The Science of Turbulence

17 October 2010
Presented by Ben Valsler, Dave Ansell.

It's a bumpy ride on this week's Naked Scientists, as we explore the science of turbulence. We'll find out what turbulence is and why it needs some of the most powerful computers in the world to study it. We'll discover how puffs of water can terminate turbulence in tubes, and how convection keeps the temperature just right in new buildings. In the news this week, we hear about a potential new super-vaccine for TB, the comet that turned into an asteroid and the prospect of new low-cost gold-free leads for your hi fi. Plus, in Question of the Week, we find out why some people prefer not to be backwards in travelling forwards...

In this episode

01:35 - New Vaccine Could Protect Against Resistant TB

A new vaccine against tuberculosis could not only boost the effectiveness of the existing childhood BCG vaccine, but could also offer protection against multidrug resistant forms of the...

New Vaccine Could Protect Against Resistant TB

A new vaccine against tuberculosis could not only boost the effectiveness of the existing childhood BCG vaccine, but could also offer protection against multidrug-resistant forms of the disease.

TB is a global problem, with the World Health Organisation estimating that almost 1 billion people will be infected by 2020.  Multidrug-resistant strains of the bacteria are also becoming an increasing problem.

Scale bar, 20 micrometers. See figure 5 in the manuscript for more information.

Image courtesy of Science/AAAS (c) Image courtesy of Science/AAAS' alt='New TB vaccine shows promise' >The standard vaccine, the bacillus Calmette-Guérin or BCG that most of us will have had at some point, is very good at protecting against severe disease in children, but actually offers little protection for adults.  Attempts to boost the immunity by re-administering the BCG have not been successful, so any new vaccine would have to boost this immunity, as well as offer protection against drug resistant strains.

This new vaccine, which has been tested and found to be effective in a range of animals, consists of four proteins joined together along with a chemical known as an adjuvant, which helps to create an immune response.  Each of these proteins has been shown to give partial protection against TB on their own but as there are so many different strains of TB out there, no single protein is enough for a vaccine.  By combining proteins into one super-molecule, the vaccine offers protection against a range of different strains of Mycobacterium tuberculosis, the bacterium responsible for TB, including a strain known to be resistant to multiple drugs.

In guinea pigs that had previously been immunised with a short-term BCG, the new vaccine not only offered its own protection, but also stimulated the release of immune components that were originally activated by the BCG.  This makes it a good candidate for boosting immunity from the BCG, which has been given to millions of people worldwide since its first use in humans back in 1921.

03:28 - The Comet that Never Was

In a case of cosmic forensics that could rival even an episode of CSI, scientists have pinpointed a moment 18 months previously when two asteroids collided...

The Comet that Never Was

In a case of cosmic forensics that could rival even an episode of CSI, scientists have pinpointed a moment 18 months previously when two asteroids collided.

In January 2010, the LINEAR (Lincoln Near Earth Asteroid Research) study, which is a robotic scan of the sky, picked up a strange object in the asteroid belt out beyond Mars.  Dubbed P/2010 A2, it had a long comet-like tail but, oddly, no central nucleus (body). And, orbiting as it does amongst the asteroid belt, it's also in the wrong place and moving the wrong way to fit the classical cometary description.

Asteroid collisionHowever, the mystery has now been solved in two stunning papers presented this week in the journal Nature.  In one, University of California Los Angeles astronomer David Jewitt and his colleagues describe the results of turning the Hubble space telescope on the odd object, whilst the second paper, by Max Planck Institute for Solar System Reserach scientist Colin Snodgrass, provides an off-world perspective by imaging P/2010 A2 from outer space using the Rosetta probe.  By comparing these two views it's possible to reconstruct the shape and distribution of the tail and of the object itself.

What this reveals is that the object is about 120 metres across and streaming in its wake is a dusty cloud 200,000 km long and made of gravel particles measuring millimetres to centimetres across.  This confirms a case of mistaken cosmic identity, because P/2010 A2 isn't a comet at all but an asteroid that has experienced a recent close encounter with another of its own kind, a smaller 3-5m diameter asteroid travelling several miles per second in the opposite direction!

Under the influence of light from the Sun, the team have found, the tail is spreading out, with the smaller particles being pushed further than the heavier ones.  By measuring how far the particles have moved, it's therefore possible to wind back the cosmic clock to pinpoint exactly when the collision between the two objects must have taken place.

Incredibly, unlike most of the collisions in the asteroid belt, which we tend to this of as happening millions of year ago, this shows that the impact occurred on February 10th 2009, give or take a week!

These observations are important because we need to know where the dust in the Solar System comes from, including how much of it originates from colliding asteroids rather than outgassing comets.  According to David Jewitt, "we can also apply this knowledge to the dusty debris discs around other stars, because these are thought to be produced by collisions between unseen bodies in the discs. Knowing how the dust was produced will yield clues about those invisible bodies..."

07:20 - All that Contacts doesn't need to be Gold

Research is discovering cheaper alternatives to gold plated connectors...

All that Contacts doesn't need to be Gold

More and more of our lives is becoming dependent on electronics, and that electronics is dependent on wires and cables. A cable needs a plug and producing a good contact on a plug is quite challenging. The problem is that you want to make a connector out of a metal which is strong, conductive and resistant to wear, and of course cheap, however all the metals that fulfil these constraints, like copper, brass, etc. will oxidise in air. This wouldn't be a problem in itself, but the oxides are insulating, so you cover your nice conducting contact with an insulating layer. The standard solution to this problem is to cover the contact in a thin layer of a noble metal which doesn't oxidise like gold. The problem is of course that gold is very rare and so its expensive, and because of historical reasons it is used as a secure investment when markets are feeling insecure, so at the moment it is even more expensive than usual.

A 1Kg Gold KeyWhilst it is not possible to stop cheap base metals corroding and oxidising, Mark Aindow and colleges at the university of Connecticut have been approaching the problem from the opposite direction. They have been trying to make the oxide more conductive so it doesn't matter. They have been using a variety approaches, to do this, alloying the original metal with one that has a conductive oxide so there are some scales of conductive oxide on the surface, and they have been adding metals to the alloy which effectively dope the oxide, adding or removing electrons allowing current to flow.

The results are promising, they have increased the conductivity of copper contacts by a factor of 3 by adding lanthanum, Iron by a factor of over 200 by adding Vanadium and adding ruthenium to nickel improves it by a factor of about 300, so that the contact resistance only gets 20-30 times worse after a thousand hours in an oxidising atmosphere, rather than over 10000 times worse.

They are very encouraged by this result as they are only using a few 2 metal alloys, and expect further improvements with more work. This approach has the other advantage that there is no problem with the surface coating rubbing off, so in the future cables might not have to be gold plated - though I am not sure it will stop the HiFi cable manufacturers.

09:59 - Cold Gas Fed Early Galaxies

Observations with the Very Large Telescope have shed light on how early galaxies grew, by funnelling cold primordial gas into their core...

Cold Gas Fed Early Galaxies

Observations with the Very Large Telescope have shed light on how early galaxies grew, by funnelling cold primordial gas into their core.

Giovanni Cresci, from the Arcetri Astrophysical Observatory, Florence, along with colleagues across Europe, used SINFONI (or Spectrograph for INtegral Field Observations in the Near Infrared) to observe three distant galaxies that formed only 2 billion years after the big bang.  Using near-infrared spectroscopy, they were able to map the distribution of elements throughout these galaxies.

Artistic impression of gas accretion on a high redshift galaxy.Galaxies are thought to grow through a process of collision and merging - smaller galaxies colliding and becoming one, larger galaxy.  However, this doesn't account for all galactic growth.  The three galaxies observed in this study showed very regular rotation patterns, as you would only expect to see in galaxies with little or no history of collision.

These observations were looking for gradients in "metallicity" - the abundance of elements heavier than hydrogen or helium.  "Modern" galaxies tend to have high metallicity in their central regions, with fewer heavy elements towards the edges.  However, these galaxies showed the opposite gradient - lower metallicity in the central, star forming regions, getting higher towards the edges.

Writing in Nature, the authors argue that this points to the "cold flow" model of galaxy growth, in which cold, primordial gas fresh from the big bang, lacking heavy elements, is funnelled in to the centre of the galaxy, fuelling star formation.

11:35 - The Rolls-Royce Science Prize

This week, the winner of the Rolls-Royce Annual Science Prize was announced during a special ceremony held at the science museum in London. Chris Smith was there to hear who won...

The Rolls-Royce Science Prize
with Prof. Rick Parker, Vaughan Lewis & Neil Glover, Rolls-Royce, Robert Aspden, Teesdale School

This week, the winner of the Rolls-Royce Annual Science Prize was announced during a special ceremony held at the science museum in London.  Chris Smith was there to hear who won...

James Dyson -   In 2010, Japan filed 330,000 patents, America 240,000, Britain 17,000.  Other countries are now dwarfing our technology output and produce far more engineers than we do.

Chris -   The numbers make quite sobering listening, don't they?  But the main point that cyclonic vacuum cleaner inventor Sir James Dyson was making during his keynote speech at this year's Rolls-Royce Science Prize Award Ceremony is that if we're to keep British engineering open for business and internationally competitive, then we desperately need to invest in the education and nurturing of the scientists and engineers of tomorrow.  It's a view that's shared by many leading industrialists and specialist manufacturers, including Rolls-Royce themselves as the group's director of research and development, Professor Rick Parker explains.

Rolls-Royce Trent 900 on the prototype Airbus A380. This aircraft carries four.Rick -   We're very worried about the quality of science teaching and the enthusiasm for science amongst young people today.  They weren't going into science courses, they're often being put off science at a very young age, so there was no chance of them going on to university to do science or engineering because they just haven't done the right subjects in the run up to leaving school.

Chris -   Rick Parker.  To tackle this problem, the company have set up a prize, targeting teachers.  Vaughan Lewis

Vaughan -   The Science Prize is an annual competition we've been running for teachers for the past 6 years.  It was launched on our anniversary year and the idea is we asked teachers how they would improve science education in their school or college with £6,000 from Rolls-Royce.  We worked through the science learning centre network to get those entries and each year, we get from 1,500 to 2000 schools that put an entry in.  From those, the 50 best is selected to win £1,000 as a short list and from that short list, we choose 9 finalist schools, and those schools receive a further £5,000 to go ahead with their project ideas over the following academic year.

Chris -   Why did you think there was a need to do this?

Vaughan -   At Rolls-Royce, we are very committed to ensuring that the next generation of students coming through will have the right skills, the right understanding, the right knowledge to be able to work in companies such as Rolls Royce.  High-tech and high value added companies.  So they've got to have an understanding of the basic science behind things, and be enthusiastic about it and want to go on and study at degree level and beyond, so that they can come and work for us, or want to come and work for us through apprenticeships and really get their hands on with real science and engineering.

Chris -   It's telling though isn't it, if a company like Rolls-Royce has to start putting together prizes to try and stimulate what many people would argue ought to be going on in schools anyway.  That's what schools are for, to try and get people interested in sciences and development so that Britain carries on as a manufacturing nation.

Vaughan -   What I say about that is - I mean, the teachers do a great job, there's a lot of very good teachers out there, encouraging a lot of students to do this, but what we were trying to do with this money is allow them to do something above and beyond what they normally do.  And so, with £6,000, if you are in a primary school, that's a lot of money.  Our winners last year was a primary school and they received a further £15,000 pounds from us and so they received £21,000 pounds from us.  We spoke to the science coordinator of that school.  His budget for the whole school was £700 for the following year.  So, we're able, through what we are doing is just to give them a big boost and allow them to do more than they would do, just to really raise the profile of science and engineering in the school and make it fundamental to what the people are studying.

Chris -   Vaughan Lewis.  So that's the theory, but what about in practice?  Well, here's this year's winner to tell us.

Robert -   My name is Robert Aspden.  I'm a science teacher working at Teesdale School.  I run a club called the STEM club, that's for Science, Technology, Engineering, and Maths.  It's supposed to encourage students to want to take on those careers in the future because England and Great Britain are getting behind a little bit with that and we're trying to make sure that doesn't happen and we stay the great nation for engineers that we are.  So the Rolls-Royce prizes allowed my club to push the limits of what the students could achieve.

Chris -   What was the project you did that won you the prize?

Robert -   I had the students designing, building and researching enrichment devices for a captive group of primates, some mandrills at Chester Zoo.  There's a big issue at the moment about zoos and the lifestyle that animals have, so we were setting about trying to encourage and develop the lives of these animals to stop them from going insane in the captive situations.  So we work with Chester Zoo who do lots of work with their animals, trying to encourage them to work for their food, and prevent these insanity behaviours that can develop.

Chris -   So what did the students actually have to do and what do you think they learned from doing this?

Robert -   The first part of the project, we visited the zoo so they got to see the animals and we have them studying the behaviours of the animals, so that then when they went on to design the feeders that we made, they actually made them link to the animals.  So, after we did the research using obviously the internet and other resources, they had a design phase where they designed and built the feeders.  We then gave those devices to Chester Zoo and a former colleague of mine who works for Salford University is doing an extended longitudinal study on whether we have or haven't actually benefited the animals because we wanted to prove, scientifically, that we have actually enriched their lives, rather than just say, we built these toys, we gave in to them, we've done our job, we wanted to actually prove that.

Chris -   What about the children who took part in the study.  How old were the students and what do you think they got out of it?

Robert -   Well there's two parts to that answer, I suppose.  The club is a key stage 3 club, so that's students of ages 11 up to about 14.  The benefits to them was to do things like, just encourage their thinking behaviour, their team working, their skills about technology, and science, so they can see how all that actually links together in an applied sense.

Chris -   What about in terms of the long term goal for Rolls-Royce? I've spoken with Rolls-Royce, they tell me that their aim is to try to get teachers like you to stimulate students to become the engineers of tomorrow.  Are you seeing evidence that the students that you have got involved in this project are going to go in to research to benefit Britain in the future?

Robert -   As part of the project, we actually did some analyses through questionnaires where we asked the students their opinion of STEM related subjects, STEM careers, and whether they were interested in moving in to those areas.  Quite a lot of them said, as a result of this project, we'd either encouraged them to take on STEM related subjects at A-level, possibly university.  There was definitely a positive relationship in the number of students who then thought they would actually be interested in careers in that.  And there were a number of students who actually said, because we've done something unusual looking at primates and biology, they didn't realise just how much engineering could be related to that side of science as well, so they were quite interested in doing that too.

Chris - Science teacher at Teesdale School in Durham, that was Robert Aspden who won this year's Rolls-Royce Science Prize.  There are more details about the prize on the web at science.rolls-royce.com.

So that's the schools side of the equation, but what about higher level training that will turn university students into the specialist engineers and materials scientists of tomorrow?  Well, in the last 12 months, Rolls-Royce have also launched a multi-million pound initiative called the strategic partnership, which aims to do just that.

Neil -   I'm Neil Glover, I'm a materials scientist at Rolls-Royce, and I'm responsible for the content and execution of the research programme within the company.  The strategic partnership is a partnership between Rolls-Royce and the EPSRC, the Engineering and Physical Sciences Research Council, that enables us to fund research in our preferred universities within the UK, on a whole range of materials science topics that underpins all of our technology going in to engines.

Chris -   People might say "Rolls-Royce is a big company, why doesn't it fund it's own research?"  Why do you have to work with universities for that?

Neil -   Well the universities enable us to provide a level of deep independent research on the more academic aspects of materials science that can then underpin the work that we do in company to deploy that technology into engines.  So it's the freedom for the academics to think, to explore, to check out new technologies, and to investigate problems and detailed issues of materials understanding that we just simply don't have time for in the day to day business.

Chris -   So looking at the nuts and bolts of how the partnership works, is this just a research exercise or is the aim here also to try to get people in a position where they could then go on to have progression in Rolls-Royce if they chose to do so?

Neil -   It's absolutely that.  Rolls-Royce is very much dependent on the recruitment of highly skilled materials scientists, and traditionally that has always come largely from our own internal supply chain, through the universities and through the research base.  And so the strategic partnership, as much as it develops technology, also develops people who we can recruit into the company, or who can go into academia.

21:56 - Planet Earth - Plastics on the Beach

Walk along any beach almost anywhere in the world and you'll find plastics washed up on the shore. From plastic bags to lighters, bottle tops to flip-flops. Plastics have even turned...

Planet Earth - Plastics on the Beach
with David Barnes, British Antarctic Survey

Walk along any beach almost anywhere in the world and you'll find plastics washed up on the shore.  From plastic bags to lighters, bottle tops to flip-flops.  Plastics have even turned up on the coast of Antarctica!  But it's not only the visual effect of this human detritus that's a problem.  Plastics actually carry pollutants and even life around the globe, sometimes having serious consequences.  Planet Earth podcast presenter Richard Hollingham joined David Barnes from the British Antarctic Survey on the Pebble Beach in the wind and rain sadly at Cley in North Norfolk...

Plastic BagsDavid -   So we're going to walk along two strandlines.  One, the storm line where the last time there was a big storm there's deposited lots of natural debris, but also man-made things, and along the strandline from the last big tide. And we're going to walk along and look at some of the more persistent items of debris particularly plastic, and see what it tells us about the ocean, far away from these shores.

Richard -   So we've got here a plastic bag and I think we can probably tell from the green lettering which supermarket that's from.  It's torn and it's gone translucent, but it's still retained its essential plastic bagness.

David -   Yeah.  Plastic bags are made of very, very thin plastics, so they break down relatively easily with salt spray and UV light, as long as they stay in the top layer.  But even this could've travelled quite some distance and we can see that looking at it closely, you can see all sorts of things have started to get a grip on top of it, including foraminiferans.  So life has started to colonise this plastic bag.

Richard -   That's amazing.  You've also got down here - I think this is part of the heel of a shoe.

David -   What will happen with lots of these materials is when it starts to get broken up, the surface has got a very good texture for settlement because it slows down the water over it.  Its boundary effect will be slightly stronger than the smooth pebbles and other smooth things that are typically floating.  So life can get a grip and then it can be carried around.  But it's not just life.  These plastics will absorb all sorts of things such as toxic chemicals and transport those around as well.  And so, various groups over the UK and elsewhere have been studying what plastics can carry, how far they're carrying them, and what sort of effects they have.

FlotsamRichard -   And that for you is almost a bigger issue than that they're not aesthetically pleasing to see on a beach.  Here, we've got a green bottle top, there's a little bit of string here, the plastic bag, but it's the fact that not only these spread toxic chemicals in the environment.  They carry life around the environment.

David -   Yeah and we can see on this piece of plastic twine that we've actually got two different species of hydroids, one on the base there and another one halfway up, and they're actually reproductively active.  They'll be releasing larvae, so this is not just transporting adults around.  These will be producing larvae that will settle wherever this goes.

Richard -   So these tiny little - almost twig-like projections on the side, they're alive and so, they kind of mingle in.  They almost become part of the twine there.

David -   Yeah and actually, I've found a 3rd species, so we've got three species just on this insignificant little piece of twine.  These species are probably native to the UK, but there'll be other species that come in that aren't and that's where the problem is.  It can make a big difference to local aquaculture and fisheries if some alien pest gets in, becomes established, and then really starts to outcompete or eat our native fauna.  But these pieces of plastic travelling for many years, perhaps decades, that that means we can get animals and algae, and other organisms from all over the world, landing on our shores.

Richard -   It doesn't, I suppose matter that there's this twine with life clinging onto it here, but it might matter in other parts of the world.

David -   It's still a problem here, but it is more a problem elsewhere.  We have to remember that looking along the strandline here, we can see lots of natural material.  We can see seaweeds and we can see bryozoans, and other animals especially crab shells that have floated here naturally and they've been doing this for a long period of time.  For hundreds of millions of years, life has been floating around this part of the world, but in the Polar regions where there aren't lots of things that float, there aren't shells that naturally float, there aren't seeds, and logs, and other material that we would naturally see on our strandline, then plastic there and other floating manmade debris has made a huge difference because it's created this new environment of things floating on the surface, transporting organisms that wasn't there before.

27:39 - What is Turbulence?

When most people hear the word ‘turbulence’ they immediately think of being thrown around inside an airplane and perhaps, needing to use those little paper bags that they supply us with....

What is Turbulence?
with Dr Fred Marquis, Imperial College London

Ben -    When most people hear the word 'turbulence' they immediately think of being thrown around inside an airplane and perhaps, needing to use those little paper bags that they supply us with.  But the way that it affects flights is just one aspect of a very large and a very complicated subject, as I found out when I spoke to Dr. Fred Marquis from Imperial College London.

Fred -   Turbulence itself is actually quite difficult to define as is.  It's actually a mixture of things and we usually try and classify it.  Some people actually call it a syndrome, but  turbulence is characterised I think by three main things:  disorder, mixing, and vorticity.  By disorder, I mean that if we look at a flow that we call turbulent flow, if we look at a point in that flow and measure very carefully what's going on at that particular point in terms of either temperature or the velocity of the flow, if we look at the data that we get out from that point, we see that it's got very small, sometimes large, seemingly chaotic motions that seem to vary almost randomly as a function of time.  But if we measure for long enough, what we can see is over that long period of time, a mean value appearing as we take our measurements.  So the measurements, if we average them all out, seem to come to a mean or a steady value.

Ben -   What do we mean when we say mixing, when we're talking about turbulence?

The Bowl at the EndFred -   Okay, probably an easy one for you to visualise is ink mixing into water. If you start off with a nice blob of blue ink and you've obviously got the colourless water.  The blue ink is very easy to see and the blueness of the ink propagates out into the rest of the fluid, and the colour of fluid will become more even over time.  That takes a long time because the mixing that is taking place there is by molecular motion.  If however we take a spoon, we stir up the ink and water, you will see the colour will even out extremely quickly, and that is because as we stir the fluid we're imparting turbulence into the fluid which causes this very high rate of mixing.

Ben -   So, to make something turbulent, do we actually have to add energy?

Fred -   Spot on.  That's exactly what you have to do.  And it's worse than that.  To keep the turbulence going, you have to continually supply that energy because over time, the natural viscosity of the fluid, which you can think of as fluid friction, tends to slow or absorb the turbulent motions, and ultimately convert those into heat.

Ben -   Okay, so that's turbulence and mixing.  The other one you mentioned was vorticity.  Now vortices are these - most people think of them as smoke rings, aren't they?

Fred -   Yes.  That's a good analogy to look at.  A smoke ring is a very well defined vortex.  Again, if you look at a turbulent flow and it could be off the front of a ship or it could be around the bridge support, you will see quite pronounced curly motions.  I mean, even Leonardo DaVinci, in some of his sketching, he was actually sketching these vortical motions within the fluid.  And of course, with our eye, what we tend to see is the big motions, but in actual fact, what's happening is that these big vortices are getting stretched over time.  As they get stretched over time, they get thinner.  Because they're getting thinner, they're getting faster and as these vortices get faster and faster, and longer and longer, they become unstable.  They break up into smaller vortices, so you get a whole cascade of vortices and we call it a cascade of energy.

Wake Vortex Study at Wallops Island

Ben -   So obviously, turbulence is a very interesting thing, a very difficult thing to study, but why is it important that we do actually research it?

Fred -   Turbulence is important.  The two that I'm perhaps most familiar with are in things like the chemical and pharmaceutical industry, and also the motor industry, in the design of combustion engines and indeed, gas turbines for aero engines.  Now, one of the processes that's going on in combustion is obviously combustion - you're mixing fuel and an oxidant, usually air, together to produce heat and you want that heat to be able to ultimately do work and push your airplane forward or move your car along the motorway.  But how that air and the fuel mix within the combustion chamber determines quite critically the efficiency of the combustion process going on within the combustion chamber.  The efficiency is measured in two ways, if you like.  One way is how much the fuel gets turned into useful heat and the other thing that happens is, during those chemical reactions, pollutants get formed.  The amount of those that you produce is dependent on the levels of turbulence and how efficient your mixing is within the combustion chamber.

Ben -   You also mentioned the pharmaceutical and chemical industries.  In what way do they need to understand turbulence?

Fred -   In exactly the same way, in the chemical and pharmaceutical industry, you've got chemical reactions going on.  Sometimes you've even got competing chemical reactions going on, so for example, if I'm trying to make a drug and I mix two chemicals together, I may well get my drug which is what I want, but I may also get some side products which I don't want.  The amount of side products can be critically dependent upon the levels of mixing that are going on in the pharmaceutical process.  And by controlling the levels of turbulence, for example by controlling how you actually do the mixing in what's called a mixing vessel, you can tune your process to produce the chemical or the drug that you want to produce and try and minimise a side product.

Ben -   So, clearly understanding turbulence is very important, but how do we actually go about researching it?

Fred -   Okay.  If we actually want to try and understand what's going on within turbulence or within a turbulent flow, I suppose you really got sort of two avenues of approach.  The first is an experimental method, so you actually set up your flow, and you carry out measurements.  These measurements are usually laser-based, for example, and we like to use laser-based methods because they don't disturb the flow.  Another way of trying to understand what is going in turbulent flow is that we can actually try to model the turbulence.  When we model this turbulence, we have to resolve all of those chaotic motions that I was talking about earlier.  This means that there's a very high computational cost.  In fact, one of the biggest computers in the world is modelling the weather.  And these weather modelling computers - computer programs, essentially, they have to be large because they're trying to resolve all the turbulent flow within the atmosphere.

34:35 - Terminating Turbulence in Tubes

Turbulence occurs in fluids and one way we often transport fluids around is in pipes. Tobias Schneider from Harvard explains that in order to reduce turbulence in pipes, it helps to add...

Terminating Turbulence in Tubes
with Tobias Schneider, Harvard University

Dave -   Turbulence occurs in fluids and one way we often transport fluids around is in pipes.  Tobias Schneider is a researcher at Harvard University and he studies turbulence in pipes.  Hello, Tobias.

Tobias -   Hey!

Dave -   Is turbulence a big problem in pipes?

Tobias -   I guess it is.  If you transport oil through pipelines or water through city water mains then it just costs much, much more energy if the flow is turbulent, so you want to terminate turbulence in these kind of systems.

DTurbulenceave -   I guess, turbulence, it swirls.  That's happening in three dimensions, so is it difficult to study because it's hard to kind of see and separate everything out?

Tobias -   Sure.  So what you have to do basically is set up a simple experiment to do that and that's basically what we did.

Dave -   So what exactly where you doing?

Tobias -   So we were setting up an experiment where we had water flowing through a glass pipe - a long glass pipe about 12 metres long and 3 centimetres in diameter or something like that, and then we created a patch of turbulence in this pipe.  So a local region, which is also known as a turbulent puff, that moves downstream within an otherwise laminar flow.

Dave -   So, did you have to actually trigger the turbulence yourself or would it develop naturally?

Tobias -   No.  In this case, it didn't develop naturally so if you have a very smooth pipe then you have to perturb the flow, so we injected a little bit of water into the pipe that then generated this turbulent patch that travels down the pipe.

Dave -   So it's one of these things where you start all off the little disturbance and the disturbance gets bigger, and bigger, and bigger, and in the end, you get problems.

Tobias -   Exactly.  So, our question then was how we can make this turbulent patch that we generated, how can we make that vanish?  That was basically the goal of our study.

Dave -   So what did you end up doing?

Tobias -   So we have this one turbulent puff and the answer of how to make that vanish sounds somewhat paradoxical.  So, to an eliminate turbulence, we basically had to create more turbulence and what I mean by that is - we had this one puff and we created more puffs at regular intervals and the trick then is that if these puffs get close to each other, close enough to each other, then they start affecting each other, and they decay.  So really, more turbulence can lead to less or in our case, really no turbulence.

Dave -   So were you actually getting rid of all the turbulence or just changing the scale of it?

Tobias -   No.  In our case, we were actually getting rid of all the turbulence.  So the flow at the end of the pipe was completely laminar and that's kind of the thing that we are really excited about.  We didn't only reduce the level of turbulence, but completely eliminated it.

Dave -   So would it work in an actual real-world pipe which isn't as smooth as your one or would it just work in this beautifully smooth pipe?

Tobias -   Okay.  That's a slightly difficult question.  So in our experiment, it works for pipes up to a certain flow speed.  At higher flow speeds, we were, up to now, not able to do it in an experiment yet, but we have computer simulations which suggests that similar things might also work in pipes when the velocity is higher, so it could actually work in real pipes.

Dave -   So the puffs you're adding, where they exactly the same as the puff you were using to create the turbulence in the first place or was there anything special about them?

Tobias -   No.  The puffs are basically the same and that basically leads to kind of a question of, why can we destroy turbulence or what is the mechanism about that?  We tried to investigate exactly that and in order to do that, what we basically did is we did an experiment where we visualise the flow.  We measured the flow using a laser-based measurement technique and we looked at basically how this puff works, so where it gets it's energy from.  So what we basically found is that the turbulent vortices and eddies that power turbulence are created at the rear end of this path.  So they are created at the interface between laminar and turbulent flow.  So it turns out that the energy source that basically powers this turbulence requires laminar flow behind the puff.  There is no longer laminar flow, but you add a second puff, then this energy input ceases and the turbulent vanishes.

Dave -   So you have to keep on adding these puffs forever or can you somehow get it weaker and weaker, and weaker and actually kill it in the pipe entirely?

Tobias -   Basically, you have to add these puffs at a certain spot in the pipe locally, but then for the downstream of the pipe, you don't have any puffs anymore.  One can probably explain that with this imperfect analogy, but think of this turbulent puffs as a sailboat in a regatta.  At a certain point, what we do is we constantly start new sailboats that enter this race.  After some time these boats sail so closely behind each other, that the succeeding boat takes the wind of the sails of the boat in front of it and that basically means that the whole entire regatta comes to a halt, and we don't find any boats downstream, or in our case, we don't find puffs downstream.  So we have to add the puffs all the time, but downstream, you don't have any more.

Dave -   So in a real world situation, you would just sort of envisage having a pipe with a whole series of these kind of puff inducers and therefore, so it would break up the turbulence every kind of 100 metres or so, or whatever, and therefore, everything would flow a lot more easily.

Tobias -   Exactly.  So that's the idea here.

Dave -   Would this work in any other examples or other place where turbulence is a problem sort of in cars, aerodynamics, and things like that?

Tobias -   Yeah.  That's basically what we hope we can do.  Right now, this is a proof of concept experiment.  So right now, we can only do that in pipe flow and we cannot yet do it in more relevant problems like the wing of an airplane or something like that.  But we're super excited about our results because it shows that, in principle, turbulence can be completely eliminated.  It's still a very difficult task and don't expect to have cars with reduced turbulence in the next 10 years or so, but we are very confident that we can get closer to this.

41:17 - Naked Engineering - Natural Ventilation

Meera and Dave explore how the principles of convection can be manipulated to cool down, and warm up, our buildings...

Naked Engineering - Natural Ventilation
with Alan Short, Andy Woods, BP Institute

Meera -   This week on Naked Engineering, we're going to be looking into how simple principles of physics can be used when designing buildings to cool them down without the need for energy consuming air-conditioning.  To investigate the design of these low energy buildings, Dave and I have come along to the BP Institute here at the University of Cambridge.  Now Dave, to start things off, this all basically uses the principles of convection, doesn't it?

Dave -   Yeah.  Convection is a major thing which drives fluid flows all over the universe in fact, from the sun to the deepest ocean and in a house.  So I've got a nice little experiment here to show you what's going on.  I've got a little bottle filled with red fluid.  It's just some hot water with a little bit of food colouring in it.  When water heats up, it expands.  It becomes slightly less dense.  So we then put it in this small fish tank full of cold water and then take the lid off, the slightly less dense red warm water should float up...

Meera -   Yes, the red food colouring is moving up through the water towards the top.

Dave -   That's right and so, the warm fluid will float upwards and because its place is takenLanchester Library, coventry by cold water, it's pushed up and you get a circulation.

Meera -   Well whilst that colouring spreads through, let's look into how this principle can be used to actually design naturally ventilated buildings.  So joining us this week are Alan Short,  Professor of Architecture here at University of Cambridge, and Andy Woods, Professor of Fluid Mechanics.  Now Alan, how do you actually adapt this when designing buildings and therefore, take into account air movement for the first low energy building?

Alan -   Well we had a wonderful opportunity to make quite a big building in Malta.  A very simple building.  It was an industrial building for a brewery.  We had to make one big space that would stay as cool as possible through the summer in Malta where the temperature cheerfully gets up to 40 degrees centigrade quite often.  The building didn't have very many people in it at any one moment so we didn't need to supply them with much fresh air during the day.  So our strategy was to make a very heavy weight building, very difficult to change its temperature quickly, and then as soon as the sun went over the horizon, we'd open up our chimneys and vent it as fast as we could.  We achieved up to 12 full air changes an hour by about 2 o'clock in the morning and the building starting to actually respire - you could see it breathing and then by dawn in the following morning, when the temperature inside is exactly the same as outside, you shut it up as quickly as you can, and you live off the coolness of the rest of the day.

Meera -   And Andy, you look into the fluid mechanics really behind this flow of air.  So how does it actually work and what controls the flow of air through a building?

Andy -   Well essentially, the dominant principle driving the flow is a combination of wind driving forces and buoyancy forces, and the buoyancy forces arise from the fact that air inside the building is typically warmer than the air outside the building.  And so, that air wants to rise, relative to the air outside the building because it's less dense.  So if you put an opening at the top of the building and the base of the building, the warm air will tend to rise out the upper opening, bringing in cooler air from the outside.

Meera -   And to encourage this, you use stacks on your building?

Andy -   And so stack is really like a large chimney on the top of a building.  It provides an extra vertical distance through which you can have the warm air as it rises out through the building.  And essentially, this increases the pressure driving the flow between the inside and the outside of the building.

Dave -   So this is actually the reason why we have chimneys anyway.  So you have a big column of hot air above a fire which floats upwards very strongly, so you get a big draft going up the chimney which pulls all the smoke out.

Andy -   That's exactly the case.  And of course, with the chimney, you want the smoke to vent out of the building, but in natural ventilation, we're also trying to bring air into the building.

Meera -   So to actually show this in action, you've got a water model here.  So it's a plastic box which represents a room, but it's tipped upside down so the floor is at the top and then the ceiling is at the bottom.  But coming in through the floor, you've got a vent and out of the ceiling, there are two chimneys essentially and you can see a good movement of water through here.  What's actually happening in this model then?

Andy -   Okay, so this is what we call a water bath model of natural ventilation.  We're using salt as an analogue system to model the density changes associated with heating and cooling air.  So, salt water is heavier than fresh water and so, salt water sinks.  And so, if we turn the system upside down as we have in the experiment, we can put a source of salt water to model a heat source, and so we've dyed that red and you can see a turbulent plum starts rising off this little model radiator, and it rises to the ceiling in the tank, taking up a lot of the air in the room as it goes.  We've also got a hole in the floor of the tank and you can see blue fluid is coming through there and this is the cold outside air, providing fresh air into the building.  This is what's called a conventional displacement ventilation system that you'd use in the summer.  This is a very effective way of removing the heat from the building because as you see, this red plum of heat goes all the way to the ceiling.  And even though it does mix, it produces a layer near the ceiling of hot air which goes straight out of the space and the occupants near the floor are actually immersed in this colder layer of outside air that you can see below that interface.

Meera - Now this model is great, it clearly shows the movement of air through a building. But Alan, how do you actually adapt this for different conditions of different buildings?

Manchester Contact TheatreAlan - there are many different recipes for different types of climate, and different times of the year. You can manipulate the air before it gets into the space at the bottom, if you capture it in a plenum first you can pre-cool it or pre-warm it naturally or help it along a bit. If you back up to the top, you can pre-cool air as it comes in and drop it into the space, which is very effective. You can do that in a dry version or you can mist water vapur into it which is even more effective. And you can the exhaust stacks as well just to try and induce a flow.

Dave - So this is increasing the flow because you're making the air in the stacks even hotter than it would be naturally, driving more comvection and more flow.

Meera -   And coming back to your water model Andy, I can see here that the hot air, or hot water in this case, is still moving through the building and out of the stacks at the top. But what about during winter?  Could this be changed to keep a building warm as well?

Andy -   Winter presents a very interesting challenge because if you look at the way this system is running at the moment, the air is coming in through the low level in the building from the outside and then the warm air is venting out through the ceiling.  And so, we're actually losing a lot of heat out through the ceiling to the outside.  In many buildings in the UK today, the heat generated in the building from the occupants of the building who may be using computers, or doing other activities in building, is typically sufficient to actually keep the building warm during the occupied times.  And so, what we don't want to do is actually vent away that heat through the stacks.  And so, in winter, there's a very simple thing we can do to actually change the whole of this process and I'll show you this by just putting this bung in, essentially closing the window at low level, and what you see now is that the inflow at the floor has stopped and the two stacks are now doing something very different.

Meera -   There's actually water flowing out of one and it's coming in through the other one now.

Andy -   That's right because we stopped the air supply at the floor level, but we still have a space which is warmer than the outside, the warm air leaves through one of the stacks and to replace all that air, air has to come in through the other stack. This is very interesting because the air that's coming into the building there develops a cold plume that you can see falling towards the floor.  And as it falls towards the floor, it mixes with the warm air in the space, so by the time it's reached the floor, it's actually much warmer than the outside temperature.  So this is a way of ventilating the building in winter when you've got a lot of heat generated in the building without needing to provide additional heat to preheat the ventilation air.  And so, that provides a low energy solution to actually keeping the natural ventilation in the winter.

Meera -   Now Alan, we've discussed here then that buildings can be cooled down and also a higher temperature can be maintained, but just how much - so what are the actual temperatures we're talking about here?

Alan -   Well, it's surprisingly effective. The library that we all designed in Coventry, the Lanchester library for the university.  I think the temperature there has never been recorded as being above 26 ½ degrees centigrade, but we know that it's got to about 33 ½ outside, so that's quite a lot of free cooling.  It's 110,000 square feet - that building.

Meera -   And as well as monitoring the temperature though, it's a low energy building.  So how much of a reduction is there in the energy consumption say, compared to using air-conditioning?

Alan -   Well air-conditioning is a huge energy guzzler because you refrigerate the air before you heat it back up again, so a naturally ventilated office building should be using about 1/3 of the energy of the full fruit salad air-conditioned thing.

Meera -   But this is all for the design of new buildings.  What about the current buildings that we have?  Would it be possible to retro-fit these buildings in order to give them natural ventilation as well?

Alan -   Yes, it certainly is and in fact, that's much more important than the design of new buildings because most of the buildings that we know now will be here in 50 years time.  We're very interested in '60s and '70s buildings which tend to be concrete framed with lightweight envelopes.  You can easily start adding new things to the facade.  This is a fantastically rich and interesting area of work.

Meera -   Okay, well that's great.  Thank you, Alan and thank you, Andy.  That's it from Dave and I this week, but look out for more of these naturally ventilated buildings in the future.

Dave -   That was Meera Senthilingam who joined me with Alan Short and Andy Woods from Cambridge University's BP Institute.

51:06 - How do heat pumps work to warm up a house?

How do heat pumps work to warm up a house?

Dave - Well a heat pump basically is a pump of heat. It does matter how big the temperature difference is, but essentially it will pump heat from somewhere which is cold and somewhere which is warm. So essentially, what your fridge is doing, it's pumping heat from the cold inside your fridge to the hot outside of your fridge. It does it by compressing and expanding gases. And so basically, the way they're rigging these heat pumps is essentially they're taking the inside of your fridge and putting it outside so that taking heat from outside and putting the hot pipes at the back of your fridge, where the heat gets pumped to, inside your house so it keeps it nice and warm. The bigger the temperature difference is though, the more electricity you need. Ben - So, when you're dealing with a whole house scale, can that really work? Obviously, a fridge is quite small, it's well insulated, it's very contained. Can you really heat a house using that technique? Dave - You certainly can people do and you can shift sort of between three and five times as much heat as electricity would've produced, just by running an electric heater inside. Ben - I certainly noticed when we bought a freezer and installed it in our garage that our garage seemed to be a lot warmer than it used to be. Same sort of thing going on? Dave - Yeah. Basically, it's pumping heat from inside the freezer to outside and it's not very efficient, so you need sort of extra electricity in there, so that electricity goes heating up your house.

52:46 - Could turbulence be used as a renewable energy source?

Could turbulence be used as a renewable energy source?

We posed this question to Tobias Schneider from Harvard University...

That's a very good question, but I think that this would be hard because basically, what turbulence does, is it consumes energy, so it increases the dissipation of energy. Nevertheless, it's important to understand and to control turbulence in ideas of how to harvest renewable energy simply to make for example, wind turbines or things like that more efficient, but turbulence in itself is not an energy source, so that shouldn't work.

53:32 - Are there harmonic patterns in turbulence?

Are there harmonic patterns in turbulence?

We posed this question to Tobias Schneider from Harvard University...

Tobias - Typically there are not. Turbulence - one of the features of turbulence is that it has structure on all scales. So typically, you have vortices, and a large vortex creates a smaller vortex, and so on and so forth. So you typically don't have regular patterns. However, if you're looking at theoretical work done right now which tries to understand turbulence, then there are these regular patterns, harmonic patterns which you observe in solutions of the underlying equations which are computed with big computer programs, and they seem to be very important. At pipe flow, they have even been observed in 2004, so yeah, there are these patterns, but the patterns are extremely special. Typically, turbulence, doesn't have any structure. Dave - I guess the one place where you can sort of use some of the noise and these vibrations from turbulence is something like a flute where you've got a resonator attached to something which is producing a turbulent airflow, and that turbulent airflow drives the resonator in the flute to the air, so it rushes in and back, in and out, and that connects to the turbulence being produced as you blow over the top of the flute. And so, you can get a big sort of resonance, big airflow, in and out and vibrations which you hear as sound, but I guess that's a property of the resonator more than the actual turbulence itself. Ben - It's that effect related to noisy pipe syndrome? Is it something similar going where you turn your hot water on and you get this horrendous noise or buzzing or a low hum for awhile? Dave - I think that again, yes something very similar. You get sort of turbulence in the pipe which produces lots of different frequencies of vibration. It's quite low level and then if you've got some air trapped inside the pipe somewhere, that can compress and expand, and it acts like a spring. You've got a mass and a spring so that will vibrate and resonate, and build up to quite a large vibration even if the original turbulence with lots of different frequencies to start off with and not very large.

55:36 - How can corn starch and water both flow and be firm?

How can corn starch and water both flow and be firm?

Cornstarch, or corn flour, is basically made up of lots of tiny, tiny particles. They really like water so they wet very, very well. So you get lots of tiny particles surrounded by water and that water sort of lubricates them and means they can move past one another if you apply slow forces and it flows like a liquid. If you hit it quickly, you squash the whole material very fast and particles don't have time to move out of the way, so they all kind of jam into each other and lock together, and so, you get a solid line of particles which locks it all up and it behaves like a solid. And so, it's a solid when you hit it fast. When it flows slowly, it's behaving like a liquid. Of course when you cook it, it's entirely different. It's no longer particles it's polymers. They just turn into gloopy liquid which is just kind of sticky and gloopy.

56:41 - If wind has zero resistance, does it make a sound?

If wind has zero resistance, does it make a sound?

Dave - It certainly won't cause any sound if there's nothing to cause any turbulence, if there's no vibrations because sound is just a vibration of air. So, if the wind is flowing past something and that thing makes it vibrate, like if you get some swirling when it blows across the edge of a surface or something, you get some turbulence or that turbulence can drive vibrations in a bottle or something when you blow across the top of a bottle, you produce very loud sounds. If you're in a middle of an air column, so if you're up high in the air where all the air is moving the same direction very fast, there is no relative movement, so you won't get any vibrations at all. It would be very quiet. So people hot air ballooning is very, very still, even if the wind is quite high because they're moving with the wind. However, there's various interpretations of what he means by resistance. One of them is how gloopy it is, how viscous the fluid is. Ben - So that's sort of the internal resistance of the fluid itself, rather than it interacting with say, a wall. This is resistance inside the fluid. Dave - That's right. Basically, how much friction there is inside the fluid itself and fluids which are very, very viscous, things like treacle will move very, very smoothly, and you won't get any vibrations and it'll essentially be very, very quiet. However, if there's very, very low viscosity, it can't lose energy by viscous flow by friction. It can only lose it by turbulence. airflows being incredibly turbulent and very, very noisy. Ben - So that goes back to what Fred was saying about how you have to put energy in to create turbulence and then it actually loses the energy back to this internal friction. Dave - That's right and the more viscosity there is, the quicker the turbulence will die away, and if there's enough it won't form in the first place.

58:42 - Why does moving backwards make you feel ill?

Why does moving backwards make you feel ill? We will have the answer for you on our show, 17th October. Until then, what do you think?

Why does moving backwards make you feel ill?

We posed this question to Dan Parker from the University of Washington...

Dan - There are two basic reasons for motion sickness. One is, you have conflicting motion cues from the sense of balance in the inner ear, the vestibular system, and the eyes. The other problem is you get conflicting cues about your orientation. I've been studying which way is up most of my life and that's a problem. The effects of conflicting signals about how you're moving and how you're oriented are - you get dizzy. Why does riding backwards make you motion sick? The major reason is, if you're riding facing forward either in a bus, that you can see out of, in a car if you're a child, or you're on a train riding forward. If you are facing forward, you can predict, you can prepare for turns. You can see out the window and see the bank of the curve. If you're in a car, you can see where turns are coming up. You can predict what's going to happen and consequently, your ability to predict that you lean in to a turn. This reduces the disorientation that you feel. Why do people riding in backwards get motion sick? Because they can't make those predictions that you could make when you're riding forward. Diana - The same thing occurs when we get travel sick in the car. Your eyes, when looking at objects within the car tell your brain you're not moving whilst your vestibular system knows very well that you are. The two messages differ to the extent that you start to feel nauseous.

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