Tag: fluid dynamics

The Dance of the Droplets
Milk and juice vibrating on a speaker can put on a veritable fireworks display of fluid dynamics. Vibrating a fluid can cause small standing waves, called Faraday waves, on the surface of the fluid. Add more energy and the instabilities grow nonlinearly, quickly leading to tiny ligaments and jets of liquid shooting upward. With sufficiently high energy, the jets shoot beyond the point where surface tension can hold the liquid together, resulting in a spray of droplets. (Image credit: vurt runner, source video; h/t to @jchawner)
April 24, 2015
Cloud Formation
Clouds are so ubiquitous here on Earth that it’s easy to take them for granted. But there’s remarkable complexity in the mechanics of their formation. This great video from Minute Earth steps through the processes of evaporation and condensation that drive basic cloud formation. After evaporation, buoyancy lifts warm, moist air upward. That warm air expands and cools until it reaches an altitude where water droplets can condense onto dust particles in the atmosphere. These droplets form the wispy cloud we see. Turbulence mixes these droplets and helps them collide and grow. Interestingly, although we understand the basic process of cloud formation, relatively little is understood about the details, and the subject is still very much an area of active research. (Video credit: Minute Earth; via io9)
April 22, 2015
Dead Water
Sailors have long known about the “dead water” phenomenon, which can bring ships to a near-standstill, but it was only within the last century that an explanation for the behavior was found. The underlying cause is a stratification of fluids of different densities. As seen in the video above, when a boat moves by exerting a constant force, such as with propellers, it generates an internal wave along the interface between two density layers in the water. As the wave grows in amplitude, it speeds up, chasing and eventually breaking against the boat. The energy that drives the internal wave’s growth comes from the energy the boat expends for propulsion; the larger and closer the wave gets, the slower the boat goes because its energy is sapped by the wave. In the ocean, particularly near sources of freshwater run-off, like melting glaciers, the water can be extremely stratified, with many layers of different salinity and density. The end of the video simulates this with a three-fluid demonstration in which the boat’s motion generates internal waves across multiple density interfaces. (Video credit: M. Mercier et al.)
April 20, 2015
Why Joints Pop
Joints like our knuckles are lubricated with liquid called the synovial fluid. When manipulated, these joints can pop or crack audibly. For half a century, researchers have thought the cracking sound joints under tension make was the result of bubbles in the synovial fluid collapsing. But a new cine magnetic resonance imaging (MRI) study shows that the sound is generated during bubble inception and that the cavity persists after the sound. When the bones of the joint are pulled, viscous forces resist their separation. With enough force, the joints separate suddenly, causing a pressure drop in the synovial fluid that forms a vapor-filled cavity in the joint. According to the real-time MRI observations, this is when the sound is generated. The cavity does eventually dissipate, they found, but only well after the pop. The whole joint-cracking process is consistent with the tribonucleation mechanism seen in machinery. (Image credit: G. Kawchuk et al.; GIF via skunkbear, source video)
April 17, 2015
Espresso in Space
The International Space Station resupply mission launched yesterday included a long-awaited fluid dynamics experiment that offers astronauts a taste of home: the ISSpresso espresso machine. Built by two Italian companies, the specially-designed espresso maker contains a non-convectional heating system and high-pressure piping to safely enable proper brewing using real coffee while in microgravity. The machine is also ruggedized to withstand launch forces; prototypes were even dropped in drop towers to simulate microgravity brewing conditions. The machine dispenses the brewed espresso into plastic packets, but another experiment aboard the ISS, Capillary Effects of Drinking in Microgravity, includes 3D-printed cups designed to allow orbiting astronauts to sip their beverages from open containers without spilling. They’re an improvement on a design created by astronaut Don Pettit in 2008 while in orbit. The cup’s sharp interior angle causes surface tension and capillary action to wick liquid upward to the spout. (Image credits: Lavazza; NASA/Portland State University/A. Wollman)
April 15, 2015
Newtonian and Non-Newtonian Vortices
Not all vortex rings are created equal. Despite identical generation mechanisms and Reynolds numbers, the two vortex rings shown above behave very differently. The donut-shaped one, on the top left in green and in the middle row in blue, was formed in a Newtonian fluid, where viscous stress is linearly proportional to deformation. As one would expect, the vortex travels downward and diffuses some as time passes. The mushroom-like vortex ring, on the other hand, is in a viscoelastic fluid, which reacts nonlinearly to deformation. This vortex ring first furls and expands as it travels downward, then stops, contracts, and travels backward! (Image credit: J. Albagnac et al.; via Gallery of Fluid Motion)
April 13, 2015
Growing Icicles
For those from colder climates, icicles are a familiar part of winter. They come in a multitude of shapes and sizes, many of which have been captured and cataloged in the Icicle Atlas project. The site contains images, videos, and measurements of more than 230 icicles grown in the lab over the course of four years of research into how and why icicles form the way they do. One interesting result of the work was the discovery that the ripples commonly found on icicles are directly related to impurities. Icicles grown with pure water remain smooth, and only those with ionic impurities, like salt, develop ripples. Check out more images and icicle research at the Atlas. (Video credit: A. Chen and S. Morris/The Icicle Atlas; via Discover)
April 10, 2015
Blast Waves Visualized
Typically, shock waves are invisible to the human eye. Using sensitive optical techniques like schlieren photography, researchers in a lab can visualize sharp density gradients like shock waves or even the slight density variations caused by natural convection. But it takes some special conditions to make shock waves visible to the naked eye. The blast wave of the explosion in the photo above is a great example. The leading edge of the shock wave and the heat of the explosion create a strong, sharp change in density. That density change is accompanied by a change in the air’s refractive index. As light travels from the distance toward the camera, it’s distorted–more specifically, refracted–when it travels through the blast wave and its wake. And, in this case, that visual distortion is strong enough that we can clearly see the outlines of the shock waves moving out from the explosion. The apparent horizontal line through the blast wave is probably the intersection of a weaker secondary shock wave with the initial expanding shock wave. (Image credit: Defense Research and Development Canada; via io9)
April 8, 2015
Make Your Own Dancing Droplets
As a follow-up to last week’s “dancing droplet” post, here’s a video that describes how to recreate the experiment yourself at home. The droplet motion is driven by the two-component structure of the droplets, where differing evaporation rates and surface tension values between the two fluids in the drop cause the attractions and chasing behavior you see. To demonstrate this at home, you’ll need glass, fire (for sterilization), tweezers, a pipette, water, and food coloring. Looks like a fun way to spend a weekend afternoon! (Video credit: M. Prakash et al.; via io9)
April 6, 2015
Lab-borne Tornadoes
Conventional wind tunnels are great, but some aerodynamic testing requires facilities of a different nature. The video above is from the WindEEE dome, a hexagonal chamber with sixty fans on one wall, eight directional fans on the other five walls, and six fans in the upper chamber. Each is individually computer controlled, allowing the researchers to create straight flows as well as complex vortical ones. The video shows their tornado flow, which stands 5 m tall and swirls at 30 m/s. They can also move the tornado around the chamber at 2 m/s. This capability enables a kind of scale-model analysis of tornadoes and their impact that’s not possible in most facilities. You can read more about the dome at New Scientist or the WindEEE website. (Video credit: New Scientist/WindEEE; submitted by entropy-perturbation)
April 3, 2015