When differing layers of fluid move past one another, friction between them causes shear. This shear quickly transforms a simple flat interface between fluid layers into a wavy unstable boundary that resembles a series of breaking ocean waves. This effect is known as the Kelvin-Helmholtz (KH) instability. In the atmosphere, this instability causes air layers with differing temperatures and moisture content to form wave-like clouds where the two layers meet. Other examples of the effect are widespread. On earth, many ocean waves are generated by wind shearing the water; elsewhere in our solar system, the cloud bands of Jupiter are lined with spinning eddies from the KH instability. (Photo credit: H. Bondo)
Category: Phenomena
Kelvin’s Thunderstorm
In this video, Derek Muller explains how an experiment known as Lord Kelvin’s thunderstorm generates electricity from falling water. The set-up relies on a positive feedback loop that creates a separation of charge between the two streams of water. Check out the video for a great demonstration and explanation. If you prefer your science with a more dystopian flavor, there’s a second version of the video made in collaboration with the Hunger Games movies. (Video credit: Veritasium; submitted by entropy-perturbation)
The Marangoni Effect
Differences in surface tension can create Marangoni flow along an interface. Imagine a shallow bowl filled with a liquid. In the middle of the fluid, every molecule is surrounded on all sides by like molecules, which push and pull it equally in all directions. But at the surface, the fluid molecules are only acted on by similar molecules in some directions. This imbalance in molecular forces is what creates surface tension. When the surface tension is constant, the fluid surface is like a taut rubber sheet. Poke a hole in that sheet, and everything pulls away from the hole. Likewise, when the surface tension varies, fluid will move from areas of low surface tension toward areas of higher surface tension. This effect is easily demonstrated at home in a setup like the animation above. Pour milk (higher fat content is better) and food coloring in a shallow container. Then lower the local surface tension using dish soap or rubbing alcohol and watch the colors run away! (Image credit: Flow Visualization at UC Boulder, source video)
Meandering Rivulet
This rivulet is the result of a horizontal liquid jet impacting a vertical pane of glass. Gravity, surface tension, adhesion, and even surface finish can affect the path the water follows. Like the meandering path of rain on a windshield, it’s hard to predict a priori where the flow will go without accounting for a myriad of seemingly inconsequential variables governing both the liquid and solid surface. (Photo credit: T. Wang)
Phytoplankton Bloom
In satellite imagery the blue and green whorls of massive phytoplankton blooms stand out against the ocean backdrop. These microscopic organisms are part of a delicate predator-prey balance and can be very sensitive to nutrient concentrations and other environmental conditions. Their individual size is negligible, but in a bloom phytoplankton are numerous enough that they act as seed particles for the flow. As a result, differing concentrations of phytoplankton reveal the swirling, turbulent mixing of ocean waters. (Image credit: NASA/USGS; via SpaceRef; submitted by jshoer)
Turbine Blade Separation
[original media no longer available]
Maintaining consistent air flow along the contours of an object is key to aerodynamic efficiency. When air flow separates or forms a recirculation zone, the drag increases and efficiency drops. On wind turbine blades, flow often separates on the root end of the blade near its attachment point. This behavior is apparent in the video above at 0:34. The tufts in the foreground on the turning blade flap and flutter with no clear pattern because the air flow has separated from the surface. In the subsequent clip, a line of vortex generators has been attached near the leading edge of the blade. These structures–also commonly seen on airplanes–trail vortices behind them, mixing the flow and generating a turbulent boundary layer which is better able to resist flow separation. The effect on the flow is clear from the tufts, most of which now point in a consistent direction with little to no fluttering, indicating that the air flow has remained attached. (Video credit: Smart Blade Gmbh/Technische Universität Berlin)
(Source: /)Bardarbunga Eruption
I thought I was done with volcanoes for this week, but DJI’s aerial footage from Iceland’s Bardarbunga eruption is too fantastic not to share. The eruption is over a month old now and more than 25,000 earthquakes have been registered in Iceland since this eruption began. The lava field covers more than 46 square kilometers, and experts remain unsure how long the eruption will continue. The lava itself is a basalt, which is lower in viscosity than more silica-rich lava. This lower viscosity means that the gases dissolved in the rising magma can escape more easily, like carbon dioxide fizzing out of a soda. If the lava’s viscosity were higher, those dissolved gases would generate a more explosive eruption as they try to escape. (Video credit: DJI; via Wired)
Undulatus Asperatus
This surrealistic timelapse doesn’t show an ocean in the sky. These are undulatus asperatus clouds rolling over Lincoln, Nebraska. Also known simply as asperatus, this cloud formation has been proposed as but not yet recognized as a distinctive cloud type. Their speed is much slower than shown in the animation, but the wave-like motion is accurate and is the source of the cloud’s name, which comes from the Latin word aspero, meaning to make rough. Though they appear stormy, asperatus clouds do not usually produce storms. They form under conditions similar to those of mammatus clouds, but wind shear at the cloud level causes the undulations to form. (Maybe some Kelvin-Helmholtz instabilities going on there?) You can check out many more images of asperatus clouds at the Cloud Appreciation Society’s gallery. (Image credit: A. Schueth, source video; submitted by leftcoastjunkies)
Pyroclastic Flow
Saturday morning Japan’s Mount Ontake erupted unexpectedly, sending a pyroclastic flow streaming down the mountain. Many, though sadly not all, of the volcano’s hikers and visitors survived the eruption. Pyroclastic flows are fast-moving turbulent and often super-heated clouds filled with ash and poisonous gases. They can reach speeds of 700 kph and temperatures of 1000 degrees C. The usual gases released in a pyroclastic flow are denser than air, causing the cloud to remain near the ground. This is problematic for those trying to escape because the poisonous gases can fill the same low-lying areas in which survivors shelter. Heavy ashfall from the flow can destroy buildings or cause mudslides, and the fine volcanic glass particles in the ash are dangerous to inhale. The sheer power and scale of these geophysical flows is stunning to behold. Those who have witnessed it firsthand and survived are incredibly fortunate. For more on the science and history of Mount Ontake, see this detailed write-up at io9. (Image credits: A. Shimbun, source video; K. Terutoshi, source video; via io9)
Saturnian Auroras
Earth is not the only planet in our solar system with auroras. As the solar wind–a stream of rarefied plasma from our sun–blows through the solar system, it interacts with the magnetic fields of other planets as well as our own. Saturn’s magnetic field second only to Jupiter’s in strength. This strong magnetosphere deflects many of the solar wind’s energetic particles, but, as on Earth, some of the particles get drawn in along Saturn’s magnetic field lines. These lines converge at the poles, where the high-energy particles interact with the gases in the upper reaches of Saturn’s atmosphere. As a result, Saturn, like Earth, has impressive and colorful light displays around its poles. (Image credit: ESA/Hubble, M. Kornmesser & L. Calçada, source video; via spaceplasma)
