in the 2018 PyeongChang Winter Olympics. Here's how they did it... Taylor-Couette flow of a shear-thinning and viscoelastic polymer solution is visualised using white light and a small quantity of mica flakes. As Reynolds number is slowly decreased, vortices spontaneously split and form new pairs. The image is formed by compiling slices of the flow acquired continuously as the cylinder decelerates. A rotating rod is partially submerged in an aqueous solution (~1wt%) of polyacrylamide acid (PAA).  With inertia present, a Newtonian fluid moves outwards toward the container walls.  Here, however, the fluid climbs the rod due to the elastic stresses generated along the streamlines by the rotation of the rod. The photo is not a composite and has not been enhanced. This image is a projection of a 3D simulation of a massive gas cloud (tens of thousands times the mass of our Sun). The brighter regions indicate more dense areas where the gas is collapsing under gravity, and the black dots are where this chaotic collapse has created a star. We present a three dimensional direct numerical simulation of a heated chocolate fountain flow where the geometrical structure is built from primitive geometrical objects using a state distance function (positive for fluid and negative for solid). We use physical properties of melted chocolate (40°C) flowing and ambient atmosphere (20°C). We solve both Navier-Stokes and the thermal energy equation in the context of a front-tracking based multiphase method and fluid structure interaction. The left-hand half is the temperature field. The right-hand half is the streamlines on a vertical slice. The parallelization of the code is based on algebraic domain decomposition, where the velocity field is solved by a parallel GMRES method for the implicit viscous terms and the pressure by a parallel multigrain method and the communication across process threads is handled by MPI message-passing procedures. A millimeter-sized oil droplet can bounce on a vertically vibrated liquid bath for an unlimited time. It may couple to the surface wave it emits, leading to horizontal self-propulsion as an object called a walker. Its motion is captured optically with a Nikon camera while an LED light is illuminated through a diffuser (array of blue and yellow colour sheets). An SPH simulation of the giant impact of a protoplanet twice the mass of Earth crashing into the young Uranus, thought to explain the planet's unique tilted spin, using an unprecedented 100,000,000 simulation particles - 1000 times more than the current standard - coloured by their thermal energy. Rainbow colours in the wake of a British Airways Boeing 777 showing details of the wake structure. Owing to the interplay between forward Stokes drift and a backward return flow, Lagrangian particles underneath surface wavepackets can follow different trajectories. The image shows records from Particle Tracking Velocimetry experiments in which particles are illuminated by a LED light box (Van den Bremer et al., 2019, J. Fluid Mech.). <p>Kelvin-Helmholtz instability of a Giesekus jet in contraflow with a (nearly) Newtonian fluid in a doubly-periodic domain. Colour indicates internal (molecular) deformation. Simulations used an in-house multiphase incompressible Smoothed Particle Hydrodynamics code, with 115200 particles. Increasing elasticity causes higher growth rate, finer filaments and increased transverse mixing.</p> Internal flow pattern during convective stage of evaporation for two binary liquid droplets, showing the effect of gravity on controlling flow direction. Composite image created from time averaged optical coherence tomography measurements over 30 seconds overlaid with the flow pattern map from PIVLab measurements. Photo taken from inside the world's largest indoor fire experiment, performed by Imperial Hazelab and collaborators in May 2019. The fire front spreads along unburned wood in the left of the photo (the leading end) and fully develops into large blaze on the right (the trailing end). The picture depicts an oil-flow visualization experiment to analyse the qualitative properties of a highly-swept, low aspect-ratio wing. A tidal bore on the River Dee with reflections of the banks and sky. Droplets of water (blue) and water-glycerol mixture (red) are propelled along the surface of a brass cone due to pressure gradients induced by the curvature gradient of the cone with larger droplets moving faster. The surface of the cone is lubricated with silicone oil allowing easy motion. This video shows formation of monodispersed multiple emulsion droplets with controllable number of inner droplets. Droplets are composed of exactly 4 water droplets enclosed within larger monomer drops. Both inner and outer droplets are highly monodispersed. Outer drops can be polymerised to preserve the structure. A millimeter-sized oil droplet can bounce on a vertically vibrated liquid bath for an unlimited time. It may couple to the surface wave it emits; leading to horizontal self-propulsion called walker. Its motion is recorded optically with a high-speed camera (Phantom MIRO110, 2000 FPS) and used a brown color backlighting. The presented video is 0.01x slower. Turbulent flow within the draining of a thin soap film. Colours arise from interference within the thin film indicating differences in thickness. The break of fluid jets is a canonical problem in interfacial singularity two-phase flows due to topology changes. The atomisation regime is difficult to characterise experimentally due to its chaotic and stochastic behaviour due to turbulence. Hence, we have performed three-dimensional Direct Numerical Simulations of turbulent liquid jets (Re ~6520) ensuring that we resolve all the turbulent scales. We observe Kelvin-Helmholtz vortices which deform to hairpin vortical structural close to the interface, visible in the figure. Droplets evolution of the PVA (polyvinyl alcohol) - Rapeseed oil (Photo 1) emulsion after 20 mins from formation and, phase 1 (Photo 2) and phase 2 (Photo 3) droplets of the PVA-Olive oil emulsion, after 80 mins from emulsion formation. Origin: Optical Microscope Nikon Eclipse Lvision, mag20x, enhanced in ImageJ software. The video shows an effervescent vitamin tablet dissolving into water whilst illuminated by a sheet of ultraviolet light from a household security marking blacklight. The natural dyes present in the tablet fluoresce as it dissolves, highlighting flow patterns and turbulent eddies. The turbulent breakdown of an internal gravity wave in a sinusoidal shear flow. The wave is refracted by the shear, leading to a turbulent transition through a combination of convective and shear instabilities. Local kinetic energy dissipation rate is visualised on a log-scale using a cmocean colour map. Turbulent flow within the draining of a thin soap film. Colours arise from interference within the thin film indicating differences in thickness. Direct numerical simulation of stratified turbulence forced by vortical modes. A periodically tiled horizontal plane snapshot of the local kinetic energy dissipation rate is visualised on a log-scale using a cmocean colourmap. The original picture (without any rendering) shows flame holes (view on top of the center-plane) in a reactively, temporally evolving mixing layer. The fuel stream (Z=1) is composed of 50% CO, 10% H2 and 40% N2 by volume, and the oxidizer stream (Z=0) is composed of 25% O2 and 75% N2. The stoichiometric mixture fraction is Zst=0.42. The reference Reynolds number is 9075. More details can be found in https://doi.org/10.1016/j.combustflame.2019.04.038. Instantaneous turbulent flow state in a representative gas-turbine port flow. Six radial port-jets issued from outer annulus impinge onto each other in a bulk cross flow. The port Reynolds number is 22,600. Key features: highly-vortical unsteady flow, horse-shoe port vortices, central-recirculation zone, fifth-order accurate LES using Nektar++. In non-Newtonian polymer solutions, the flow between two concentric cylinders with the inner rotating (Taylor-Couette flow) transitions from azimuthal Couette flow to elasto-inertial turbulence, through various intermediate unsteady flow states. The transitions are visualised using reflective mica particles and high speed imaging (figure in false colour). The original picture (without any rendering) shows scalar gradient trajectories that intersect the stoichiometric mixture fraction (Zst) iso-surface in a reactively, temporally evolving mixing layer. The fuel stream (Z=1) is composed of 50% CO, 10% H2 and 40% N2 by volume, and the oxidizer stream (Z=0) is composed of 25% O2 and 75% N2. The stoichiometric mixture fraction is Zst=0.42. The reference Reynolds number is 9075. More details can be found in https://doi.org/10.1016/j.combustflame.2019.04.038. This image is constructed from planar particle image velocimetry data obtained in a zero-mean turbulence box facility (Re_lambda = 250) and processed to a vector resolution equal to 1.5 times the Kolmogorov scale. The projected strain-rate field, calculated as the square of the symmetric part of the reduced velocity gradient tensor, reveals beautiful patterns indicative of small scale turbulence. Realisations of the streamwise turbulent fluctuations over a sparse canopy obtained from a direct numerical simulation. The thin film which forms on the surface of a cup of Earl Grey tea breaks up in a similar fashion to icebergs in the sea. The thin-film pattern formation follows a Cahn-Hilliard-like coarsening process until the breaking up process is complete. This image was taken in the middle of the coarsening process, only colour has been adjusted for creative effect to mimic an actual iceberg. Capillary waves on a water film and pinch-off of a droplet from a Worthington jet, formed during the impact of a water droplet. Anionic surfactant, Sodium Linear Alkylbenzene Sulfonate (NaLAS), forms flow-shape, pill-shape and necklace-shape at the liquid-air interface during the cooling process. <p>Droplet nuclei transmit to wider environments as turbulent flows interact with quiescent conditions. This image shows hairpin vortex structures stretch and break away intermittently at a turbulent/non-turbulent interface, sending droplet cargo further afield. This was taken using a planar jet simulation at Re=10,000, St=1, and resolved to the Taylor microscale.</p> <p>Intricate, time-periodic motion of double-droplet structure formed in a simulation of the Moffatt problem. The work is based on novel thick-flow formulations of low-order models, and their comparisons with direct numerical simulations. This is a computer rendering of this complex motion, designed to try to elucidate the sort of motion usually only decipherable via video. Further details are available at https://doi.org/10.1017/jfm.2020.421.(Simulation results are unaltered.)</p> <p>This image illustrates the first segmented micro-CT X-ray image from the dynamics of solute (KI solution) transport in a glass-bead packing filled with oil (dark grey) and water. The X-ray image has been taken in I12 beamline in Diamond Light Sources at a spatial resolution of 3.25 microns and time resolution of 6 seconds. Results of the research have been recently published in PNAS https://www.pnas.org/content/early/2020/09/03/2011716117.</p> A millimetre-sized oil droplet can bounce on a vertically vibrated liquid bath for an unlimited time. It may couple to the surface wave it emits; leading to horizontal self-propulsion called walker. The interaction of the two walkers leads to orbiting with each other. A dyed falling droplet coalescing with an undyed sessile droplet of equal fluid properties, with pinch-off preceding permanent coalescence and internal jet formation. Video shows 25ms, played 500x slower than real time. Frame is approximately 6mm wide. <p>Current underwater vehicles are rigid in structure which limits their suitability for many tasks required for ocean exploration. Francesco Giorgio-Serchi is working with a team at the University of Southampton to design new robots based on squids and octopuses that are made entirely from silicone. They are not only more mobile, but are also more reliable and more efficient.</p> <p>Cyclists can use up to 90% of their energy overcoming drag, which was the motivation behind the work of Ivaylo Nedyalkov at the University of New Hampshire, who was able to measure the force on each individual cyclist in a train formation to determine the best position to reduce your overall drag.<br />  </p> <p>Following the Deepwater Horizon oil spill in 2010, scientists at the University of Cambridge have been studying underwater plumes to try to understand how the Earth's rotation affects the spread of oil. Their experiments revealed the important role played by conservation of angular momentum after one rotation period, emphasising the importance of a rapid response to a disaster.</p> <p>The flow of air around a sail is very different to that of a wing, but both generate significant lift force. Ignazio Maria Viola at the University of Edinburgh studied sails in numerical simulations and experiments to discover the force comes from vortices that are produced at the edges of the sail. By controlling the strength and location of these vortices he hopes to be able to produce faster and more efficient sails in the future.<br />  </p> <p>Before a tornado forms the pressure drop at the centre emits a dull tone at 5-10Hz which can be detected hours before it becomes dangerous. Brian Elbing at Oklahoma State University has devised a detection system that works up to 300 miles away from the source and can predict the size and strength of the tornado before it forms, providing advanced warning for at-risk areas.</p> <p>One of the clean-up methods used following an oil spill is to burn the fuel on the surface of the ocean. This generally works well, except in polar regions where the heat from the fire rapidly accelerates the melting of ice. Hamed Farahani at Worcester Polytechnic Institute is studying this phenomenon using laboratory experiments with the goal of improving the efficiency of combustion as a control for ocean pollution.</p> <p>There is no doubt that sea ice in the polar regions is melting, but what is the exact role that this plays in the global climate system? To understand climate change we need to understand mixing in the ocean, which is exactly what Andrew Wells at the University of Oxford comes is trying to do by studying a model for sea ice growth in the Arctic.</p> <p>The Coconut Rhinoceros Beetle is an invasive species that if left alone would decimate citrus crops across California. To prevent this from happening, John Allen and his team at the University of Hawai'i have been working to hunt the insects down before they are able to reach the West Coast of the USA. By identifying the frequency of the beetles wing beat, they are able to track them down by listening out for the unique flapping sound of their wings and alert pest control to their whereabouts.</p> <p>The leaves of stinging nettles are covered in 'pipette-like' stingers which penetrate the skin on contact and deposit a small amount of poison. The 'pipette-like' design means that almost all of the poison contained in the stinger can be injected at once if sufficient force is applied to bend the stinger to an angle of 90 degrees. This is demonstrated in laboratory experiments conducted by Kaare Jensen at the Technical University of Denmark.</p> <p>Current underwater vehicles are either difficult to manoeuvre making them unsuitable for sensitive work, or are extremely expensive. Gabe Weymouth and his team at the University of Southampton are designing new underwater robots based on the Plesiosaur - the dinosaur behind the legend of the Loch Ness Monster - which are much smaller, cheaper and more energy efficient. By copying the Plesiosaur's 4-flipper configuration, thrust is increased by up to 40% and efficiency by up to 60% in the new designs. </p> <p>Research by Gabe Weymouth at the University of Southampton. </p> <p>From rotating impellers stirring air into a liquid, to a turbulent jet entering a moving flow, mixing processes are essential to almost every industrial application you can think of. Omar Matar and his research group at Imperial College London are experts in running numerical simulations to model mixing in multiphase flows. Here, we see examples of mixing in stirred tanks, mixing through droplet collisions, and mixing at interfaces.</p> <p>Research by Omar Matar at Imperial College London. </p> <p>When an object, such as an aircraft or submarine, moves through a fluid it generates a 'wake' behind it which can be used by monitoring tools to detect its size and position. Cloaking devices work by modifying this wake to hide the object from detection, or to provide false information about its size and intention. Sam Shelley, a researcher at the University of Exeter, explains how a flexible tail can be fixed to the back of an object to modify its wake for this purpose, or even to generate energy harvesting in the ocean. </p> <p>Research by Sam Shelley at the University of Exeter. </p> <p>When catching prey underwater snakes use two main techniques: the frontal strike and the lateral strike. By studying real snakes in the lab at ESPCI/MNHN, Marion Segall was able to recreate the setup using a 3D-printed snake head and laser visualisation techniques, which allowed for the forces involved in each strike to be measured. </p> <p>Research by Marion Segall at ESPCI/MNHN. <br />  </p> <p>When navigating strong crosswinds birds adjust the angle of their body into the wind, whilst keeping their head fixed on the target destination, in order to maintain forwards motion. </p> <p>Experiments conducted by Daniel Quinn at the University of Virginia use live lovebirds in a wind tunnel painted to simulate a forest environment. The lovebirds are seen to consistently angle their body in the direction of the wind, while keeping their head oriented towards the landing perch. This behaviour is very similar to that used by a pilot of a fixed-wing aircraft, but in the case of birds they are also able to control the flapping speed of their wings to assist. The hope is to be able to use this knowledge to improve the stability of drones when flying in strong winds.</p> <p>Research by Daniel Quinn at the University of Virginia. </p> <p>The tail (flagellum) of the sperm does not simply push the body, but uses a rhythmical beating with pushing and fraying over a 4-beat pattern, as demonstrated in the video.</p> <p>The new dynamical model for a swimming sperm was developed by Kenta Ishimoto at Kyoto University using high-frame-rate microscopic images to obtain detailed information of the moving tail.</p> <p>The hope is that by understanding the swimming mechanism used by sperm, we may be able to better treat infertility issues in the future.</p> <p>Research by Kenta Ishimoto at the University of Kyoto.</p> A snapshot of a high-fidelity CFD simulation of a low pressure turbine blade with a leading edge probe. The image was obtained from a DNS of the flow over a rough surface (Reτ=395) and illustrates the complexity of the mean flow within the roughness sublayer. The streamlines were seeded upstream of a characteristic feature of the surface, some of them becoming trapped downstream of that feature before being ejected. A snapshot of a three-layer, stably stratified density field undergoing mixing in a shear flow. An instability in three-layer stratified shear flow arises when interfacial gravity waves are doppler-shifted by the shear, and resonate. At this wavelength, the resulting nonlinear dynamics show regions of intense mixing alongside more quiescent dynamics. Direct numerical simulation (various angles in a composite image) of a water drop of diameter 1 mm impacting onto a deep liquid pool at a 45 degree angle of incidence with an initial velocity of 10 m/s. The liquid-liquid interfaces are coloured using the norm of the velocity field. The underlying (adaptive) volume-of-fluid discretisation is also highlighted. This movie shows the direct numerical simulation of a two-dimensional body-forced incompressible Navier-Stokes flow with periodic boundary conditions. At early times the movie shows typical 2D turbulence at Re=200. We then apply a time-delayed feedback term in the equations (Pyragas Phys. Letters A 1992). This term allows us to stabilise the T1 travelling wave reported in Chandler & Kerswell (J. Fluid Mech. 2013). This is made possible by making use of the underlying symmetry of the target solution to avoid the well-known "odd-number" limitation, and we dynamically adapt the translation (in the streamwise direction) of the feedback term to match the phase speed of the solution. We find several solutions can be stabilised in this way. Three-dimensional coherent structures identified by instantaneous iso-surface of Q-criterion using large eddy simulation approach. The unique position of a temperature probe at the leading edge has a significant impact on the flow along the blade which has a significant impact on any further measurements downstream. All thin films investigated contain water and surfactant as base ingredients, and varying amounts of glycerol and ferrofluid. The film patterns observed are greatly influenced by the composition and whether an external magnetic field is applied to the ferrofluid-based thin films. The images have been enhanced. Time sequence images during the dripping of a water droplet from a laboratory tap showing pendant droplet formation, extension, pinch-off, recoil and thread breakup. Enhanced image from a mesoscopic simulation which demonstrates emerging pattern of red blood cell distribution in realistic microvascular networks and its underlying effect on vessel remodelling during development. This is a three-dimensional Large Eddy Simulation for oil-water emulsion in a stirred vessel (turbulence), which is filled with water (lower half) and oil (upper half).  This shows the effect of impeller speed on the spatio-temporal evolution of interface shape. Both side (left) and top (right) views are presented. The intricate dynamics of crown formation and break-up following the impact of a 1.9mm ethanol droplet at 2.1 m/s onto a thin (70 micron) layer of the same fluid, imaged in unprecedented detail with a Phantom TMX 7510 at 110k fps (1280x512 resolution). High-speed video of a dripping of a water droplet from a laboratory tap showing pendant droplet formation, extension, pinch-off, recoil and thread breakup. The Italian Frecce Tricolori aviation display team emitting a sequence of equally spaced jets of smoke. As the jet flow loses energy and begins to mix with the free stream flow, the scales of turbulent mixing can be seen. Streamlines on a wind-swept surface are visualised with an oil-flow mixture comprising of titanium dioxide, paraffin, motor oil and oleic acid. The evaporation pattern in the lower-right corner of the image formed naturally one hour after the initial experiment. Turbulent flow in a river with chaotic waves. The picture was taken at low-tide when the riverbed roughness has more pronounced effects on turbulence production. An instantaneous shot is compared to a long-exposure shot, analogous to the RANS equations where the flow characteristics are averaged over time. The chaotic patterns in a soap film are an example two-dimensional turbulence. This bubble was created using water and a weak dilution of detergent and sugar. In the thin wall of the bubble, vortex stretching is restricted, enstrophy is conserved, and different inertial range scalings occur than in 3D turbulence. The video illustrates the atmospheric plume's trajectory from a smokestack in an urban boundary layer. It shows how hot moisture (combustion product) condenses and generates a huge moving plume.  The video reveals that flow velocity (moisture distribution) displays rapid fluctuations (turbulence) and vertical mixing is strong at this layer. This video shows how the canyon geometry (the aspect ratio of building height to street width) influences the turbulent airflow structure and how the turbulent airflow transports pollutants in an urban street canyon environment. Deep canyon (with higher aspect ratio) leads to poor ventilation and worst pollution. <p>This photo shows two amazing natural materials: snail mucus and silk (which is a liquid when it is formed). They share a common function: they are used as secretions to be used outside of an animal's body. The photo was taken using a black background, a silk cocoon and a silk moth. Then a living snail (which was not killed) was placed on the silk fibres to allow it to move. This photograph shows a connection between these two materials, highlighting the drop of mucus as a fluid with extraordinary properties from antibacterial to skin regeneration.</p> Generated from a video of a natural vortex taken on Wessenden Head Moor in the Peak District. The video frames were z-projected in Image-J with naturally-occurring foam acting as tracer particles, and then this image was contrasted, cropped to an ellipse, and superimposed on the first video frame. The video depicts the scales of structures and dynamics of breakup in a spray produced from an agricultural nozzle. A high speed camera connected with a microscopic lens resolves the intricate details of perforations, ligaments and droplets formation (raw images combined). The image illustrates the pattern deposited on a surface after the evaporation of a micro-droplet which contains Rhodamine particles under a microscope. It creates an illusion of a Red Giant in a telescope upon first sight. The mysterious beauty of science emerges when people realize what they saw. The photo depicts turbulent boundary layers and flow separation over the rear wing of a model open-wheel race car in the RJ Mitchell wind tunnel at the University of Southampton. The freestream velocity is 20m/s and flow-vis oil was used to visualise the flow structures. Millimetre-sized drop impacting onto a pool of the same liquid at gentle speeds, resulting in bouncing dynamics. Ice spikes formed during the freezing of a thin water film. The video presents the movement and partial coalescence of a droplet at a wavy liquid-liquid interface, captured with a high-speed camera. The coalescence is delayed as there is a relative velocity between the droplet and the interface. The video captures the beauty of dynamic balance between the droplet and the changing waves. Upon simultaneous impacts on a solid surface, the interaction of two spreading droplets generates a central uprising sheet, which grows upwardly and sidewise with a semilunar shape and eventually deposits and/or breaks into secondary droplets.  The video shows two perspectives of the sheet evolution at 300x slower than real-time. Millimetre-sized drop impacting onto a pool of the same liquid at gentle speeds, resulting in bouncing dynamics. The experimental campaign has been complemented by direct numerical simulations, with insight from both approaches encapsulated in a new and efficient reduced-order model.