Here, a Soyuz rocket takes off in 2023, carrying three of the Expedition 70 crew to the International Space Station. This initial stage of the Soyuz launch vehicle uses four identical rocket boosters lashed around the second stage core. Each of the boosters has a rocket engine with four combustion chambers (and thus four exhaust nozzles) of its own. That creates the fiery flurry of engine plumes seen here. Most of the exhaust plumes are directed downward to provide the thrust needed to lift the rocket, but you can see a few angled slightly to either side to help stabilize the launch vehicle as it rises. (Image credit: NASA)
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“Running on Water”
In the early morning light, young photographer Max Wood captured this coot escaping a fight. With wings flapping, the bird runs across the water surface. Each slap and stroke of a foot provides a portion of the vertical force needed to stay atop the water; lift from its wings provides the rest. With enough speed, the bird will take off. Some birds, however, are born water-walkers; certain species of grebe don’t need to use their wings to run on water. (Image credit: M. Wood; via BWPA)
How Moths Confuse Bats
When your predators use echolocation to locate you, it pays to have an ultrasonic deterrence. So, many species of ermine moths have structures on their wings known as tymbals. These areas have a band of ridges, and, when the moth’s wing lifts or falls, the ridges buckle one-by-one. A nearby bald patch on the wing acts as an amplifier, making these ultrasonic snaps louder. Altogether, the mechanism deters prowling bats anytime the moth flaps its wings — without any additional effort on the moth’s part. Since the moths have no ears, they presumably don’t even know that they’re making the sound! (Image credit: Wikimedia/entomart; research credit: H. Mendoza Nava et al.; via APS Physics)
“Mason Bee at Work”
Mason bees like this one build landmarks to help them navigate as they construct a shelter for their eggs. Even hauling materials, these bees can easily stay aloft. This is in contrast to an old misconception that physics can’t explain how a bee flies. It’s true that bees don’t fly using the same mechanisms as a typical airplane — no fixed wings here! But they, like every other flyer aerodynamicists study, still produce lift and drag and thrust. The flapping of a bee’s wings generates much unsteadier quantities of these things, but at its small size, that is no hindrance to its ability to control its flight and even carry cargo. (Image credit: S. Zankl; via Wildlife POTY)
Parting a Flame
A sheet of flame splits around a cylinder in this Gallery of Fluid Motion poster. Looking at the image sequences, you can see how the flames lift up as they flow around the cylinder, following the arms of a horseshoe vortex. Researchers study situations like this one to better understand how wildfires move as they encounter obstacles. Understanding and predicting how fires flow is increasingly important with more wildfires encountering human-built infrastructure. (Image credit: L. Shannon et al.)
Turbulent Thermal Convection
In the winter, warm air rises from our floor vents or radiators, creating a complex, invisible flow in the background of our lives. Buoyancy lifts warmer air upward while cooler, denser air sinks back down. This thermal convection is everywhere: in our buildings, the ocean, the sky overhead — even in the visible layer of our sun.
In nature, these systems are so large and complex that fully measuring or simulating them remains impossible. Instead, researchers focus on a simplified system — a Rayleigh-Bénard cell — that’s essentially an idealized version of a pot on a stovetop. The lower surface of the cell is heated — like the bottom of a pan on the burner — while the upper surface of the fluid cools. Even this idealized system is a challenge, though, and neither lab-scale versions nor simulations can reach the same conditions that we find in nature.
To bridge the gap, scientists rely on mathematical models — theories built on our best understanding of the physics — and physical analogies to similar systems — like flow over a flat plate — that are “easier” to measure. For a thorough overview of recent work in the area, check out this review in Physics Today. (Image credit: A. Blass; research credit: D. Lohse and O. Shishkina in Physics Today)
A Working Wirtz Pump
In the mid-eighteenth century, pewterer Andreas Wirtz invented a spiral pump. Even today, his design is useful for small-scale, low-power pumping, as seen in this Steve Mould video. The design relies on a series of air and water plugs to build up pressure that’s then used to lift the fluids higher. In the video, Mould visits a stream-powered, home version of a Wirtz pump that regularly delivers water over eight meters in elevation. See it in action in the full video! (Video and image credit: S. Mould)
Withstanding Rocket Launches
It takes a lot of power to lift a giant rocket‘s payload all the way to orbit, and in the first moments of a rocket launch, all that energy is directed downward at a concrete pad. How do engineers design and protect launch pads? In this Practical Engineering video, Grady tackles just that question through a comparison of SpaceX’s Stage Zero and NASA’s Launch Pad 39A.
SpaceX notoriously chose to build Stage Zero without a trench or water sprayer system like the ones NASA use. Trenches deflect the rocket exhaust to reduce the impact on infrastructure beneath the engines. And water sprayers reduce the temperatures the pad experiences and disrupt shock waves that otherwise hammer the pad. Without those precautions, even special heavy-duty concretes have a hard time holding together against a launch. (Video and image credit: Practical Engineering)
Modeling Wildfires With Water
Turbulence over a burning forest can carry embers that spread the wildfire. To understand how wildfire plumes interact with the natural turbulence found above the forest canopy, researchers modeled the situation in a water flume. Dowel rods acted as a forest, with turbulence developing naturally from the water flowing past. For a wildfire, the researchers used a plume of warmer water, which buoyancy lofted into the turbulence over their model forest.
The experiment used to model wildfire flows. Dowel rods represent the forest and a plume of warm water (right side; distorting the background) represents the wildfire. The dark device in the foreground is a probe used to measure turbulence. The flow over the forest canopy naturally forms side-by-side rolls of air rotating around a horizontal axis. As the buoyant plume rises, it can be torn apart by these rollers, as well as carried downstream. Varying the turbulence, they found, did not affect the average trajectory of the plume. But the more intense the turbulence, the greater the vertical fluctuations in the plume. Those large variations, they concluded, could lift more embers into stronger winds that distribute them further and spread a fire faster. (Image credit: wildfire – M. Brooks, experiment – H. Chung and J. Koseff; research credit: H. Chung and J. Koseff; via APS Physics)
How Squall Lines Form
Summertime in the middle U.S. means thunderstorms, many of which can form long lines of storms known as squall lines. Complex convective dynamics feed such storms. Here is an illustration of one part of a squall’s lifecycle:
As rain falls and evaporates, it fuels the formation of a cold pool of air below the cloud. Incoming wind (gray arrows) blocks the cold pool from spreading. In turn, the cold pool acts as a ramp that redirects this warm, moist air upward. The vertical variation in wind speed (wind shear, shown with pink arrows) creates a positive vorticity. Together with the negative vorticity in the cold pool, this induces a vorticity dipole that lifts air and moisture, feeding the growing line of storms. As it falls, rain evaporates, cooling air near the ground and forming a cold pool. If incoming winds block the cold pool from spreading, the pool will act instead as a ramp that redirects the wind upward, carrying any warmth and moisture up into the storm cloud. Wind shear — a vertical variation in wind strength with altitude — creates positve vorticity that opposes the negative vorticity inherent to the cold pool. Together these two regions of opposing vorticity lift more air and moisture into the squall, generating more clouds and more rainfall. (Image credit: top – J. Witkowski, illustration – C. Muller and S. Abramian; see also C. Muller and S. Abramian)
