hydrodynamics

New research shows that supermassive first-generation stars may explode in supernovae without leaving behind remnants like black holes. The work is a result of modeling the life and death of stars 55,000 to 56,000 times more massive than our sun. When such stars reach the end of their lives, they become unstable due to relativistic effects and begin to collapse inward. The collapse reinvigorates fusion inside the star and it begins to rapidly fuse heavier elements like oxygen, magnesium, or even iron from the helium in its core. Eventually, the energy released overcomes the binding energy of the star and it explodes outward as a supernova. The image above is a slice through such a star approximately one day after its collapse is reversed. Hydrodynamic instabilities like the Rayleigh-Taylor instability produce mixing of the heavy elements throughout the expanding interior of the star. The mixing should produce a signature that can be observed in the aftermath as these stars seed their galaxies with the heavy elements needed to form planets. For more, see Science Daily and Chen et al. (Image credit: K. Chen et al., via Science Daily; submitted by mechanicoolest)

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The Leidenfrost Effect - Allowing water to flow uphill.

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Look closely enough at a shark’s skin, and you will find it is covered in tiny, anvil-shaped denticles (lower left). To try and discover how and why these denticles help sharks, researchers are 3D printing denticles in different patterns onto flexible sheets to create biomimetic shark skin (lower right). 

They test the artificial shark skin in a water tunnel by moving it with prescribed motions and measuring different characteristics, like the swimming speed attained and the power required. When compared to a smooth but flexible control surface, one pattern came out ahead. The staggered-overlapped denticle pattern (shown in C of the lower right figure) achieved swimming speeds 20% higher than the smooth control despite having far more surface area due to the denticles. The cost of that speed was only 13% greater than the smooth case on average, and was about equal to the smooth case for small amplitude motion. This suggests that the patterning of a shark’s skin may help it swim faster with little to no additional cost in effort.

For more on shark hydrodynamics, check out my previous posts on the topic, and if you want even more shark science, check out these great videos. (Image credit: R. Espanto; J. Oeffner and G. Lauder; L. Wen et al.; research credit: L. Wen et al., 1, 2)

The pink whipray (Himantura fai) is the only species of stingray know to engage in this sort of piggybacking behavior. In fact, multiples pink whiprays piggyback on others rays of the same species, and this is unexpected, elasmobranch, in general only interact when they try to eat each other.

The pink whipray is a large ray that occurs in coastal soft-sediment habitats in the Indian Ocean.

But apparently they love it, and pink whipray piggyback in another species. Pictures don’t lie.

The reasons for this behaviour are unknown, one possibility is that piggybacking is a predator defence strategy that allows the smaller rays to appear larger than they actually are and breaks up silhouettes on which predators can focus. There may also be some hydrodynamic or foraging advantage to the smaller rays in travelling with larger species in this manner, although this does not explain why these rays piggyback on other rays resting on the seabed or at cleaning stations. 

Like many sharks, the great hammerhead shark is negatively buoyant, meaning that, absent other forces, it would sink in water. To compensate, sharks generate lift with their pectoral (side) fins to offset their weight. Their dorsal (top) fin is used to generate the horizontal forces needed for control and turning. However, both captive and wild great hammerhead sharks tend to swim rolled partway onto their sides. The reason for this unusual behavior is hydrodynamic – it is more efficient for the shark. Unlike other species, the great hammerhead has a dorsal fin that is longer than its pectoral fins. By tipping sideways, the shark effectively creates a larger lifting span and is able to induce less drag than when it swims upright. Models show that swimming on their sides requires ~8% less energy than swimming upright! (Image credit: N. Payne et al., source)

The transom stern of HMS Vanguard during construction. Instead of having a gradual tapering off section, it was found that simply ending the hull anywhere within up to six feet of the rudder had no significant impact on performance. The loss of weight meanwhile gave an increase in speed and acceleration. It was a discovery made in the design studies for the N3-class battleships and G3-class battlecruisers of the early 1920s - colossal ships which never came to fruition.

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Sharks have evolved some incredible fluid dynamical abilities. Instead of scales, their skin is covered in microscopic structures called denticles. To give you a sense of size, each denticle in the black and white image above is about 100 microns across. Denticles are asymmetric and overlap one another, creating a preferential flow direction along the shark. When water tries to move opposite the preferred direction, the denticles will bristle, like in the animation above. The bristled denticles form an obstacle for the reversed flow without any effort on the shark’s part. Since local flow reversal is an early sign of separation, researchers theorize that this bristling tendency prevents flow along the shark’s skin from separating. Keeping flow attached, especially along the shark’s tail, is vital not only to the shark’s agility but to keeping its drag low. Researchers have even begun 3D printing artificial shark skin to try and harness the animal’s hydrodynamic prowess. For much more shark-themed science, be sure to check out this week’s “Several Consecutive Calendar Days Dedicated to Predatory Cartilaginous Fishes” video series by SciShow, It’s Okay to be Smart, The Brain Scoop, Smarter Every Day, and Minute Physics. (Image credits: J. Oeffner and G. Lauder; A. Lang et al.; original video; jidanchaomian)

Question: Why do mermaids have the tiddies?

Are they mammalian, wherein they produce milk for their young…

…or more like fish, wherein they work together to fertilise large clusters of eggs and then teach the ones that hatch how to forage for alternate foods?

If the latter, why tiddies? Certainly they would hinder swimming abilities, and are not exactly hydrodynamic…

Not to mention, if they had them… why would they bother covering them up, because that’s a human thing. Animals and other creatures don’t have specific coverings for their bodies… it is doubtful mermaids would.

If mermaids are ‘designed to have breasts’ does this mean the mermen have them too, or are they all sort of genderless and switch between based on current, age and/or demand (like some fish do)? 

And if they DO have clothing and a magical need for the underwater tiddies… why clam shells? Those things would fall off pretty flipping fast the minute you started swimming (get tugged down by the current/water resistance, etc)… they seem illogical as a choice. 

Maybe a seaweed wrap or kelpkini or something to hold them and hold them down/flatten the chest area and provide a smoother surface over which the water can glide for fast swimming…