FYFD

Category: Research

  • Ocean Mixing

    Ocean Mixing

    Movement in Earth’s oceans is driven by a complicated interplay of many factors like temperature, salinity, and Earth’s rotation. Above are results from a numerical simulation of the top 100 meters of ocean contained within a 1 km x 1 km box.  The colors indicate surface temperature. Two major processes create the motion we see. The first is convection, in which water at the surface releases heat to the atmosphere and cools, causing it to then sink due to its greater density. Warmer water rises to replace it. This process happens quickly and dominates the early part of the simulation where we see the puffy convection cells shown on the left animation.

    A slower process is in effect as well. Because of variations in the water temperature, the density of the fluid at a given depth is not constant. We can already see that at the water surface, where the temperature (and thus density) is varying significantly. Those variations in density at the same depth combined with gravity’s tendency to shift fluids create what is known as a baroclinic instability. Put simply, this instability will cause warmer water to slide horizontally past colder water. The result is the large, spinning eddy motion seen in the animation on the right. To see how the whole system develops, check out the full video below.  (Image/video credit: J. Callies)

  • Boiling on Mars

    Boiling on Mars

    Today’s Mars is cold and dry, with a thin and insubstantial atmosphere. One of the challenges facing planetary scientists is unraveling the processes behind the complex terrain we can observe on the surface. Without flowing water, how do we explain these features? A new experiment suggests that the answer lies in boiling.

    Surface conditions on Mars include atmospheric pressures low enough to be below the triple point of water* – the critical temperature and pressure where water vapor, liquid water, and ice can all exist simultaneously. This means that liquid water is unstable under Martian conditions; any water that seeped up to the surface would immediately begin to boil. That explosive boiling ejects sand particles, as seen in the animation above. The authors suggest that this hybrid process of wet percolation combined with vaporous ejection of sediment may better explain the Martian surface features we observe. (Image credit: M. Masse et al., source: Supplementary Movie 3; via Gizmodo; submitted by Paul vdB)

    * The evidence we’ve seen so far on Mars points to briny water flowing near the surface. Although brines have lower freezing temperatures than pure water, the authors’ argument holds for them, as well. The boiling is simply not as vigorous.

  • Emulsion Impact

    Emulsion Impact

    Emulsions – mixtures of two immiscible fluids – are quite common; the oil and vinegar combination used in many salad dressings is one. The image sequence above shows the first 800 microseconds of the impact of a similarly emulsified droplet. The outer drop, seen on the left, consists of a water/glycerin mixture, and inside the drop are 20 smaller perfluorohexane droplets. These smaller droplets are denser and tend to settle toward the bottom of the outer drop. When the compound droplet hits a solid surface, it spreads in a spectacular starburst pattern that depends on the number and location of interior droplets. You can see a similar impact in motion here. (Image credit: J. Zhang and E. Li; source: C. Josserand and S. Thoroddsen)

  • Bubbles and Films Merging

    Bubbles and Films Merging

    As we’ve seen before, a water droplet can merge gradually with a pool through a coalescence cascade. It turns out that the coalescence of a soap bubble with a soap film can follow a similar process! Initially, the bubble and film are separated by a thin layer of air. Once that air drains away and the bubble contacts the fluid, it starts to coalesce. But the bubble pinches off before its entire volume merges, leaving behind a daughter bubble with about half the radius of the previous bubble. This process repeats until the bubble is small enough that it merges completely. To see more great high-speed footage of this bubble merger, check out the full video below.  (Image/video credit: D. Harris et al.)

  • Crown Splash Sealing

    Crown Splash Sealing

    A sphere falling into water generates a spectacular crown
    splash at the surface. The object’s impact ejects a thin sheet of fluid
    that rises vertically. The air pulled down into the cavity by the
    sphere’s passage makes the air pressure inside the sheet lower than the
    ambient air pressure on the exterior of the sheet. This pressure
    difference is part of what draws the crown inward to seal the cavity. As
    the splash collapses inward and seals, the liquid sheet starts to
    buckle and wrinkle, leaving periodic stripes around the closing neck.
    This so-called buckling instability occurs when the radius of the neck
    collapses faster than the vertical speed of the splash. For more, see
    the research paper or this award-winning video. (Image credit: J. Marston et al., source)

  • Pinning a Drop

    Pinning a Drop

    The shape of a droplet sitting on a surface depends, in part, on its surface tension properties but also on the nanoscale roughness of the surface. Small variations in the height and shape of the surface will change the area a drop contacts as well as the contact angle the edge of the drop makes with the surface. If the contact line between the drop and surface stays the same as a droplet evaporates into the surrounding gas or dissolves into the surrounding liquid, then we say the drop is pinned. A pinned drop’s contact angle will decrease as the drop’s volume decreases. This strains the ability of the nanoscale roughness to keep the drop’s edge pinned. As individual points of contact fail, the drop’s edge may jump inward to a new contact point. This set of discrete jumps between pinned states is called a stick-jump or stick-slip mode. (Image credit: E. Dietrich et al., source; see also: E. Dietrich et al. 2015)

  • Plasma Flow Control

    Plasma Flow Control

    Engineers frequently face the challenge of maintaining control of air flow around an object across a wide range of conditions. After all, wind turbines and airplanes don’t always get to choose the perfect weather. To widen their operating ranges, designers can use active flow control to keep air flowing around an airfoil instead of separating and causing stall. One method of flow control uses plasma actuators on the upper surface of an airfoil. When activated, the plasma actuator ionizes air near the wing surface, producing the purplish glow seen above. That ionized air, or plasma, gets accelerated by the electric field of the device. The acceleration adds momentum to air near the wing surface, which helps it stay attached and flowing smoothly despite the unfavorable pressure conditions near the trailing edge of the wing. Compared to other methods of active flow control, plasma actuation is relatively simple to implement and so is actively being researched for applications in aviation and wind energy. (Image and research credit: I. Brownstein et al., source)

  • Bonbon Coatings

    Bonbon Coatings

    If you’ve ever bitten into a chocolate-covered bonbon, you may have noticed that the candy’s chocolate coating is remarkably uniform. Inspired by this observation, a group of engineers have investigated how viscous fluids poured over a curved surface flow and solidify; their findings were published this week.

    Rather than heated chocolate, the group used polymer-filled fluids that cure and harden over time. Interestingly, they found that the final shell is quite uniform and that its thickness does not depend on the pouring technique. Instead, they can predict the final shell thickness based on the radius of the mold and the rheological properties of the fluid–specifically its density, viscosity, and curing time. The reason for this is that the time it takes for the fluid to drain and coat the mold is much shorter than the time it takes for the polymer to cure. As a result, the amount of fluid that sticks to the mold depends on geometry and fluid properties – not how the fluid was poured.

    Amateur confectioners rejoice: pouring uniform chocolate coatings may be easier than you thought!  (Image credit: MIT News, video; research credit: A. Lee et al.)

  • Featured Video Play Icon

    Fluttering Feathers

    Birds do not always vocalize in order to make their songs. The male African broadbill, shown in the top video above, makes a very distinctive brreeeet in its flight displays, but as newly published research shows, the sound comes from its wings, not its voice. During the display, the broadbill spreads its primary feathers and sound is produced on the downstroke, when wingtip speeds reach about 16 m/s. By filming a broadbill wing with a high-speed camera in a wind tunnel at comparable air speeds, researchers could localize the sound production to the 6th and 7th primary feathers.

    In the second video above, you can see these feathers twisting and fluttering in the breeze. This is an example of aeroelastic flutter, a phenomenon in which aerodynamic and structural forces couple to induce oscillations. The same phenomenon famously caused the collapse of the Tacoma Narrows Bridge in 1940. In the birds, however, the flutter is non-destructive and the vibration produces audible sound which the other feathers modulate into the calls we hear. Broadbills aren’t the only birds to use this trick; some species of hummingbirds use flutter in their tail feathers during mating displays. (Video, image, and research credits: C. Clark et al.; additional videos here)

  • Bumblebees in Turbulence

    Bumblebees in Turbulence

    Bumblebees are small all-weather foragers, capable of flying despite tough conditions. Given the trouble that micro air vehicles have when flying in gusty winds, bumblebees can help engineers to understand how nature successfully deals with turbulence. Under smooth laminar conditions like those shown in the animation above, bumblebees stay aloft by beating their wings forward and backward in a figure-8-like motion. On both the forward downstroke and the backward upstroke, you’ll notice a blue bulge near the front of the bee’s wing. This is a leading-edge vortex, which provides much of the bee’s lift.

    Researchers were curious how adding turbulence would affect their virtual bee’s flight. The still image above shows the bee in moderate freestream turbulence (shown in cyan). Surprisingly, this outside turbulence has very little effect on the flow generated by the bee, shown in pink. In fact, the researchers found that the bees could fly through turbulence without a significant increase in power. Too much turbulence does make it hard for the bee to control its flight, though. The bee’s shape makes it prone to rolling, and the researchers estimated, based on a bee’s 20 ms reaction time, that bumblebees can probably only correct that roll and maintain controlled flight at turbulence intensities less than 63% of the mean wind speed. (Image credits: T. Engels et al., source; via Physics Focus)