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A new model could help stall shifting sand dunes, protecting infrastructure and ecosystems

cmm201 — Tue, 26 Oct 2021 16:33:55 +0000

̽��ֱ��team’s experiment – which featured mock-up obstacles of varying size and shape – shows that large obstacles are the most effective at halting the migration of a dune, especially when they are ridge-shaped, like a wall, rather than smooth and cylindrical, like a pipeline.

̽��ֱ��model, published in Physical Review Fluids, is the first to describe interactions between sand dunes and obstacles.

By analysing how currents are deflected in the presence of an obstacle, they were also able to develop an efficient, data-driven tool that aims to forecast how a dune will interact with its surroundings.

̽��ֱ��research could help in the design of more effective barriers that can, for instance, stop sand dunes from invading agricultural land. It could also be used to protect sand dunes and their unique ecosystems from damage.

“Moving sand dunes impact people and their livelihoods directly; across the world and in a range of environments,” said lead author Karol Bacik, who conducted the experiments as a PhD student in Cambridge’s Department of Applied Mathematics and Theoretical Physics (DAMTP). “By revealing the physics behind dune-obstacle interactions, this work gives us the guiding principles we need to divert or halt dunes – mitigating damage.”

As deserts continue to expand, sand dunes pose an increasing risk to the built environment: swallowing up roads and houses whole as they engulf the land. In a similar way, dunes on the seabed can block shipping routes and even compromise the safety of underwater cables and pipelines.

But in certain locations, rather than stopping the sand dune moving, it can be preferable for a dune to move through an obstacle as quickly as possible. Take pipelines, for instance, which can be damaged if buried under the weight of a stationary dune for too long.

Bacik’s work shows how obstacles of varying design should be selected to fit the desired outcome, “If you want the dune the pass, make the obstacle as smooth and rounded as possible – if you want to halt it, make it as sharp as possible,” said Bacik.

̽��ֱ��research is one of a series of experiments Nathalie Vriend - who is based jointly at Cambridge’s BP Institute for Multiphase Flow, the Department of Earth Sciences and DAMTP - has been leading experiments to understand why sand dunes move like they do. “Sand is fascinating: pour some from your hand and it flows like a liquid….then, when it lands, it makes a solid heap,” she said. “But toss it into the air and it blows along like a gas. Its ability to morph between states like this makes it a real challenge to model how sand moves.”

̽��ֱ��team made a ring-shaped tank to contain their sand dunes, which can travel in circuits, almost like a ‘merry-go-round’. By submerging the dunes in water, and disturbing the flow with paddles, they were able to reconstruct how the dunes are moved by water currents. They then put obstacles of varying size and shape in the path of the moving dunes to observe their effect.

“We can see evidence of sand dunes moving right in front of us, but what’s fascinating is their movement is all down to the hidden flow of water currents or wind patterns,” said Bacik, “You can’t see the curling tails of turbulence until you use a visualisation technique…and it's only then, once you have done a fluid analysis, that you can really understand why sand dunes move like they do.”

̽��ֱ��researchers’ ultimate goal is to model sand dune movements in more complex and realistic, three-dimensional, landscapes in addition to exploring the wind-blown dunes found in deserts. Ideally, they would like to be able to pinpoint a location on a map, input information on weather, air or water currents, and predict whether a dune would pass over a specific obstacle. Although these numerical simulations would be more complex, their new experiments serve as an important validation benchmark for continued exploration.

Reference:
Bacik, KA, Canizares, P, Caulfield, CP, Williams, MJ, Vriend, NM, Dynamics of migrating sand dunes interacting with obstacles, Physical Review Fluids, DOI: 10.1103/PhysReFluids.00.004300

PBS Terra Documentary, 'What makes These Dunes Sing': release date 20 October

Cambridge scientists have used downscaled laboratory models to show how sand dunes move through a landscape, revealing the conditions that determine whether they will pass through hurdles in their path – like pipelines or walls -- or get stopped in their tracks.

We can see evidence of sand dunes moving right in front of us, but what’s fascinating is their movement is all down to the hidden flow of water currents or wind patterns

Karol Bacik

What Makes These Dunes Sing? (ft. @It's Okay To Be Smart)

Pixabay

Desert under blue sky

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – as here, on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

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Sand dunes can ‘communicate’ with each other

sc604 — Tue, 04 Feb 2020 01:00:00 +0000

Using an experimental dune ‘racetrack’, the researchers observed that two identical dunes start out close together, but over time they get further and further apart. This interaction is controlled by turbulent swirls from the upstream dune, which push the downstream dune away. ̽��ֱ��results, reported in the journal Physical Review Letters, are key for the study of long-term dune migration, which threatens shipping channels, increases desertification, and can bury infrastructure such as highways.

When a pile of sand is exposed to wind or water flow, it forms a dune shape and starts moving downstream with the flow. Sand dunes, whether in deserts, on river bottoms or sea beds, rarely occur in isolation and instead usually appear in large groups, forming striking patterns known as dune fields or corridors.

It’s well-known that active sand dunes migrate. Generally speaking, the speed of a dune is inverse to its size: smaller dunes move faster and larger dunes move slower. What hasn’t been understood is if and how dunes within a field interact with each other.

“There are different theories on dune interaction: one is that dunes of different sizes will collide, and keep colliding, until they form one giant dune, although this phenomenon has not yet been observed in nature,” said Karol Bacik, a PhD candidate in Cambridge’s Department of Applied Mathematics and Theoretical Physics, and the paper’s first author. “Another theory is that dunes might collide and exchange mass - sort of like billiard balls bouncing off one another - until they are the same size and move at the same speed, but we need to validate these theories experimentally.”

Now, Bacik and his Cambridge colleagues have shown results that question these explanations. “We’ve discovered physics that hasn’t been part of the model before,” said Dr Nathalie Vriend, who led the research.

Most of the work in modelling the behaviour of sand dunes is done numerically, but Vriend and the members of her lab designed and constructed a unique experimental facility which enables them to observe their long-term behaviour. Water-filled flumes are common tools for studying the movement of sand dunes in a lab setting, but the dunes can only be observed until they reach the end of the tank. Instead, the Cambridge researchers have built a circular flume so that the dunes can be observed for hours as the flume rotates, while high-speed cameras allow them to track the flow of individual particles in the dunes.

Bacik hadn’t originally meant to study the interaction between two dunes: “Originally, I put multiple dunes in the tank just to speed up data collection, but we didn’t expect to see how they started to interact with each other,” he said.

̽��ֱ��two dunes started with the same volume and in the same shape. As the flow began to move across the two dunes, they started moving. “Since we know that the speed of a dune is related to its height, we expected that the two dunes would move at the same speed,” said Vriend, who is based at the BP Institute for Multiphase Flow. “However, this is not what we observed.”

Initially, the front dune moved faster than the back dune, but as the experiment continued, the front dune began to slow down, until the two dunes were moving at almost the same speed.

Crucially, the pattern of flow across the two dunes was observed to be different: the flow is deflected by the front dune, generating ‘swirls’ on the back dune and pushing it away. “ ̽��ֱ��front dune generates the turbulence pattern which we see on the back dune,” said Vriend. “ ̽��ֱ��flow structure behind the front dune is like a wake behind a boat, and affects the properties of the next dune.”

As the experiment continued, the dunes got further and further apart, until they form an equilibrium on opposite sides of the circular flume, remaining 180 degrees apart.

̽��ֱ��next step for the research is to find quantitative evidence of large-scale and complex dune migration in deserts, using observations and satellite images. By tracking clusters of dunes over long periods, we can observe whether measures to divert the migration of dunes are effective or not.

Reference:
Karol A. Bacik et al. ‘Wake-induced long range repulsion of aqueous dunes.’ Physical Review Letters (2020). DOI: 10.1103/PhysRevLett.124.054501

Even though they are inanimate objects, sand dunes can ‘communicate’ with each other, researchers have found. A team from the ̽��ֱ�� of Cambridge has found that as they move, sand dunes interact with and repel their downstream neighbours.

We’ve discovered physics that hasn’t been part of the model before

Nathalie Vriend

Sand dunes can ‘communicate’ with each other.

Karol Bacik, Nathalie Vriend

Sand dune

Yes

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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