UKFN Videos 30/11/18: Recent Episodes
Cambridge University
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.
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.
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.
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.
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.
A 50 mm long magnet placed at the bottom of a petri dish with 1 mm layer of ferrofluid. Following the gradient of magnetic field, the fluid forms a hump over the magnet. First, the film of flowing liquid becomes unstable forming "jets". Then, jets become unstable breaking up into droplets. Field of view approximately 70x70 mm. Camera frame rate 250 fps, playback 30 fps.
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.
Thermoacoustic (395 Hz) and lean blowoff (7.8 Hz) instabilities were observed using high-speed (4000 Hz) OH* chemiluminescence in a premixed methane-air swirl flame. The thermoacoustic instability results from a coupling of heat release fluctuations with burner resonant frequency. The lean blowoff instability is characterized by bulk extinction and reignition events.
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.
Dye visualisation of a synthetic jet in cross flow in water, 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 and vortex pinch off.
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.
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.