̽��ֱ�� of Cambridge - gas

What makes a sand dune sing?

sc604 — Fri, 04 Nov 2016 08:50:28 +0000

For Marco Polo, the desert could be a spooky place, filled with evil spirits. Writing in the 13th century, he described the famous singing sands, which “at times fill the air with the sounds of all kinds of musical instruments, and also of drums and the clash of arms.” But the low, loud rumbles coming from the dunes were not the work of spirits. They were the work of physics.

As grains of sand slide down the side of certain dunes, they create vibrations that can be heard for miles around. ̽��ֱ��sand avalanches trigger the dune’s natural resonance, but only when conditions are just right. It can’t be too humid, and the grains of sand need to be just the right size and contain silica. Only then will an avalanche cause the dunes to start singing.

An avalanche, whether it’s made of sand or snow, is an example of a granular flow, when solid particles flow like liquids, colliding, bouncing around, interacting, separating and coming back together again. Granular flow processes can be found everywhere from the world’s highest mountains to your morning bowl of cereal.

Dr Nathalie Vriend, a Royal Society Dorothy Hodgkin Research Fellow in the Department of Applied Mathematics and Theoretical Physics, is a specialist in granular flows. Her PhD research at the California Institute of Technology unravelled some of the physics at work in the same singing sands that mystified Marco Polo. At Cambridge, her research focuses both on sand dunes and on avalanches, and how to quantify their behaviour, which can have practical applications in industries including pharmaceuticals, oil and gas. Vriend’s work relies as much upon laboratory experiments and fieldwork as it does on mathematical models.

“An avalanche can behave as a solid, liquid or gas, depending on various factors, which is what makes them so difficult to model mathematically,” says Vriend. “For me, modelling their behaviour starts with observation, which I then incorporate into a model – it’s nature where I get my inspiration from. That, and curiosity – I see something and I want to try to explain it.

“Since there are particles which collide and interact in a granular flow, there is a certain degree of randomness to the process, so how do you incorporate that into a model? You try to translate what you’re seeing into a physical description, and then you perform numerical or theoretical simulations to see if the behaviour you get from the models is the same as you observe in nature.”

Despite their somewhat chaotic nature, avalanches and other types of granular flows share some distinct patterns. Owing to a phenomenon known as segregation, larger particles tend to rise to the top in an avalanche, whereas smaller particles sink to the bottom, falling into the gaps between the larger particles. A similar phenomenon can be seen in your breakfast cereal: the smaller, tastier bits always seem to end up at the bottom of the bowl. Larger grains are also pushed to the side and the front, forcing the flow of the avalanche into channels.

Similar processes are at work in sand dunes. As wind blows across a dune, there is segregation of the individual grains of sand, as well as small avalanches taking place on the granular scale. But for a sand dune that is 40 m high, there are also processes taking place on the macro scale. ̽��ֱ��entire dune itself can move and race across the desert floor. Small dunes migrate faster than large dunes, as if playing a “catch me if you can game”.

A cross section of a typical sand dune would reveal a slope on one side of a ridge, and a sharp drop on the other. As the wind blows across the dune, it pushes grains of sand up the slope, where they gather in a heap. When the heap gets too big, it becomes unstable and tumbles over the other side, causing an avalanche, eventually coming to a stop. This process happens again and again, causing layers to form within the dune. “A sand dune may look like a monolithic mass of sand, but there are multiple layers and structures within it,” says Vriend.

How does this understanding of the anatomy and movement of a sand dune translate into practical applications? Understanding granular flows can be useful in the pharmaceutical industry, where two different active ingredients may need to be mixed properly before a pill is made. Granular flows are also highly relevant to the oil and gas exploration process, and with this in mind Vriend is working with Schlumberger, the oilfield services company.

Sand dunes are major sources of noise in seismic surveys for oil and gas in deserts, which are conducted to probe the location and size of underground oil and gas reserves. ̽��ֱ��surveys use an acoustic pulse from a source and carefully placed receivers at different points to listen to the signal that is received, which can then be used to calculate what is hidden underground. ̽��ֱ��problem encountered by surveyors is that the sand dunes are composed of loose sand and therefore have a much lower wave velocity than the rocky desert floor, and as a result they act as traps of wave energy: the energy keeps reverberating and creates a source of noise in the post-processing of the seismic surveys. As part of a secondment at Schlumberger, one of Vriend’s PhD students is performing numerical simulations to understand the origin and features of this noise.

Another industrial problem that Vriend’s group is currently working on is the phenomenon of ‘honking’ grain silos. As grains are let out of the bottom of a silo, the friction of the pellets on the walls of the silo makes a distinctive ‘honking’ sound. Annoying for the neighbours perhaps, but hardly dangerous. However, when the vibrations get loud enough, it can cause a resonance within the silo, leading to structural failure or collapse. Vriend’s students are attempting to understand what affects the way that silos honk, which could someday be used to minimise noise, or even to prevent collapse.

̽��ֱ��phenomenon behind honking silos on a busy farm is similar to that which causes massive desert sand dunes to sing, although one could be perceived as an annoyance while the other is considered captivating. For Vriend, however, it’s the real-world observations and the opportunity to spend time in nature that motivate her.

She explains: “What I love about my research, whether it’s looking at silos or avalanches, is that you can observe it, see it, feel it, touch it.”

Inset images: Avalanche Zinal, credit: dahu1. Slip face GPR, credit Matthew Arran.

When solids flow like liquids they can make sand dunes sing, and they can also result in a potentially deadly avalanche. Cambridge researchers are studying the physics behind both of these phenomena, which could have applications in industries such as pharmaceuticals, oil and gas.

A sand dune may look like a monolithic mass of sand, but there are multiple layers and structures within it

Nathalie Vriend

̽��ֱ��District

Sand dune

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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Galactic gas caused by colliding comets suggests mystery ‘shepherd’ exoplanet

fpjl2 — Thu, 06 Mar 2014 19:05:00 +0000

Astronomers exploring the disc of debris around the young star Beta Pictoris have discovered a compact cloud of carbon monoxide located about 8 billion miles (13 billion kilometers) from the star. This concentration of poisonous gas – usually destroyed by starlight – is being constantly replenished by ongoing rapid-fire collisions among a swarm of icy, comet-like bodies.

In fact, to offset the destruction of carbon monoxide (CO) molecules around the star, a large comet must be getting completely destroyed every five minutes, say researchers.

They suggest the comet swarm is most likely frozen debris trapped and concentrated by the gravity of an as-yet-unseen exoplanet.

This mystery ‘shepherd’ exoplanet – so-called for its capacity to corral the swarms of comets through its gravitational pull, like Jupiter in our own solar system – is likely to be about the size of Saturn.

"Detailed dynamical studies are now under way, but at the moment we think this shepherding planet would be around Saturn's mass and positioned near the inner edge of the CO belt," said Mark Wyatt, from Cambridge’s Institute of Astronomy, who proposed the shepherd model – currently the favoured hypothesis because it explains so many puzzling features of the Beta Pictoris disc.

"We think the Beta Pictoris comet swarms formed when the hypothetical planet migrated outward, sweeping icy bodies into resonant orbits."

Paradoxically, the presence of carbon monoxide – so harmful to humans on Earth – could indicate that the Beta Pictoris planetary system may eventually be a good habitat for life. If there is CO in the comets, then there is likely also water ice – meaning that the cometary bombardment this system’s planets are probably undergoing could also be providing them with life-giving water.

̽��ֱ��findings are published today in the journal Science Express.

̽��ֱ��clump was discovered when an international team of astronomers, led by ALMA-based ESO astronomer Bill Dent, along with Wyatt, used the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to map the millimeter-wavelength light from dust and carbon monoxide molecules in the disc surrounding Beta Pictoris, a star located about 63 light-years away and only 20 million years old.

Beta Pictoris is considered one of the best examples of a typical young solar system, and hosts one of the closest and brightest debris discs known – making it an ideal laboratory for studying the early development of planetary systems. ̽��ֱ��latest findings could help us understand what conditions were like during the formation of our own solar system.

Much of the carbon monoxide is concentrated in a single clump located about 8 billion miles (13 billion kilometers) from the star, or nearly three times the distance between the planet Neptune and the sun. ̽��ֱ��total amount of the gas observed exceeds 200 million billion tons – equivalent to about one-sixth the mass of Earth’s oceans, say researchers.

̽��ֱ��presence of all this gas is a clue that something interesting is going on because ultraviolet starlight breaks up CO molecules in about 100 years, much faster than the main cloud can complete a single orbit around the star. “So unless we are observing Beta Pictoris at a very unusual time, then the carbon monoxide we observed must be continuously replenished,” said Bill Dent, ESO astronomer based at ALMA and lead author on the paper.

̽��ֱ��researchers calculate that a large comet must be completely destroyed every five minutes, and only an unusually massive and compact swarm of comets could support such an astonishingly high collision rate.

"Although toxic to us, carbon monoxide is one of many gases found in comets and other icy bodies," said team member Aki Roberge, an astrophysicist at NASA’s Goddard Space Flight Center. "In the rough-and-tumble environment around a young star, these objects frequently collide and generate fragments that release dust, icy grains and stored gases."

Because we view the disc nearly edge-on, the ALMA data cannot determine whether the carbon monoxide belt has a single concentration of gas or two on opposite sides of the star. Further studies of the gas cloud's orbital motion will clarify the situation, but current evidence favors a two-clump scenario, which in turn points to a shepherding planet.

In our own solar system, Jupiter's gravity has trapped thousands of asteroids in two groups, one leading and one following it as it travels around the sun. A giant planet located in the outer reaches of the Beta Pictoris system likewise could corral comets into a pair of tight, massive swarms.

Astronomers have already directly imaged one giant exoplanet, Beta Pictoris b, with a mass several times greater than Jupiter, orbiting much closer to the star. While it would be unusual for a giant planet to form up to 10 times farther away, as required to shepherd the massive comet clouds, the hypothetical planet could have formed near the star and migrated outward as the young disc underwent changes. Indeed, this outward motion is needed to corral the comets.

A brief animation expanding on this can be viewed here.

If, however, the gas actually turns out to form a single clump, Wyatt’s recently graduated Cambridge PhD student Alan Jackson, also a co-author on the paper, suggested an even more violent alternative scenario. A crash between two Mars-sized icy planets about half a million years ago would account for the comet swarm, with frequent ongoing collisions among the fragments gradually releasing carbon monoxide gas.

Either way, Beta Pictoris clearly has a fascinating story to tell, say the scientists, one that could provide insight into the early development of our own solar system.

Latest research has uncovered a massive clump of carbon monoxide in a young solar system. ̽��ֱ��gas is the result of near constant collisions of icy comets – suggesting vast swarms of tightly packed comets in thrall to the gravitational pull of an as-yet-unseen exoplanet.

We think the Beta Pictoris comet swarms formed when the hypothetical planet migrated outward

Mark Wyatt

NASA's Goddard Space Flight Center/F.Reddy

At the outer fringes of the system, the gravitational influence of a hypothetical giant planet (bottom left) captures comets into a dense, massive swarm (right) where frequent collisions occur.

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Galactic ‘vapour trails’ uncovered in giant cluster

fpjl2 — Fri, 20 Sep 2013 09:44:57 +0000

Unusual gas filament ‘arms’ have been found in the central region of the Coma cluster, a large collection of thousands of galaxies located about 300 million light years from Earth - and one of the largest structures in the Universe held together by gravity.

These remarkably long arms – which bear resemblance to vast galactic vapour trails - glow in X-ray light, and tell astronomers about the collisions that took place between Coma and other galaxy clusters over the last billion years.

A team of astronomers from Cambridge and the Max Planck Institute discovered the enormous X-ray vapour trails – spanning at least half a million light years – in Coma by using data from NASA’s Chandra X-ray Observatory as well as ESA’s XMM-Newton. ̽��ֱ��elongated filaments of hot gas were revealed after enhancing the detail in Chandra X-ray images, shown in purple above.

Researchers think that these arms were most likely formed when smaller galaxy clusters had their hot gas stripped away while merging with the larger Coma cluster. This would have left a trail of superheated gas behind them similar to a jet leaving behind trails of water vapour as it moves across the sky.

Coma is an unusual galaxy cluster because it contains not one, but two giant elliptical galaxies near its centre. These two giant elliptical galaxies are probably the trace remains of each of the two largest galaxies that merged with Coma in the past. There are also other signs of past collisions and mergers that the researchers were able to uncover in the data.

̽��ֱ��newly discovered X-ray arms are thought to be about 300 million years old, and they appear to have a rather smooth shape. This gives researchers some clues about the conditions of the hot gas in Coma. Most theoretical models expect that mergers between clusters like those in Coma will produce strong turbulence, like ocean water that has been churned by passing ships. Instead, the smooth shape of these lengthy arms points to a rather calm setting for the hot gas in the Coma cluster, even after many mergers.

“Coma is like a giant cosmic train wreck where several clusters have collided with each other. We hadn’t expected that these rather delicate straight filaments would survive in that environment,” said lead author Dr Jeremy Sanders, who conducted much of the research whilst at Cambridge’s Institute of Astronomy alongside Professor Andrew Fabian.

“ ̽��ֱ��existence of these long straight structures appears to point towards the centre of the Coma cluster being a much calmer environment than we had expected.”

Elongated structures of hot gas found after enhancing the detail in images taken with the Chandra (pink) and on larger scales XMM-Newton (purple) X-ray observatories

Two of the arms appear to be connected to a group of galaxies located about two million light years from the centre of Coma. One or both of the arms connects to a larger structure seen in the XMM-Newton data, and spans a distance of at least 1.5 million light years. A very thin tail also appears behind one of the galaxies in Coma. This is probably evidence of gas being stripped from a single galaxy, in addition to the groups or clusters that have merged there.

Galaxy clusters are the largest objects held together by gravity in the universe. ̽��ֱ��collisions and mergers between galaxy clusters of similar mass are the most energetic events in the nearby universe. These new results are important for understanding the physics of these enormous objects and how they grow.

Large-scale magnetic fields are likely responsible for the small amount of turbulence that is present in Coma. Estimating the amount of turbulence in a galaxy cluster has been a challenging problem for astrophysicists. Researchers have found a range of answers, some of them conflicting, and so observations of other clusters are needed.

These new results on the Coma cluster, which incorporate over six days worth of Chandra observing time, appears in the latest issue of the journal Science.

Text adapted from a NASA Chandra press release

Astronomers have discovered enormous smooth shapes that look like vapour trails in a gigantic galaxy cluster. These ‘arms’ span half a million light years and provide researchers with clues to a billion years of collisions within the “giant cosmic train wreck” of the Coma cluster.

Coma is like a giant cosmic train wreck where several clusters have collided with each other. We hadn’t expected that these rather delicate straight filaments would survive in that environment

Jeremy Sanders

NASA Chandra

Revealed elongated filaments of hot gas found after enhancing the detail in Chandra X-ray images (purple), also showing the optical light galaxies in cluster (taken from the Sloan Digital Sky Survey)

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