Ever seen a squid fly? Not many have, but the behavior may be more common than you think. Thanks to a set of photos from an amateur photographer, scientists have managed to estimate the velocity and acceleration of squid as they propel themselves out of the water by squirting a jet behind them. Researchers found that their speeds in air are roughly five times that in water, thanks to decreased drag. Previously it was thought that the flying behavior might be linked to escaping predators, but some now suggest that it enables migration over long distances by saving energy.
Category: Research
The Gobbling Drop
A little polymer goes a long way when it comes to changing a fluid’s behavior. Normally, a falling jet of fluid will develop waviness and be driven by surface tension and the Plateau-Rayleigh instability to break up into a stream of droplets. We see this at our water faucets all the time. But when traces of a polymer are dissolved in water, the behavior is much different. The viscoelasticity of the polymer chains creates a force that opposes the thinning effects caused by surface tension. So, instead of thinning to the point of breaking into droplets, a drop is able to climb back up the jet until it reaches a critical mass where it reverses direction, accelerates downward due to gravity and eventually breaks off the jet. Then the whole process begins again with a new terminal drop. (Video credit: C. Clasen et al)
Making Mixed Emulsions
Ever tried to mix oil and vinegar? Anyone who has ever dealt with salad dressings knows the difficulty of evenly distributing immiscible fluids; the key is to shake them and create an emulsion, where droplets of one fluid are distributed throughout another. In this video, researchers create a double emulsion–oil in water in oil–without touching the two fluids. First they suspend a drop of water on a wire and then coat it with oil. Below, they place a bath of silicone oil, which they vibrate. When the oil-coated droplet falls onto the bath, it bounces on the surface rather than coalescing because a thin layer of air–constantly refreshed due to the vibration of the surface–separates the droplet from the bath. When the amplitude of the vibration is large enough, the oil coating penetrates the water during the bounce, leaving behind a tiny droplet and creating the emulsion. (Video credit: D. Terwange et al; Research paper)
Water Drops on Sand
This high-speed video captures the impact of liquid droplets onto a granular surface. While there is some similarity to liquid-solid and liquid-liquid impacts, the permeability of the granular surface helps to “freeze” the splash rather quickly. Energy is dissipated in the initial impact, causing a splash of grains. Then the surface tension, viscosity and inertia of the droplet compete in causing the deformations seen in the video. The deformation appears strongly dependent on the kinetic energy with which the droplet hits the surface (i.e. proportional to the height from which it is dropped). (Video credit: G. Delan et al)
Smoke Flow Viz
Smoke visualization, illuminated by a laser sheet, shows a 2D slice from an axisymmetric jet as it breaks down to turbulence. The flow is laminar upon exiting the nozzle, but the high velocity at the edge of the jet and low velocity of the surrounding air causes shear that leads to the Kelvin-Helmholtz instability. This instability leads to the formation of small vortices that grow as they are advected downstream until they are large enough to interrupt the jet and it breaks down into fully turbulent flow. (Video credit: B. O. Anderson and J. H. Jensen)
Supersonic Flow Around a Cylinder
This numerical simulation shows unsteady supersonic flow (Mach 2) around a circular cylinder. On the right are contours of density, and on the left is entropy viscosity, used for stability in the computations. After the flow starts, the bow shock in front of the cylinder and its reflections off the walls and the shock waves in the cylinder’s wake relax into a steady-state condition. About halfway through the video, you will notice the von Karman vortex street of alternating vortices shed from the cylinder, much like one sees at low speeds. The simulation is inviscid to simplify the equations, which are solved using tools from the FEniCS project. (Video credit: M. Nazarov)
Hawk Moth Hovering
The hawk moth (Manduca sexta) flies quite similarly to a hummingbird, able to hover over the flowers from which it feeds by rotating its wings as it flaps. This constant change in angle of attack allows it to maintain lift while remaining stationary in space. Researchers study the stability of such miniature hovering flight by destabilizing the moths and studying how they react to disturbances like being struck with a miniature clay cannonball. By testing how the moths recover from disturbances, we can learn how to build better robots and micro air vehicles (MAVs). (via supercuddlypuppies)
Supersonic Stellar Jets
Astronomers studying stellar jets–massive outflows of gases and particles pouring from the poles of newborn stars–are finding reasons to turn to fluid dynamicists to understand the timelapse videos they’ve stitched together from multiple exposures from the Hubble telescope. Usually astronomical events unfold on such a slow timescale that our only view of them is as a snapshot frozen in time. Stellar jets can move relatively quickly, though, with portions of the jet flowing at supersonic speeds. Over the course of Hubble’s lifetime, these jets have been imaged multiple times, allowing astronomers to create movies that reveal swirling eddies and shock wave motion previously unseen. (submitted by sakalgirl)
Vortex Ring Collision
Two vortex rings collide head-on in this video. If their vorticities and velocities are matched in magnitude and opposite in direction, their collision results in a stagnation plane–essentially a wall across which the fluid does not pass. In reality, there are slight variations that result in non-zero velocities where the vortices meet, so some mixing occurs, but the overall symmetry remains striking. The collision breaks up the vortex ring into filaments, some of which cross-link with the other vortex’s filaments, resulting in the little halo-like eddies around the perimeter. Videos of the same experiment at different Reynolds numbers can be found here. (Submitted by Charlie H; Video credit: T. Lim and T. Nickels)
Freezing in a Microchannel
Fluid mechanics at the microscale can behave quite differently than in our everyday experience. Microfluidic devices–sometimes known as labs on a chip–are becoming increasingly important in research and daily life. For example, the test strips used by diabetics to check their blood sugar levels are microfluidic devices. In this video, researchers use a microfluidic channel to observe the freezing of supercooled water droplets. As the droplet first passes into the cold zone of the channel, it flash freezes, filling from the inside out with ice crystals. As it continues through the cold zone, the drop freezes fully, beginning at the outside surface and working inward. As it does so, the ice droplet fractures due to stresses. (Video credit: Stan et al)
