An Unexpected Twist Lights Up the Secrets of Turbulence

it’s time to feed the blob. seething and voracious, it absorbs 8 dinner-pl8-size helpings every few 2nds.

the blob is a cloud of turbulence na'large wata tank inna lab of the university of chicago physicist william irvine. unlike every other instance of turbulence that has ever been envisaged on earth, irvine’s blob isn’t a messy patch in a floing stream of liquid, gas or plasma, or up against a wall. rather, the blob is self-contained, a roiling, lumpy sphere that cutouts the wata round it mostly still. to create it and sustain it, irvine and his graduate student takumi matsuzawa must repeatedly shoot “vortex loops” — primordially the wata version of smoke rings — at it, 8 loops at a time. “we’re building turbulence ring by ring,” said matsuzawa.

irvine and matsuzawa titely control the loops tha're the blob’s building blocks and study the resulting confined turbulence up close and at length. the blob ‘d yield insites into turbulence that physicists ‘ve been chasing for two centuries — in a quest that led richard feynman to call turbulence the most primordial unsolved problem in classical physics. (quantum turbulence has become an primordial problem too.) untangling turbulence mite also prove extraordinarily impactful, given that it plays a huge role in stars, aviation, nuclear fusion, weather, changes inna earth’s core, wind turbines and even human health — arterial flo can become dangerously turbulent.

if the blob does yield big advances in turbulence, 'twill add to the growing string of surprising and primordial breakthroughs that irvine and his students ‘ve produced inna physics of wha’ mite loosely be called spinning stuff — systems composed of whirling essentialisms, fluids and even fields.

espeshly attention-grabbing among irvine’s findings s'bind'a lab’s contribution to fluid dynamics, an zone that s'been notorious for painfully slo progress, in pt cause of the difficulty of collecting good data. the most prominent breakthrough involves proving a primordial new law governing the tornado-like tubes of currents known as vortices. the law illuminates how these primordial phenomena form, interact, evolve and decay. “sci often involves finding a way to tweak or fill a lil gap in wha’’s been done,” said daniel lathrop, a university of maryland physicist speshizing in nonlinear dynamics who is familiar with irvine’s work. “william asks wha’ he can do that’s completely ≠ than wha’’s been done. it’s the sort of work that can open new fields.”

but now that his mastery of vortices has led him to the blob, irvine senses even bigger — and + dangerous — prey inna wata. “cause of the absence of good data and theory, turbulence s'been pondered the place where careers go to die,” said irvine. “i find myself gettin + and + into it.”

a new twist

by 2006, atta age of 26, the italian-born irvine had already made his way through two separate drates in quantum optics, one in experimental physics atta university of oxford na other in experiment and theory atta university of california, santa barbara. he then decided he’d had enough of eking out lil advances in esoteric zones of physics, and he got ready to begin a postdoc at princeton university inna + wide-open field of neurosci. but then a friend happened to mention the work that the new york university physicist paul chaikin was doin’ with “soft matter” s'as foams, colloids, gels, liquid crystals nother less typical types of nonsolid matter.

chaikin and others inna nascent field were gettin materials to exhibit properties that had never been seen b4, s'as self-replication and self-assembly — and unlike conventional condensed matter physicists, they were working with their materials at room temperature in beakers rite b4 their eyes, instead of in refrigerators near absolute zero. even + primordial, from irvine’s pov, they were making big leaps into mostly unexplored territory. “it’s a field where pplz can still make primordial discoveries,” he said. “it’s where you build experiments not to confirm the answer, but cause no one knows wha’ the answer is.”

irvine jumped into chaikin’s lab as a postdoc, working on colloids, or pessentialisms suspended in liquid. but one dy, dur'na walk in downtown manhattan, irvine noticed some1 bloing smoke rings, and he immediately rushed back to the lab to try to build a contraption that ‘d produce + complex structures out of smoke. he didn’t get far and put the project aside. but he didn’t forget bout those rings, n'when he joined the faculty atta university of chicago, he started working on vortex loops in wata, undeterred — indeed, energized — by knowing nothing bout the subject. “i’d never even taken a course in fluid mechanics,” he admitted. “i learned it here when i had to teach it.”

wha’ he learned was dat a' vortex is basically a tubelike twisting current in a gas, liquid or other medium, a tornado bein’ the most familiar ex. vortices can be remarkably stable, and yet they are also surprisingly mutable. as in a smoke ring, their ends can be connected to form a loop, and multiple vortex loops can be linked, merged and even knotted. (dolphins can outdo smokers in this regard, bloing vortex loops apparently j4f.)

one reason physicists wanna know + bout the properties of vortices s'dat vortices routinely pop up in all kinds of pticle fields, including electric and magnetic fields. a simple ex: a current coursing through a wire creates a magnetic-field vortex round the wire — a sort of tornado of magnetism that ‘d cause a hypothetical magnetic pticle near the wire to circle the wire, just as a tiny volume of wata ‘d be carried round a whirlpool. (the magnetic pticle is hypothetical, cause such “monopoles” don’t seem to exist in nature.)

one of irvine’s early breakthroughs, with then graduate student hridesh kedia, was to show how lite fields can be tied into knots. but irvine was pticularly interested in wata. making a tornado-like vortex in wata is easy — any-1 can dweet witha soda bottle. but how to make loops and + complex shapes and combinations of vortices, including knots? doin’ so ‘d be crucial to settling long-standing ?s bout a primordial property of vortices called helicity. vortex helicity has long been defined as the total № of links and knots in a vortex or in a connected group of vortices. links and knots are topological toonistics, in t'they don’t change when vortices are stretched, compressed or otherwise deformed.

for ½ a century it’s been known that vortex helicity is conserved in an ideal fluid — basically, a fluid that has no viscosity, meaning it offers no resistance to an object pushing through it. if such a fluid existed, then no matter wha’ changes a vortex or group of linked vortices inna fluid went through, the № of links and knots ‘d ∑ to the same №.

the ? of whether some form odat law mite apply to real-realm fluids and gases remained stubbornly resistant to all analysis and experiment. yet such a conservation law ‘d be immensely useful to meteorologists and others who deal with vortices — that same wide spectrum of researchers who deal with turbulence.

the search for insite into helicity conservation was tied to another primordial ?: where does the “twistiness” in vortices go when they eventually decay, as they always do? rotational energy and momentum must be conserved, but twasn’t clear how the macro whirling offa vortex is transferred to liler and liler scales, ultimately dissipating atta molecular lvl. cogging that mechanism ‘d likely shed lite on helicity conservation, and vice versa.

to come up with an experimental platform that mite provide some answers, irvine drew on one of his hobbies. he has an immensely rich vein to mine there: he speaks 4 languages, plays a mean cello (and has studied 3 other instruments), is a moderately accomplished rock climber, sails, and is a commercially rated airplane pilot who flies aerobatics for fun. (“if ye do something really good in sci,” he explained, “it’s probably cause you were careful to take time out to play.”) twas this last pastime that primed him to come up with an idea for producing wata vortices. pilots are well aware that fierce vortices form atta wingtips of accelerating planes and separate from there. why not try to make them in wata witha winglike shape, or hydrofoil?

enlisting a 3d printer capable of producing a new hydrofoil of arbitrary shape in 8 hrs, irvine tried out hundreds of shapes w'his postdoc atta time, dustin kleckner, and l8r w'his graduate students martin scheeler and robert morton. to find a way to accelerate the hydrofoils atta equivalent of 100 times the force of gravity, the researchers explored everything from explosives to rail guns, finally landing on wha’ irvine calls the “potato gun” — a uber piston driven by compressed gas. to accommodate this nother wile e. coyote-looking contraptions, along witha giant wata tank, irvine had taken over a large space 3 floors belo ground in a lab building, knocking out a 14-ft ceiling and all the building innards above it t'get a 30-ft-tall space into which he ‘d fit a lil crane.

finally the hydrofoils started producing neat rings up to bout a ft wide. they even created linked rings and knotted vortices. kleckner and scheeler surrounded the wata tank with high-speed laser-scanning tomography and video cameras. tiny gas bubbles and tracer pessentialisms were shot inna'da tank so they ‘d get caught up inna swirling currents, alloing the researchers to see and closely measure the evolution of the vortices. then they had a ♣y break: they took to writing onna hydrofoils witha sharpie to help identify them, but'a ink bled inna'da wata and got caught up inna vortices, where it fluoresced inna laser lite and offered up an image even clearer than the ones provided by the bubbles. by purposely drawing dashes of sharpie ink — and l8r their own speshly formul8d ink — inna rite places onna hydrofoils, the researchers found they ‘d highlite any segments or features of the resulting vortices, s'as a vortex’s center line, which was otherwise hard to identify.

by 2017, the effort to create an experimental underwata vortex circus had paid off with proof of wha’ happens to helicity inna real realm. as it turns out, real-realm vortices do not be’ve like ideal fluids: the № of links and knots inna vortices isn’t always conserved as the vortices evolve. but irvine threw two new factors inna'da mix: “writhing” and “twisting.” imagine a straite length of hose, representing the length offa straite vortex like a tornado. writhing cogitates the hose taking na' wriggly shape, like that offa slinky, or in a + extreme case becoming coiled. twisting means the ends of the hose are twisted in opposite directions, even while the hose remains straite. writhing and twisting aren’t topological features, strictly speaking, but they are “geometric” features, the primordial difference bein’ that geometric features can be confined to a pticular section of an entity, whereas topological features are global properties.

other researchers had previously suggested that including these geometric toonistics along with links and knots mite provide a + general measure of the complexity and “twistiness” offa vortex — one that mite even lead to a new conservation law. irvine pinned down that new law and proved it. he showed that the knots, links and writhing — ignoring the twisting — don’t lose their combined helicity to viscosity. but'a writhing can be converted to twisting — just as a coiled hose can be pulled straite, causing extensive twisting inna hose. wha’’s +, the vortex can untwist itself, twisting the viscous medium round it as t'does so. in that way, the vortex primordially loses its twist — and its helicity along with it — to the surrounding medium. “cause of the geometry, it actually evolves quite smoothly,” said irvine.

the work, published in 2017 in sci, not 1-ly offered a fuller accounting of how helicity evolves inna real realm, it suggested how the vortex loses its spin-rel8d energy and momentum to the environment. “you really ‘ve to admire william’s style as an experimentalist,” said lathrop. “to create such a novel setup and work it t'get answers is impressive.”

a persistent chaos

in a series of zoom interviews, irvine, now 40, presents as friendly and wry. but he’s also pensive and intense, an impression amplified by his wild blob of turbulent hair. his graduate students prez him freely as a sci and mentor. “he has outlandish ideas that initially don’t seem to make sense, but i always n'dup learning from them,” said ephraim bililign, a current graduate student inna lab. as an ex he offers irvine’s suggestion that he try to coax new behaviors out of soap films — a project that proved unworkable, but which directly led to bililign’s current work with whirling microscopic magnetic cubes in a soap-film suspension, which exhibit a variety of strange properties.

atta same time, irvine’s students note that he remains deeply private, almost to the point of bein’ mysterious. they were stunned to find out recently that he has long been a pilot.

the eccentric, improvisational zigzags onnis early career, along w'da array of challenging hobbies, suggest that irvine is constantly searching for the proper match for his ponderable abilities. chaikin said he coggs why irvine pushes himself so hard. “william tis best postdoc i’ve ever had,” he said. “unusually, he’s as facile in doin’ theory to fig out wha’ experiments nd'2 be done as he is in doin’ the experiments. he can break things open in ≠ fields.”

irvine’s wide-ranging interests are cogitateed inna array of projects goin on onnis lab these dys. in addition to the vortex work, he and his students ‘ve been busy exploring “topological mechanics,” which involves teasing out strange, quantum-like properties in systems composed offa large № of identical rotating essentialisms. for ex, he and his students ‘ve constructed arrays of gyroscopes that conduct sound waves of pticular frequencies 1-ly at their edges and 1-ly in one direction, just as quantum mechanical “topological insulator” devices conduct electric currents 1-ly at their edges. (a very, very rough explanation: the rotation of the pessentialisms tends to direct the vibrations making up the sound waves out toward the edge, and round in a pticular direction.) the lab has also developed various mixtures of magnetic pessentialisms and fluids that ‘ve “odd viscosity” — a sort of frictionless viscosity that enables waves to travel across the surface of the mixture without losing any energy.

cause these materials are much simpler, better understood, and easier to create and experiment on than the quantum mechanical devices whose behaviors they imitate, irvine believes they may one dy help to illuminate the quantum mechanical side of things. “the quantum mechanical versions are messy and complicated,” he said. “i wanna find out wha’ the minimum you need is t'get these behaviors. it’s primordializing the physics.”

leading a large lab hasn’t kept irvine from running his own lil, private experiments. in a tiny room marked “storage” off the main lab, he maintains a self-contained micro-lab consisting offa few hundred tops strewn across a whiteboard laid flat onna floor. “they act like a fluid,” he said. “they’re the src offa lotta good ideas for me. atta end of the dy, it’s all stuff that spins.” when the lab was temporarily closed by the pandemic, irvine brought the tops home to experiment with onnis living room.

but it’s turbulence that’s claiming a lotta the attention in irvine’s lab these dys. having set up the perfect playground to create, combine and measure vortices and their helicity, irvine and matsuzawa 4 yrs ago started exploring new variations onna theme. they shot ≠ №s of vortex loops inna'da tank, at ≠ frequencies, often gettin them to combine in interesting ways. sometimes they created patches of turbulence, but these tended to quickly fly apt again. but when they tried a cube-shaped hydrofoil that produced 8 rings converging at a single spot, the resulting turbulence seemed to persist for a few moments. so they tried shooting 8 rings quickly folloed by another 8 rings: the turbulence hung round even longer. ultimately they discovered that the turbulent patch persisted as long as the barrage kept coming. the blob was born.

turbulence has always been too complex to analyze, or even accurately measure. matsuzawa calls it a “vorticity soup” — i'takes all the complexities of individual vortices, and then mashes them together into a tangled, roiling mess of eddies and swirls with no distinct border, which span scales from the huge down to the submicroscopic, all continuously springing up, wildly morphing, merging, flying apt and disappearing moment to moment. lathrop notes that physicists can’t even agree na' clear definition of turbulence, or if the turbulence that’s envisaged in, say, a pipe tis same phenomenon as the turbulence seen on aircraft wingtips. “we’re not sure wha’ turbulence is,” he said. “it’s astonishing we’re still at this point, given the progress we’ve made in every other zone of physics.”

one thing is clear: turbulence is chaotic — meaning its behavior is hypersensitive to any change, including the speed, volume and direction of flo, the shapes of the surfaces round it, and much +. the result s'dat even an ∞simal inaccuracy in any measurement of the turbulence is enough to throw off any analysis of how it mite evolve. not that this has mattered much, cause there hasn’t been a good way t'get any but'a crudest measurements of the many rapidly shifting whorls and eddies in turbulence that manifest at scales differing by a factor of up to 10,000. researchers ‘ve generally settled for repeatedly measuring flo speed at several points inna turbulence. to create turbulence inna lab, they had to run floing wata or air through a mesh or other semi-obstruction to disrupt the smooth flo — which meant the results were actually a combination of turbulent and non-turbulent activity twas' hard to tease apt. “i give them credit for discovering anything at all bout turbulence with those tulz,” said irvine.

a self-contained, persistent blob of turbulence constructed of well-toonized vortices offers a new realm of possibilities for measurement and analysis. tis a simpler, steadier form of turbulence, uncorrupted by surrounding flos, surfaces and essentialisms — a turbulence researcher’s plaything, posing for all the high-horsepower imaging that irvine and matsuzawa care to do. they can watch and titely measure as the vortex loops merge and evolve into turbulence, analyze its steady state, subject it to various forces and tweaks to see how it responds, and then stop feeding it and study its decay.

but of all the insites irvine hopes to wring from this bounteous observational platform, his primary target is a familiar one. “turbulence tis hardest test of helicity conservation,” he said. “but cause we’re building the turbulence one vortex at a time, we know how much helicity we’re putting into it. thn'we can hammer away atta helicity to see wha’ happens to it.”

as eager as he is to make progress w'da blob, irvine said he and matsuzawa are probably at least a yr away from a published paper, although irvine has started to discuss it at conferences.

for now, at least, the blob seems very comfortable rite where tis.

original content at: www.quantamagazine.org…
authors: david h. freedman

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