The Science of Turbulence

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.

Whilst 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.

结果是有前途的,他们增加了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.

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.

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?

罗伯特-作为项目的一部分,我们确实ome 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.

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?

Fred - 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?

弗雷德——是的。这是一个很好的类比。一个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.

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?

弗雷德——在完全相同的方式,在化学d 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.

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 takenby 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.

戴夫,所以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?

Alan - 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.

戴夫,所以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?

艾伦,这是令人惊讶的有效的。的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?

艾伦- guzz空调是一个巨大的能量ler 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.

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.

玉米淀粉、玉米面粉,基本上是由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.