FYFD

Category: Research

  • Shock Waves in the Solar Wind

    Shock Waves in the Solar Wind

    The empty space of our solar system is not truly empty, as we’ve discussed previously. For one, there’s a fast-moving flux of charged particles – the solar wind – that flows constantly from the Sun. Sometimes these solar wind particles encounter their interstellar equivalents – charged ions from outside our solar system – and exchange energy.

    One predicted mechanism for this energy swap is a solar wind shock wave, which occurs when a faster-moving clump of charged particles plows into a slower-moving one. Scientists hypothesized in the mid 1990s that far from the Sun, solar wind shock waves would lose their energy by passing it to these interstellar ions, in a process known as pickup. Data from the New Horizons spacecraft has finally provided evidence for this theory.

    In October 2015, instruments on the spacecraft recorded a shock wave when the speed of solar wind ions nearby jumped from 380 km/s to 440 km/s. Comparing the energies of solar and interstellar ions before and after the event, researchers found that interstellar pickup ions became 30% more energetic while solar ions lost 85% of their energy. It’s an important confirmation of theoretical predictions and should help us better understand high-energy particle physics at the edges of our solar system. (Image credit: NASA; research credit: E. Zirnstein et al., via J. Ouellette)

  • Heating from Cavitation

    Heating from Cavitation

    When cavitation bubbles collapse, they can produce temperatures well over 2,000 Kelvin. Since cavitation near a surface can be so destructive, researchers have long wondered whether the high temperatures inside the bubble can be transmitted to nearby surfaces. A new set of numerical simulations provides some insight into that process. The researchers found that collapsing cavitation bubbles raised nearby wall temperatures in two ways: bubbles that were further away sent shock waves that heated the material, and nearby bubbles could contact the surface itself as they collapsed.

    Heat transfer requires time, however; this is part of why quickly dunking your hand in liquid nitrogen and pulling it out likely won’t damage you. (Still, we don’t recommend it.) The cavitation bubbles could only transmit these high temperatures for less than 1 microsecond, which means that most materials won’t actually heat up to their melting temperature. The researchers did conclude, however, that softer materials exposed to frequent bubble collapses could show localized melting under the barrage. (Image credit: L. Krum; research credit: S. Beig et al.)

  • The Driver of Hydraulic Jumps

    The Driver of Hydraulic Jumps

    You’ve seen it a million times. When you turn on your kitchen faucet, the falling water forms a distinctive ring – known as a hydraulic jump – in the bottom of your sink. First described by Leonardo da Vinci, this phenomenon has been studied for centuries, and, for nearly all of that time, scientists assumed that gravity played a major role, even in kitchen-sink-sized hydraulic jumps. But that’s not the case.

    A newly published study shows that gravity can’t be a major player in setting the radius of these small-scale hydraulic jumps because they form the same whether the jet impinges from above, below, or sideways. Instead, the researchers found that surface tension and viscosity are the parameters that determine the jump’s formation. It’s not every day that you get to overturn a centuries-old theory in physics! (Image credit: J. Kilfiger; research credit: R. Bhagat et al.; via Silicon Republic; submitted by Patrick D.)

  • The Jumping Flea

    The Jumping Flea

    Nearly every lab has a magnetic stirrer for mixing fluids, but this ubiquitous tool still holds some surprises, like its ability to unexpectedly levitate. Magnetic stirrers consist of two main parts, a driving magnet that creates a rotating magnetic field, and a bar magnet – commonly referred to as the flea – that is submerged in the fluid to be stirred. When the driver’s rotating field is active, the flea will spin at the bottom of its container, keeping its magnetic field in sync with the driver.

    But if you place the flea in a viscous enough fluid, the drag forces on the flea can pull it out of sync with the driver’s field. Above a certain speed, the flea will jump so that its field repulses the driver’s. That makes the flea levitate as it spins. Depending on the interplay of viscous and magnetic forces, that spin can be unstable (left) or stable (right). The researchers suggest that this peculiar behavior could help artificial swimmers propel themselves or lead to new methods for measuring fluid viscosity. (Image and research credit: K. Baldwin et al.; via APS; submitted by Kam-Yung Soh)

  • Zones and Stars

    Zones and Stars

    Large-scale rotating flows, like planetary atmospheres, tend to organize themselves into zones. Within a zone, flow remains essentially in an east-west direction and serves as a barrier that keeps heat or other elements from mixing from one zone to another. This is, for example, how the tropical trade winds work here on Earth.

    Stars, on the other hand, don’t show this kind of zonal behavior. The reason, it turns out, is their magnetic fields. When there’s no magnetic influence, even weak shear in a rotating flow is enough to start organizing turbulent fluctuations and grow a zonal flow. This tendency toward growth is known as the zonostrophic instability. But when you add a magnetic field, instead of organizing the hydrodynamic disturbances, that weak shear strengthens the magnetic ones, which in turn suppress the flow fluctuations. As a result, the hydrodynamic disturbances cannot grow and no zonal flow forms.

    Researchers think this mechanism can explain both why stars have no zonal flows and just how deep zones can penetrate inside the atmospheres of gas giants like Jupiter and Saturn before their planet’s magnetic field suppresses them. (Image credit: NASA; research credit: N. Constantinou and J. Parker, arXiv; via LLNL News; submitted by Stephanie N.)

  • Swirling the Wrong Way

    Swirling the Wrong Way

    When you swirl wine, you create a rotating wave that travels in the direction that you’re moving the glass. You would expect that anything floating atop that fluid would travel in the same direction of rotation. But it turns out, for a large, thin raft floating atop the rotating fluid, that’s not the case.

    Above you can see a swirling container, rotating counter-clockwise, with a raft of foam. This is from a timelapse where only one photo is taken per rotation, so that it’s easier to see how the foam is rotating relative to the container. And, once enough foam covers the surface, it starts rotating in a clockwise direction – opposite the container! It works for more than foam, too. The researchers show that the same holds for powders or beads. The key to the counter-rotation is that the raft needs to be coherent; it has to be able to transmit friction and internal stress among its constituents. Otherwise, the raft will just drift along with the swirling wave. (Image and research credit: F. Moisy et al., source, arXiv; via Improbable Research; submitted by David H. and Kam-Yung Soh)

  • Using Paper to Avoid Splashback

    Using Paper to Avoid Splashback

    Daily life and countless pool parties have taught us all that objects falling into water create a splash. Sometimes that splash is undesirable, and while there are many ways to tune a splash – by adding surfactants or changing the fluid’s viscosity – there’s a relatively common one that’s escaped scientific study until now. Researchers looked at how splashes change when you add a thin, penetrable fabric – commonly known as toilet paper – to the water surface. 

    Now, the common assumption is that adding a sheet of toilet paper can prevent splashback, but the story is not quite that simple. On the left, you see a splash generated without toilet paper. Because the ball is hydrophilic (water-loving), it does not pull any air into a cavity as it passes. There’s a nice axisymmetric Worthington jet formed, and it doesn’t splash very high, although some of the satellite droplets go quite a bit higher.

    On the right, we see a splash with a single sheet of toilet paper. In this case, the impact of the sphere penetrates the paper, and the way the paper deforms causes air to get sucked down into a cavity behind the ball. That creates a wider, amorphous jet that rebounds higher than the jet in clean water, though it does not shed satellite drops. 

    The researchers found that single and even double sheets of toilet paper can actually increase the height of the splash jet if the object penetrates them. The hole the object makes actually helps focus the jet. Adding a couple more layers, though, can eliminate splashing completely. (Image and research credit: D. Watson et al.)

  • The Sensitivity of a Seal’s Whiskers

    The Sensitivity of a Seal’s Whiskers

    Harbor seals and their brethren have a superpower that lets them track their prey even without sight or sound. It’s their whiskers, which are sensitive enough to follow the trail left by a single fish thirty seconds earlier. The secret to the whisker’s sensitivity lies in its shape. Instead of a uniform, circular cross-section, the seal’s whisker is oval-shaped and its width varies along the length in a wavy pattern. So unlike a straight cylinder, which vibrates when towed through water, the seal’s whiskers are unperturbed by their own movement. They shed only weak vortices and do not vibrate as a result.

    But, if you expose the whiskers to any external turbulence, like the vortices trailing a fish, the whisker ‘slaloms’ back-and-forth in time with the wake. That motion gets transmitted to the nerves in the seal’s cheek, carrying potential information about both the size and speed of the wake’s originator. Researchers hope similar bio-inspired whiskers could help underwater vehicles track schools of fish or locate underwater drilling leaks. (Image credit: M. Richter; video credit: MIT; research credit: H. Beem and M. Triantafyllou; via the Economist; submitted by Russ A. and Kam-Yung Soh)

  • The Protection of the Peloton

    The Protection of the Peloton

    It’s well-known by professional cyclists that sitting in the middle of the peloton requires little effort to overcome aerodynamic drag, but now, for the first time, there’s a scientific study to back that up. Researchers built their own quarter-scale peloton of 121 riders to investigate the aerodynamic effect of cycling in such a large group versus riding solo. Through wind tunnel studies and numerical simulation, they found that riders deep in the peloton can experience as little as 5-10% of the aerodynamic drag of a solo cyclist. 

    Tactically, this means teams should aim to position their protected leader or sprinter mid-way in the pack, where they’ll receive lots of shelter without risking one of the crashes common near the back of the peloton. It also suggests that teams wanting to isolate another team’s leader should try to push them toward the outer edges of the peloton rather than letting them sit in the middle. It will be interesting to see whether pro teams shift their race strategies at all with these numbers in hand.

    Of course, this study considers only a pure headwind. But other groups are looking at the effects of side winds on cyclists. (Image credit: J. Miranda; image and research credit: B. Blocken et al.; submitted by 1307phaezr)

  • Coalescence

    Coalescence

    Simple acts like the coalescence of two droplets sitting on a surface can be beautiful and complex. As the droplets come together, they form a thin neck between them, and the curvature of that surface causes capillary forces that drive fluid into the neck. For two dissimilar droplets, like the ones above, there can be additional forces. Here, the upper drop is pure water, but the lower one has added surfactants, which reduce its surface tension. That difference in surface tension creates a Marangoni flow that tends to pull fluid away from the neck. The result is that full coalescence takes longer. Depending on other factors in this tug-of-war between capillary action and Marangoni flow, the process of coalescence can look very different. In this example, there’s a fingering instability that occurs as the neck spreads. Change the circumstances slightly and the drops may chase each other instead of merging or will merge with a perfectly smooth contact front. (Image and research credit: M. Bruning et al.)