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Citrus fruits like oranges house tiny pockets of oil in their peels. When squeezed, the oils jet out in tiny micro-jets that are about the width of a human hair. Despite their small size, the jets reach speeds of about 30 m/s and quickly break into a stream of droplets. When exposed to the flame of a lighter, like in the animation above, those microdroplets combust easily, creating a momentary fireball used to augment some cocktails. For more on how the citrus peel generates these jets, check out this previous post. (Image credit: Warped Perception, source; research credit: N. Smith et al.)

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Reducing the forces of water entry - YouTube


As anyone who’s jumped off the high board can tell you, hitting the water involves a lot of force. That’s because any solid object entering the water has to accelerate water out of its way. This is why gannets and other diving birds streamline themselves before entering the water. But even for non-streamlined objects, like a sphere, there are ways to reduce the force of impact.

This video explores three such techniques, all of which involve disturbing the water before the sphere enters. In the first, the sphere is dropped inside a jet of fluid. Since the jet is already forcing water down and aside when the sphere enters, the acceleration provided by the sphere is less and so is the force it experiences.

The second and third techniques both rely on dropping a solid object ahead of the one we care about. In the second case, a smaller sphere breaks the surface ahead of the larger one, allowing the big sphere to hit a cavity rather than an undisturbed surface. Like with the jet, the first sphere’s entry has already accelerated fluid downward, so there’s less mass that the bigger sphere has to accelerate, thereby reducing its impact force.

In the third case, the first sphere is dropped well ahead of the second, creating an upward-moving Worthington jet that the second sphere hits. In this case, there’s water moving upward into the sphere, so how could this possibly reduce the force of entry? The key here is that the water of the jet wets the sphere before it enters the pool. Notice how very little air accompanies the second sphere compared to the first one. That’s because the second sphere is already wet. It’s also been slowed down by the jet so that it enters the water at a lower speed, all of which adds up to a lower force of entry. (Image and research credit: N. Speirs et al.)

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Massive Bee Colony Buzzing In Sync To Scare Off Predators | BBC Earth - YouTube


Not everything that behaves like a fluid is a liquid or a gas. In particular, groups of organisms can behave in a collective manner that is remarkably flow-like. From schools of fish to fire-ant rafts, nature is full of examples of groups with fluid-like properties. 

One of the most mesmerizing examples are these giant honeybee colonies, which essentially do “the wave” to frighten away predators like wasps. Researchers are still trying to understand and mimic the way these groups coordinate such behaviors. Can even complicated patterns be generated by a simple set of rules an individual animal follows? That’s the sort of question active matter researchers investigate. Check out the video above to see a whole cliff’s worth of bee colonies shimmering. (Image and video credit: BBC Earth)

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Living near the Rocky Mountains, it’s not unusual to look up and find the sky striped with lines of clouds. Such wave clouds are often formed on the lee side of mountains and other topography. But even in the flattest plains, you can find clouds like these at times. That’s because the internal waves necessary to create the clouds can be generated by weather fronts, too.

Imagine a bit of atmosphere sitting between a low-pressure zone and a high-pressure zone. This will be an area of convergence, where winds flow inward and squeeze the fluid parcel in one direction before turning 90 degrees and stretching it in the perpendicular direction. The result is a sharpening of any temperature gradient along the interface. This is the weather front that moves in and causes massive and sudden shifts in temperature. 

On one side of the front, warm air rises. Then, as it loses heat and cools, it sinks down the cold side of the front. The sharper the temperature differences become, the stronger this circulation gets. If the air is vertically displaced quickly enough, it will spontaneously generate waves in the atmosphere. With the right moisture conditions, those waves create visible clouds at their crests, as seen here. For more on the process, check out this article over at Physics Today. (Image credit: W. Velasquez; via Physics Today)

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On a hot day, it’s not unusual to catch a glimpse of a shimmering optical illusion over a hot road, but you probably wouldn’t expect to see the same thing 2,000 meters under the ocean. Yet that’s exactly what a team of scientists saw through the cameras of their unmanned submersible as it explored hydrothermal vents deep in the Pacific Ocean.

At these depths, the pressure is high enough that water can reach more than 350 degrees Celsius without boiling. The hot fluid from the vents rises and gets caught beneath mineral overhangs, forming a sort of upside-down pool. Since the index of refraction of the hot water is different than that of the colder surrounding water, we see a mirror-like surface at some viewing angles. Be sure to check out the whole video for more examples of the illusion. (Image and video credit: Schmidt Ocean; via Smithsonian; submitted by Kam-Yung Soh)

Microbial Mysteries - Big Pagoda Pools - Hydrothermal Vent Structure - YouTube
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In “Aurora”, artist Rus Khasanov uses fluids to create a short film full of psychedelic color and cosmic visuals. As in a soap bubble, the bright colors – as well as the pure black holes – come from the interference of light rays. The colors directly relate to the thickness of fluid, and they allow us to see all the subtle flows caused by variations in surface tension. (Video and image credit: R. Khasanov)

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Soft systems like this bubble raft can retain memory of how they reached their current configuration. Because the bubbles are different sizes, they cannot pack into a crystalline structure, and because they’re too close together to move easily, they cannot reconfigure into their most efficient packing. This leaves the system out of equilibrium, which is key to its memory. 

By shearing the bubbles between a spinning inner ring (left in image) and a stationary outer one (not shown) several times, researchers found they they could coax the bubbles into a configuration that was unresponsive to further shearing at that amplitude. 

Once the bubbles were configured, the scientists could sweep through many shear amplitudes and look for the one with the smallest response. This was always the “remembered” shear amplitude. Effectively, the system can record and read out values similar to the way a computer bit does. Bubbles are no replacement for silicon, though. In this case, scientists are more interested in what memory in these systems can teach us about other, similar mechanical systems and how they respond to forces. (Image and research credit: S. Mukherji et al.; via Physics Today; submitted by Kam-Yung Soh)

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Periodically, our sun releases plasma in a coronal mass ejection. Afterwards, the local magnetic field lines shift and reorganize. We can see that process in action here because charged particles spin along the magnetic lines, outlining them as bright loops in this imagery. This sequence – one of the best examples of this phenomenon to date – was captured by NASA’s Solar Dynamics Observatory in early 2017. To understand behaviors like these, scientists use magnetohydrodynamics, a marriage of the equations of fluid mechanics with Maxwell’s equations for electromagnetism. (Image credit: NASA SDO, source)

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Drops that impact a very hot surface will surf on their own vapor, and ones that hit a very cold surface will freeze almost immediately. But what happens when the temperature differences aren’t so extreme? Scientists explored this (above) by dropping room-temperature water droplets onto a cool surface – one warmer than the freezing point but cooler than the dew point at which water condenses. 

They found that impacting drops formed a triple halo of condensate (right).  The first and brightest ring forms at the radius of the drop’s maximum extent during impact. The second band forms from water vapor that leaves the droplet at impact. As that vapor cools, it condenses into a second band. The final, dimmest band forms as the droplet stabilizes and cools. It’s the result of water vapor near the droplet continuing to cool and condense. (Image and research credit: Y. Zhao et al.; via Nature News; submitted by Kam-Yung Soh)

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How a Fire Sprinkler Works at 100,000fps - The Slow Mo Guys - YouTube


Most of us have probably never given much thought to how a fire sprinkler works, but fortunately, the Slow Mo Guys have used their high-speed skills to answer that question anyway. Sprinkler systems of this variety are constantly pressurized by a full pipe line of water that’s held back by a thin metal disk and a colored glass ampule containing a fluid like alcohol. The color of ampule indicates the temperature at which the system is designed to activate. As the ampule heats up, the fluid inside expands, breaking the ampule at or near the critical temperature. That allows the metal disk to fall away and releases a torrent of water, which falls onto the gear-like disk at the bottom of the sprinkler and gets flung out over a wider area. Despite appearances, that bottom disk is stationary, not spinning; its shape alone is what distributes the water. (Image and video credit: The Slow Mo Guys)

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