̽��ֱ�� of Cambridge - fluid dynamics

Giant underwater waves affect the ocean’s ability to store carbon

sc604 — Thu, 16 Mar 2023 15:00:53 +0000

An international team of researchers, led by the ̽��ֱ�� of Cambridge, the ̽��ֱ�� of Oxford, and the ̽��ֱ�� of California San Diego, quantified the effect of these waves and other forms of underwater turbulence in the Atlantic Ocean and found that their importance is not being accurately reflected in the climate models that inform government policy.

Most of the heat and carbon emitted by human activity is absorbed by the ocean, but how much it can absorb is dependent on turbulence in the ocean’s interior, as heat and carbon are either pushed deep into the ocean or pulled toward the surface.

While these underwater waves are already well-known, their importance in heat and carbon transport is not fully understood.

̽��ֱ��results, reported in the journal AGU Advances, show that turbulence in the interior of oceans is more important for the transport of carbon and heat on a global scale than had been previously imagined.

Ocean circulation carries warm waters from the tropics to the North Atlantic, where they cool, sink, and return southwards in the deep ocean, like a giant conveyer belt. ̽��ֱ��Atlantic branch of this circulation pattern, called the Atlantic Meridional Overturning Circulation (AMOC), plays a key role in regulating global heat and carbon budgets. Ocean circulation redistributes heat to the polar regions, where it melts ice, and carbon to the deep ocean, where it can be stored for thousands of years.

“If you were to take a picture of the ocean interior, you would see a lot of complex dynamics at work,” said first author Dr Laura Cimoli from Cambridge’s Department of Applied Mathematics and Theoretical Physics. “Beneath the surface of the water, there are jets, currents, and waves – in the deep ocean, these waves can be up to 500 metres high, but they break just like a wave on a beach.”

“ ̽��ֱ��Atlantic Ocean is special in how it affects the global climate,” said co-author Dr Ali Mashayek from Cambridge’s Department of Earth Sciences. “It has a strong pole-to-pole circulation from its upper reaches to the deep ocean. ̽��ֱ��water also moves faster at the surface than it does in the deep ocean.”

Over the past several decades, researchers have been investigating whether the AMOC may be a factor in why the Arctic has lost so much ice cover, while some Antarctic ice sheets are growing. One possible explanation for this phenomenon is that heat absorbed by the ocean in the North Atlantic takes several hundred years to reach the Antarctic.

Now, using a combination of remote sensing, ship-based measurements and data from autonomous floats, the Cambridge-led researchers have found that heat from the North Atlantic can reach the Antarctic much faster than previously thought. In addition, turbulence within the ocean – in particular large underwater waves – plays an important role in the climate.

Like a giant cake, the ocean is made up of different layers, with colder, denser water at the bottom, and warmer, lighter water at the top. Most heat and carbon transport within the ocean happens within a particular layer, but heat and carbon can also move between density layers, bringing deep waters back to the surface.

̽��ֱ��researchers found that the movement of heat and carbon between layers is facilitated by small-scale turbulence, a phenomenon not fully represented in climate models.

Estimates of mixing from different observational platforms showed evidence of small-scale turbulence in the upper branch of circulation, in agreement with theoretical predictions of oceanic internal waves. ̽��ֱ��different estimates showed that turbulence mostly affects the class of density layers associated with the core of the deep waters moving southward from the North Atlantic to the Southern Ocean. This means that the heat and carbon carried by these water masses have a high chance of being moved across different density levels.

“Climate models do account for turbulence, but mostly in how it affects ocean circulation,” said Cimoli. “But we’ve found that turbulence is vital in its own right, and plays a key role in how much carbon and heat gets absorbed by the ocean, and where it gets stored.”

“Many climate models have an overly simplistic representation of the role of micro-scale turbulence, but we’ve shown it’s significant and should be treated with more care,” said Mashayek. “For example, turbulence and its role in ocean circulation exerts a control over how much anthropogenic heat reaches the Antarctic Ice Sheet, and the timescale on which that happens.”

̽��ֱ��research suggests an urgent need for the instalment of turbulence sensors on global observational arrays and a more accurate representation of small-scale turbulence in climate models, to enable scientists to make more accurate projections of the future effects of climate change.

̽��ֱ��research was supported in part by the Natural Environment Research Council (NERC), part of UK Research and Innovation (UKRI).

Reference:
Laura Cimoli et al. ‘Significance of Diapycnal Mixing Within the Atlantic Meridional Overturning Circulation.’ AGU Advances (2023). DOI: 10.1029/2022AV000800

Underwater waves deep below the ocean’s surface – some as tall as 500 metres – play an important role in how the ocean stores heat and carbon, according to new research.

Turbulence plays a key role in how much carbon and heat gets absorbed by the ocean, and where it gets stored

Laura Cimoli

Laura Cimoli/GLODAP

Map of depth-integrated anthropogenic carbon

̽��ֱ��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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Fluid mechanics and the energy transition

Anonymous — Fri, 14 Oct 2022 09:24:46 +0000

Decarbonisation of the energy system is the greatest challenge we face. At Cambridge’s Institute for Energy and Environmental Flows, world-leading researchers in fluid mechanics, thermodynamics and surface science are working to develop the solutions we need to replace fossil fuels and protect our planet.

Two-metre COVID-19 rule is ‘arbitrary measurement’ of safety

sc604 — Tue, 23 Nov 2021 15:54:09 +0000

A team of engineers from the ̽��ֱ�� of Cambridge used computer modelling to quantify how droplets spread when people cough. They found that in the absence of masks, a person with COVID-19 can infect another person at a two-metre distance, even when outdoors.

̽��ֱ��team also found that individual coughs vary widely, and that the ‘safe’ distance could have been set at anywhere between one to three or more metres, depending on the risk tolerance of a given public health authority.

̽��ֱ��results, published in the journal Physics of Fluids, suggest that social distancing is not an effective mitigation measure on its own, and underline the continued importance of vaccination, ventilation and masks as we head into the winter months in the northern hemisphere.

Despite the focus on hand-washing and surface cleaning in the early days of the pandemic, it’s been clear for nearly two years that COVID-19 spreads through airborne transmission. Infected people can spread the virus through coughing, speaking or even breathing, when they expel larger droplets that eventually settle or smaller aerosols that may float in the air.

“I remember hearing lots about how COVID-19 was spreading via door handles in early 2020, and I thought to myself if that were the case, then the virus must leave an infected person and land on the surface or disperse in the air through fluid mechanical processes,” said Professor Epaminondas Mastorakos from Cambridge’s Department of Engineering, who led the research.

Mastorakos is an expert in fluid mechanics: the way that fluids, including exhaled breath, behave in different environments. Over the course of the pandemic, he and his colleagues have developed various models for how COVID-19 spreads.

“One part of the way that this disease spreads is virology: how much virus you have in your body, how many viral particles you expel when you speak or cough,” said first author Dr Shrey Trivedi, also from the Department of Engineering. “But another part of it is fluid mechanics: what happens to the droplets once they’re expelled, which is where we come in. As fluid mechanics specialists, we’re like the bridge from virology of the emitter to the virology of the receiver and we can help with risk assessment.”

In the current study, the Cambridge researchers set out to ‘measure’ this bridge through a series of simulations. For example, if a person coughed and emitted a thousand droplets, how many would reach another person in the same room, and how large would these droplets be, as a function of time and space?

̽��ֱ��simulations used refined computational models solving the equations for turbulent flow, together with detailed descriptions of droplet motion and evaporation.

̽��ֱ��researchers found that there isn’t a sharp cut-off once the droplets spread beyond two metres. When a person coughs and isn’t wearing a mask, most of the larger droplets will fall on nearby surfaces. However, smaller droplets, suspended in the air, can quickly and easily spread well beyond two metres. How far and how quickly these aerosols spread will depend on the quality of ventilation in the room.

In addition to the variables surrounding mask-wearing and ventilation, there is also a high degree of variability in individual coughs. “Each time we cough, we may emit a different amount of liquid, so if a person is infected with COVID-19, they could be emitting lots of virus particles or very few, and because of the turbulence they spread differently for every cough,” said Trivedi.

“Even if I expel the same number of droplets every time I cough, because the flow is turbulent, there are fluctuations,” said Mastorakos. “If I’m coughing, fluctuations in velocity, temperature and humidity mean that the amount someone gets at the two-metre mark can be very different each time.”

̽��ֱ��researchers say that while the two-metre rule is an effective and easy-to-remember message for the public, it isn’t a mark of safety, given the large number of variables associated with an airborne virus. Vaccination, ventilation and masks – while not 100% effective – are vital for containing the virus.

“We’re all desperate to see the back of this pandemic, but we strongly recommend that people keep wearing masks in indoor spaces such as offices, classrooms and shops,” said Mastorakos. “There’s no good reason to expose yourself to this risk as long as the virus is with us.”

̽��ֱ��research team are continuing this research with similar simulations for spaces such as lecture rooms that can help assess the risk as people spend more time indoors.

Mastorakos is a Fellow of Fitzwilliam College, Cambridge. ̽��ֱ��research was supported in part by the Engineering and Physical Sciences Research Council (EPSRC), part of UK Research and Innovation (UKRI).

Reference:
Shrey Trivedi et al. ‘Estimates of the stochasticity of droplet dispersion by a cough.’ Physics of Fluids (2021). DOI: 10.1063/5.0070528

A new study has shown that the airborne transmission of COVID-19 is highly random and suggests that the two-metre rule was a number chosen from a risk ‘continuum’, rather than any concrete measurement of safety.

We strongly recommend that people keep wearing masks in indoor spaces - there’s no good reason to expose yourself to this risk as long as the virus is with us

Epaminondas Mastorakos

How droplets spread in a cough

Pedro de Oliveira

Visualisation of droplets from a cough

Yes

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

Yes

Algae use their ‘tails’ to gallop and trot like quadrupeds

sc604 — Tue, 03 May 2016 14:12:54 +0000

Long before there were fish swimming in the oceans, tiny microorganisms were using long slender appendages called cilia and flagella to navigate their watery habitats. Now, new research reveals that species of single-celled algae coordinate their flagella to achieve a remarkable diversity of swimming gaits.

When it comes to four-legged animals such as cats, horses and deer, or even humans, the concept of a gait is familiar, but what about unicellular green algae with multiple limb-like flagella? ̽��ֱ��latest discovery, published in the journal Proceedings of the National Academy of Sciences, shows that despite their simplicity, microalgae can coordinate their flagella into leaping, trotting or galloping gaits just as well.

Many gaits are periodic: whether it is the stylish walk of a cat, the graceful gallop of a horse, or the playful leap of a springbok, the key is the order or sequence in which these limbs are activated. When springboks arch their backs and leap, or ‘pronk’, they do so by lifting all four legs simultaneously high into the air, yet when horses trot it is the diagonally opposite legs that move together in time.

In vertebrates, gaits are controlled by central pattern generators, which can be thought of as networks of neural oscillators that coordinate output. Depending on the interaction between these oscillators, specific rhythms are produced, which, mathematically speaking, exhibit certain spatiotemporal symmetries. In other words, the gait doesn’t change when one leg is swapped with another – perhaps at a different point in time, say a quarter-cycle or half-cycle later.

It turns out the same symmetries also characterise the swimming gaits of microalgae, which are far too simple to have neurons. For instance, microalgae with four flagella in various possible configurations can trot, pronk or gallop, depending on the species.

“When I peered through the microscope and saw that the alga was performing two sets of perfectly synchronous breaststrokes, one directly after the other, I was amazed,” said the paper’s first author Dr Kirsty Wan of the Department of Applied Mathematics and Theoretical Physics (DAMTP) at the ̽��ֱ�� of Cambridge. “I realised immediately that this behaviour could only be due to something inside the cell rather than passive hydrodynamics. Then of course to prove this I had to expand my species collection.”

̽��ֱ��researchers determined that it is in fact the networks of elastic fibres which connect the flagella deep within the cell that coordinate these diverse gaits. In the simplest case of Chlamydomonas, which swims a breaststroke with two flagella, absence of a particular fibre between the flagella leads to uncoordinated beating. Furthermore, deliberately preventing the beating of one flagellum in an alga with four flagella has zero effect on the sequence of beating in the remainder.

However, this does not mean that hydrodynamics play no role. In recent work from the same group, it was shown that nearby flagella can be synchronised solely by their mutual interaction through the fluid. There is a distinction between unicellular organisms for which good coordination of a few flagella is essential, and multicellular species or tissues that possess a range of cilia and flagella. In the latter case, hydrodynamic interactions are much more important.

“As physicists our instinct is to seek out generalisations and universal principles, but the world of biology often presents us with many fascinating counterexamples,” said Professor Ray Goldstein, Schlumberger Professor of Complex Physical Systems at DAMTP, and senior author of the paper. “Until now there have been many competing theories regarding flagellar synchronisation, but I think we are finally making sense of how these different organisms make best use of what they have.”

̽��ֱ��findings also raise intriguing questions about the evolution of the control of peripheral appendages, which must have arisen in the first instance in these primitive microorganisms.

This research was supported by a Neville Research Fellowship from Magdalene College, and a Senior Investigator Award from the Wellcome Trust.

Reference:
Kirsty Y. Wan and Raymond E. Goldstein. ‘Coordinated beating of algal flagella is mediated by basal coupling.’ PNAS (2016). DOI: 10.1073/pnas.1518527113

Species of single-celled algae use whip-like appendages called flagella to coordinate their movements and achieve a remarkable diversity of swimming gaits.

As physicists our instinct is to seek out generalisations and universal principles, but the world of biology often presents us with many fascinating counterexamples.

Raymond Goldstein

Kirsty Y. Wan & Raymond E. Goldstein

Microscope images showing two species of algae which swim using tiny appendages known as flagella

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

Yes

How the kettle got its whistle

tdk25 — Thu, 24 Oct 2013 07:28:57 +0000

It may come as a surprise to some, but in all the years that people have been brewing tea, no-one has ever quite been able to work out why kettles whistle. In a basic sense, of course, the reasons are pretty clear, but the physical source of the noise and the specific reason for the whistling sound have both remained elusive.

Elusive, that is, until now. Writing in the October issue of the academic journal, ̽��ֱ��Physics Of Fluids, two Cambridge ̽��ֱ�� researchers claim to have solved the conundrum, and in the process developed the first accurate model for the whistling mechanism inside a classic stove kettle.

Perhaps reassuringly for those who never felt that this was a significant problem, the ramifications reach far beyond kettles themselves. Using the knowledge gained from the study, researchers could potentially isolate and stop similar, but far more irritating whistles - such as the noise made when air gets into household plumbing, or damaged car exhausts.

“ ̽��ֱ��effect we have identified can actually happen in all sorts of situations - anything where the structure containing a flow of air is similar to that of a kettle whistle,” Ross Henrywood, from the ̽��ֱ�� of Cambridge Department of Engineering, and the study’s lead author, explained.

“Pipes inside a building are one classic example and similar effects are seen inside damaged vehicle exhaust systems. Once we know where the whistle is coming from, and what’s making it happen, we can potentially get rid of it.”

Henrywood carried out the research for his fourth-year project as part of his engineering degree, under the guidance of his supervisor, Dr Anurag Agarwal, a lecturer in aeroacoustics. Drawing on previous research by Agarwal, which identified the source of noise in jet engines, the pair were able to show how sound is created inside a kettle as the “flow” of steam comes up the spout.

Having identified the source of the sound itself, they were then able to pinpoint two separate mechanisms, which not only create the sound but specifically cause a kettle to whistle, rather than making the rushing noise a flow might create in other household items, such as a hairdryer.

A basic kettle whistle consists of two plates, positioned close together, forming a cavity. Both plates have a hole in the middle, which allows steam to pass through.

Although the sound of a kettle is understood to be caused by vibrations made by the build-up of steam trying to escape, scientists have been trying for decades to understand what it is about this process that makes sound.

As far back as the 19th century, John William Strutt, 3rd Baron Rayleigh and author of the foundational text, ̽��ֱ��Theory Of Sound, was trying to explain it. In the end, he posited an explanation that Henrywood and Agarwal have proven to be flawed. And Lord Rayleigh was forced to concede that “much remains obscure as regards the manner in which the vibrations are excited.”

Henrywood and Agarwal started by making a series of slightly simplified kettle whistles, then tested these in a rig, in which air was forced through them at various speeds and the sound they produced was recorded.

This enabled them to plot the frequency and amplitude of the sound, and the data was then subjected to a non-dimensional analysis, effectively a set of calculations using numbers without any units, which allowed them to identify trends in the data. Finally, they used a two-microphone technique to determine frequency inside the spout.

Their results showed that, above a particular flow speed, the sound itself is produced by small vortices – regions of swirling flow – which at certain frequencies can produce noise.

As steam comes up the kettle’s spout, it meets a hole at the start of the whistle, which is much narrower than the spout itself. This contracts the flow of steam as it enters the whistle and creates a jet of steam passing through it. ̽��ֱ��steam jet is naturally unstable, like the jet of water from a garden hose that starts to break into droplets after it has travelled a certain distance. As a result, by the time it reaches the end of the whistle, the jet of steam is no longer a pure column, but slightly disturbed.

These instabilities cannot escape perfectly from the whistle and as they hit the second whistle wall, they form a small pressure pulse. This pulse causes the steam to form vortices as it exits the whistle. These vortices produce sound waves, creating the comforting noise that heralds a forthcoming cup of tea.

Henrywood and Agarwal also explain why this effect makes a whistle, rather than another noise, by showing that the mechanism is similar to that seen in an organ pipe or flute. A specific frequency dominates among the sound waves because the note is determined by the size and shape of the opening, and the length of the spout. ̽��ֱ��longer the spout, the lower the note will be.

̽��ֱ��researchers also found, however, that kettles will whistle below the flow-rate at which the vortices emerge. Just as the water begins to boil, they found an entirely different mechanism, which also makes a sound. ̽��ֱ��difference was that the tone at this stage was fixed at one frequency.

“ ̽��ֱ��fixed frequency was intriguing and not something that we had expected to see,” Henrywood said. “We eventually established that below a particular flow rate the whistle behaved like a Helmholtz resonator – the same mechanism which gives you a tone when you blow over an empty bottle.”

When air is blown over the open neck of a bottle, the Helmholtz resonator mechanism causes sound to radiate from the neck. ̽��ֱ��air just inside the neck is bouncing up and down – the air in the main body of the bottle being compressed and released each time like a spring.

For the kettle, the spring is the air inside the whistle, while the air within the whistle opening reverberates like the air in the neck of a bottle. “In a kettle, of course, the air is blown through, rather than over, the neck – the effect is similar to whistling by mouth,” Henrywood added. “In some kettles, both these mechanisms are happening. Because our study enables us to assess the mechanisms in action, we can potentially make modifications to stop the noise – if we want to.”

Henrywood and Agarwal are now working on a project to make quieter high-speed hand-dryers, by looking at how the jet of air released by these devices creates noise. Their paper on kettles - ̽��ֱ��Aeroacoustics Of A Steam Kettle – can be found in the October issue of ̽��ֱ��Physics Of Fluids.

For more information about this story, please contact Tom Kirk, Tel: +44 (0)1223 332300, thomas.kirk@admin.cam.ac.uk

Researchers have finally worked out where the noise that makes kettles whistle actually comes from – a problem which has puzzled scientists for more than 100 years.

Once we know where the whistle is coming from, and what’s making it happen, we can potentially get rid of it

Ross Henrywood

Kaitlin Foley, via Flickr

Tea kettle whistle. Homepage banner image: Dwayne Bent (Att-SA)

This work is licensed under a Creative Commons Licence. If you use this content on your site please link back to this page.

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