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  • Snowflake Velocimetry

    Snowflake Velocimetry

    In our era of remote learning, students don’t always have a chance to do hands-on lab experiments in the usual fashion. But that doesn’t mean they can’t explore important flow diagnostic techniques. Here a simple smartphone video of snow falling gets turned into a lesson on particle image velocimetry, or PIV, a major technique for measuring flow velocities.

    A nearby house acts as a fixed backdrop, and by comparing snowflake positions from one frame to the next, students can measure the instantaneous flow patterns in the snowfall. Of course, that’s a tedious task to do by hand, but luckily there are computer programs that do it automatically. Simply run the smartphone video through the software, and analyze the patterns it reveals!

    As a bonus, students don’t have to get distracted by the complexities of laser sheets and flow seeding that are normally a part of PIV. Instead, the flow and the lighting are already right outside their window, and they can concentrate instead on learning the principles of the technique and how to use the software. (Image and submission credit: J. Stafford)

  • This Is Your Brain

    This Is Your Brain

    The human brain, like an egg, consists of soft matter bathed in a fluid and encased in a hard shell. To better understand how our brains respond to sudden accelerations, researchers looked at how egg yolks behave. In a purely translational impact (Image 1), the egg yolk deforms very little. But rotational motions (Images 2 and 3) cause major effects because of the imbalance between pressure forces outside the yolk’s membrane and the centrifugal forces within it. Rotational deceleration was particularly potent (Image 3).

    The researchers’ findings are consistent with concussion research, which has shown that impacts with rotational acceleration/deceleration inside the skull are the most damaging. Based on the yolk’s deformation, such impacts likely stretch neurons and disturb their delicate network. (Image credit: cracked egg – K. Nielsen, others – J. Lang et al.; research credit: J. Lang et al.; via Physics World; submitted by Kam-Yung Soh)

  • Strings of Swirls

    Strings of Swirls

    Von Karman vortex streets are the rows of alternating vortices shed off isolated objects interrupting a flow. Here, the volcanic peaks of Cabo Verde disrupt an atmospheric flow accustomed to an empty ocean. In a steady wind, air wraps around the volcanoes and detaches first on one side, creating a vortex, then from the other side, making a vortex of the opposite rotation. Although these structures are always present, we only see them when they stir up the cloud layer, leaving these strings of swirls for hundreds of kilometers behind the islands. (Image credit: L. Dauphin/NASA; via NASA Earth Observatory)

  • Flexible Wings Aid Butterfly Flight

    Flexible Wings Aid Butterfly Flight

    Butterflies are some of the oddest flyers of the insect world, given the large size of their wings relative to their bodies. That could be a recipe for inefficient flight, but a new study shows that butterflies’ large flexible wings actually help them take off quickly.

    When lifting their wings, butterflies use an unusual clapping motion, with the leading edges of their wings coming together before the rest of the wings. This motion helps cup and direct air, creating most of the butterfly’s thrust, according to the researchers. The wings’ flexibility is key to this. Using artificial wings — both stiff and flexible — researchers found that the flexible wings generated 22% more useful impulse and were 28% more efficient. For a tiny flyer with frequent take-offs, that’s an enormous savings! (Image, video, and research credit: L. Johansson and P. Henningsson; via BBC; submitted by Kam-Yung Soh)

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    Chasing Tornadoes

    Tornadoes are some of the most powerful storms on Earth. Their difficult-to-predict nature means that we still have a relatively scant understanding of exactly how they form. We know the conditions that promote their development — warm, moist rising air, wind shear, and rotation — but how and when those translate into a dangerous funnel cloud is harder to pin down. In this video, we hear from one of National Geographic’s storm researchers, Anton Seimon, who chases these storms in search of answers. (Image and video credit: National Geographic)

  • Inside Drying Wood

    Inside Drying Wood

    Wood must dry before it can be used in most applications, but with its complex internal structure exactly how wood dries out has been unclear. New experiments combining MRI and x-ray imaging reveal a process quite different than expected.

    Inside hardwoods like poplar — the species studied here — wood contains both solid structures and pores where water can gather. The pores do not form a fully interconnected network, so capillary action alone is unable to carry water through the pores and out to a surface where it can evaporate.

    Instead, researchers found that water evaporating at the surface came from so-called “bound water” in the wood’s solid structures. As the bound water evaporated, it caused water in the wood pores to diffuse into the solid walls, becoming bound and continuing to feed the evaporation. (Image and research credit: H. Penvern et al.; via APS Physics)

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    Permeable Pavement

    Controlling storm water is a major challenge in urban environments, where many surfaces are impermeable. In a city, rain cannot simply soak into the ground and filter into the water table. One potential solution is permeable pavement, which uses the same ingredients as its common counterpart minus the sand that usually packs into gaps between the gravel. Without the sand, the final pavement allows water to soak through, as seen above. In practice, the water sinks into a porous reservoir beneath the pavement that helps store and regulate the water’s discharge into the soil.

    Unfortunately, this solution has its limitations. Permeable pavement is not as strong as the regular variety, so it doesn’t work for highly trafficked areas like roadways. It’s also not well-suited to colder areas, where freezing and thawing may disrupt its operation. But it is another tool in engineers’ toolboxes when it comes to keeping urban environments in harmony with nature’s needs. (Image and video credit: Practical Engineering)

  • Cutting Coronavirus Risk in Cars

    Cutting Coronavirus Risk in Cars

    Even in a pandemic, it’s sometimes necessary to share a car with someone outside one’s bubble. When that’s the case, it’s important to know how to limit risks of coronavirus exposure. For this study, researchers used computational fluid dynamics to simulate flow around and inside a Prius-like four-door sedan with a driver and a single passenger located in the rear passenger-side seat. Assuming the air conditioner was on and the car was moving at 50 miles per hour, the researchers found that the baseline flow of air inside the car moves from the back of the cabin toward the front. With the windows closed, the simulation suggested that 8-10% of the aerosol particles exhaled by one passenger could reach the other.

    Opening the car’s windows increases the ventilation and reduces exposure risk. The best configuration the researchers found opened two windows: the front passenger-side window and the rear driver-side window. By opening the window opposite each person, the airflow in the car creates a sort of curtain between the two that reduces aerosol exposure to only 0.2-2% of what’s exhaled by the other occupant. (Image credit: rideshare – V. Xok, CFD – V. Mathai et al.; research credit: V. Mathai et al.; via NYTimes; submitted by Kam-Yung Soh)

    Computed streamlines for flow through a sedan with a driver and one rear passenger, with each opposite window opened.
  • Bubbles Affect Lava Flow

    Bubbles Affect Lava Flow

    During the 2018 eruption at Kilauea, scientists noticed that the lava flowed very differently depending on how bubbly it was. In this experiment, researchers used corn syrup as a lava analogue and studied how bubbly and particle-filled bubbly flows differed from bubble-free ones. They found that bubble-free syrup flowed fastest, while particle-filled bubbly flows were by far the slowest.

    The bubbles also affected the structure of the flows. Large bubbles gathered near the surface of the flow’s leading edge, allowing faster flow beneath. And in the particle-filled flow, the corn syrup developed channels that flowed at different speeds. The authors hope that their relatively simple experimental set-up will inspire more research on bubbly lava flows. (Image and research credit: A. Namiki et al.; via AGU Eos; submitted by Kam-Yung Soh)

  • Rocking From The Waves

    Rocking From The Waves

    Not all seismic activity stems from earthquakes. In fact, much of Earth’s measured seismic waves come from interactions of the ocean and atmosphere with solid ground. Some of the strongest vibrations come from interactions of ocean waves, which transmit pressure waves that don’t attenuate with depth before passing into the solid Earth.

    How those waves propagate and scatter inside the Earth has been a matter of contention for decades, but recent simulations are beginning to uncover the mechanisms that lead to the waves seismologists measure. (Image credit: I. Mingazova; via Physics Today)

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