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Particle flow instability

Steven Wang & Ben Xu (Northumbria University)

Visualization of the trapping of finite Stokes number particles in a laminar flow. Three particles (in this case, LED capsules) 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.

Finding Nessie: the structure and origin of confined Holmboe waves

Adrien Lefauve (University of Cambridge)

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.

Interfacial waves in the two-layer rotating annulus

Paul Williams (University of Reading)

Two immiscible layers represent the stratified mid-latitude atmosphere, and internal interfacial waves develop and interact nonlinearly. A wide range of wave behaviours are observed: for example, the left-hand image shows two packets of spontaneously emitted inertia-gravity waves in the troughs of a large-scale baroclinic wave, while the right-hand image shows a rotationally modified Kelvin-Helmholtz wave generated from shear instability at sub-critical Richardson number. An optically active liquid generates different colours according to interface height.

Curvature Gradient Induced Droplet Motion

John McCarthy (University of Oxford)

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.

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Kármán vortex street in the wake of an oscillating circular cylinder in laminar flow conditions

Elvin Hii (BMT UK 2 Limited)

Flow visualization for a circular cylinder section in laminar flow conditions within a wind tunnel.

Jetting of droplet induced by ZnO-Si surface acoustic waves

Richard Fu (Northumbria University)

'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 effect of stratification on Kelvin-Helmholtz instability

Chris Howland (University of Cambridge)

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.

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Induced Faraday instability by two-frequency forcing

Naresh Sampara (University of Nottingham)

As a liquid bath is perturbed, Faraday waves appear on the surface. 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.

Two walkers’ interaction leads to orbiting

Naresh Sampara (University of Nottingham)

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.

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Frozen in Time

Andrew Edwards (Nottingham Trent University)

Capillary waves on a water film and pinch-off of a droplet from a Worthington jet, formed during the impact of a water droplet.

Elastic deformation revealed by colours

Jian Guan (University of Oxford)

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.

Controlling vortex breakdown in swirling pipe flow

David Dennis (University of Liverpool)

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.

Fabulous, flowing, and folding fountain of chocolate

Lyes Kahouadji (Imperial College London)

A three-dimensional direct numerical simulation of a heated chocolate fountain flow, including temperature and flow fields in a vertical slice. The geometrical structure is built from primitive geometrical objects using a state distance function (positive for fluid and negative for solid). Both the Navier-Stokes and thermal energy equations have been solved in the context of a front-tracking-based multiphase method and fluid structure interaction. Physical properties of melted chocolate (40°C) flowing and ambient atmosphere (20°C) have been used.

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Liquid crystal seagull

Nikita Solodkov (University of Leeds)

‘"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.’

Acoustic eruption

Elijah Nazarzadeh (University of Glasgow)

'This video shows the transfer of mechanical energy and radiative pressure from Surface Acoustic Waves (SAWs) traveling along 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.'

Spatio-temporal feathering in elastic channels

Callum Cuttle (University of Manchester)

‘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.’

‘Halo’ vortex ring lifts dandelion seeds

Cathal Cummins, Madeleine Seale, Enrico Mastropaolo, Naomi Nakayama & Ignazio Maria Viola (University of Edinburgh)

(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, showing the formation of a drag-enhancing vortex unlike any other observed before in nature, which we term a 'halo' vortex.

Fluid dynamics at home and keeping healthy during lockdown

Richard Hodgkinson (University of Sheffield)

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.

A Wave Torn Apart

Chris Howland (University of Cambridge)

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.

Iceberg in Earl Grey (hot!)

Li Shen (Imperial College London)

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.

Wake partita

Lian Gan (Durham University)

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.

Shock-waves over a flexible surface in a supersonic wind tunnel

Michela Gramola (Imperial College, London)

'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.'

Wrinkly impact

Maxime Inizan, Finn Box, Alfonso Castrejon-Pita & Dominic Vella (University of Oxford)

‘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 — all due to an interplay between the elasticity of the sheet and the inertia of the fluid.’

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The Polymer-oil emulsion cells Galaxy

Abubakar Gunta & Dominika Zabiegaj (Northumbria University)

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.

(Accidental) double-diffusive instability

Megan Davies Wykes (University of Cambridge)

‘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.’

Mixing during coalescence with pinch-off

Thomas Sykes (University of Leeds)

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.

The hydrodynamics of plesiosaurs

Luke Muscutt (University of Southampton)

‘A combination of data from simulations and experiments on tandem flapping flippers.’

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Rod climbing: The Weissenberg Effect

Henry Ng & Rob Poole (University of Liverpool)

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.

Bead formation on a viscoelastic drop’s tail

Haonan Xu (University of Liverpool)

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.

Rainbow Contrail

Ron Smith

Rainbow colours in the wake of a British Airways Boeing 777 showing details of the wake structure.

Formation of complex monodispersed droplets by 3D flow focusing

Goran Vladisavljevic (Loughborough University)

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.

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Turbulent jets

Cristian Ricardo Constante Amores (Imperial College London)

The breakup 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 structures close to the interface, visible in the figure.

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Petal-like fluid-driven fracture patterns

Niall O’Keeffe (University of Cambridge)

‘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.’

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Walker: a droplet is surfing on a vibrating liquid bath

Naresh Sampara (University of Nottingham)

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 captured optically with a Nikon camera whilst a LED light is illuminated through a diffuser (array of blue and yellow colour sheets).

Forces in a photoelastic avalanche

Amalia Thomas (University of Cambridge)

‘Photoelastic discs are rolled down a narrow, inclined chute. The forces exerted when discs interact can be inferred from the intensity of the light they transmit. This information can be used to investigate how stresses are distributed within avalanches.’

Walker: a droplet is surfing on a vibrating liquid bath

Naresh Sampara (University of Nottingham)

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.

Simulated sperm swimming in synchrony

Simon Schöller (Imperial College, London)

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.

Demonstration of Flowave

Clive Greated (University of Edinburgh)

‘Demonstration of the capabilities of Flowave. The principles of three-dimensional wave generation on which it is based were researched back in the early 80's by Bryden and Greated (J. Phys. D: Appl. Phys. 17, 1984).’

Droplet splashing on super-hydrophobic substrate (experiment or simulation?)

Kensuke Yokoi (Cardiff University)

A numerical simulation of droplet splashing on a super-hydrophobic substrate. The numerical method is based on CLSVOF, VSIAM3, CIP-CSL scheme, density-scaled balanced CSF model.

Storm in a glass

Jason Stafford (Imperial College, London)

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.

Storm in a Soap Film

Andrew Edwards (Nottingham Trent University)

Turbulent flow within the draining of a thin soap film. Colours arise from interference within the thin film indicating differences in thickness.

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Knocking over an ice giant

Jacob Kegerreis (Durham University)

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.

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Storm in a Soap Film

Andrew Edwards (Nottingham Trent University)

Turbulent flow within the draining of a thin soap film. Colours arise from interference within the thin film indicating differences in thickness.