Bouncing Bombs and Blacksmiths

Small scale turbines that can harness energy locally from winds, tides, and rivers can be a very valuable asset in bringing power to remote populations which makes the technology that a company called 'Green Tide Turbines' are working on potentially very valuable.  Meera and Dave went to meet Green Tide Turbines' CEO, Michael Evans and his colleagues to find out a bit more.

Michael -   Well, if we're talking about a tidal turbine, we're developing technologies that have to survive some of the most aggressive environments in the whole planet.  We've got heavy moving water which contains huge amounts of energy, often in very remote locations which are very hard to access because obviously, you've got these very strong tidal currents, and also, some extremely large forces, which is the whole reason why we want to put turbines there, to tap some of that.  So, the emphasis really needs to be on devices that are really, really simple, very easy to maintain, very robust, and also, quite cheap.  

Dave -   I guess the different thing to do would be to take the main technology like take a wind turbine and stuff it underwater.

Michael -   Absolutely.  A lot of our competitors have followed that very philosophy.  Basically, they've just got wind turbines with shorter blades that are made to be waterproof, and they dump them under the sea.  The sea just isn't water.  There's lots of stuff that it carries in it.  Things like fishing nets, even submerged containers that have been washed off ships for instance.  These all find their way into these fast moving flows of water and they're basically flung at these devices.  So you've got these wind turbines with their delicate blades spinning around in a flow regime and when these things get hit by these objects, they're not going to survive particularly well.

Meera -   To tell us more about how the Green Tide Turbine design approaches these challenges is Tom Clark who is the Research Director...

Tom -   Well we have a different kind of concept which is moving away from the "wind turbines underneath the sea".  What we do is we put much smaller rotors inside a duct which allows us to capture a similar amount of energy, but using a much smaller and more robust rotor.

Dave -   I guess the first thing I'd say if you've got a very small rotor and you're squirting lots of water past it, surely if it is moving very fast, this turbine should be spinning very quickly and kind of mash anything which it hits.

Tom -   That's right.  But what we've been able to do, Dave, is use both the stator and a rotor.  So, a stator is essentially like a rotating turbine but fixed in position.  What that does is it causes a swirl of fluid inside the duct and then the rotor takes out that swirl and generates energy by doing so.  So, although the rotor is moving fairly fast, it's not moving as quickly as perhaps a design without a stator.

Meera -   So you're regulating the flow of water approaching the rotor?

Tom -   That's right.  That has the additional benefit actually that any turbulence and gustiness in the incoming stream is actually straightened and aligned as well.  A lot of people don't realise it, but the tides don't move just uniformly.  There's a lot of turbulence, swirling eddies, and so on, and currents which upset the behaviour of turbines in the current.  The benefit of our design really is that you can actually stack our turbines closer together in a tidal farm than with open tide turbine designs.

Dave -   So your system's causing the water to spin and then you're changing the direction of that spinning water completely using your rotor, so because you changed direction a lot, you don't need to be moving the rotor as quickly.

Tom -   That's right, yes because we have a swirling motion in the inside of our duct, our rotor can be moving much more slowly.  Effectively, it moves with the water.  So there's less of a relative speed between the water and the rotor.  So, it's much less damaging for fish and it's much less likely to get debris and other objects caught in the flow through the turbine.

Meera -   You mentioned that they can be situated quite close to each other, but what's the actual dimensions of the turbines?

Tom -   Our tidal turbines in general are in the range of 10 to 15 metres in outside diameter.  But the ducts are quite big so the rotors are actually substantially smaller than that, of the order 6 metres.

Meera -   We're at one of your test sites now which is alongside a river and you've got a tunnel through which water is flowing through.  It's about 1.5 to 2 metres wide, and you've got one of your turbines placed in the middle.  It's about 1 foot wide in diameter.  Tell me a bit more about how this setup works.

Tom -   Well it's called a Venturi arrangement.  Essentially, there's a nozzle which accelerates flow into our turbine for the purposes of testing because we need a higher flow rate to accurately test the turbine.  So, the flow rates that we reach in the test section of our water tunnel are around about 2 metres per second.  The action of the duct around the turbine actually accelerates fluid through the turbine disc which means that the velocity inside the duct is slightly higher than the mean flow rate.  It's about 2 ½ or 3 metres per second.  We have a part of the flow of a river diverted through our test facility.  We can control the facility, we can control the flow rate of the river using a sluice gate up in front of our turbine.

Meera -   Can we see it in action then?

Tom -   You certainly can.  All we need to do is open up the sluice gate.

Meera -   Right.  So the sluice gates are open and the water is now just flooding through!

Tom -   That's right.  Flowing through the tunnel is about 3 cubic metres per second of water.  We've got water travelling up to 2 metres per second.

Meera -   Going through then the turbine, how much power will be generated here?

Tom -   That corresponds to a power of about 500 watts.  It really isn't very much.  That's about half a household kettle, but I should say that it's a very, very small model that we're using.  The energy capture scales up dramatically as you increase the diameter up to a tidal turbine scale.  Our commercial scale turbines typically produce around 1 megawatt of power which is useful, I think that generates enough power for about 300 homes.

Meera -   Having seen this in action then, now moving back to you Michael, where is it hoped these turbines will be used?  What are the upcoming projects?

Michael -   Well we've had a great deal of interest from developing countries where there's lots of rural communities well away from established infrastructure needing power and relatively small amounts of power.  Generating power for lighting, for running mobile communication systems, and things like that.  One country in particular who were very keen on our technology is Brazil.  So this will be on a scale of about 1 metre in diameter installed in probably Northern Amazon region initially.  That'll be generating around about 5 kilowatts of energy.

Dave -   If you're going to put diesel generators up there, you still have to ship the diesel up there regularly.  So, this is even less hassle than that.

Michael -   Absolutely and on top of that, you know, you've got a lot of enthusiasm for clean technologies, and other alternatives such as photovoltaics and wind wouldn't really work in a rainforest type of environment, where obviously you've got lots of clouds most of the day.  And also, with the trees, there's not very much wind at usable level.  So, the river generation technology is ideal for this sort of environment.

Dave -   So although you started off building a tidal turbine, you ended up putting it in rivers instead.

Michael -   Making smaller turbines is a lot cheaper obviously and we need to prove our technology.  The river turbine is a fantastic test bed to actually develop the technology.

Meera -   This week on Naked Engineering, Dave and I have returned to the Structures Lab in the engineering department here at the University of Cambridge, but this time, we're looking into shape changing structures.  So, as the name suggests, these are structures that change their shape, but Dave, tell me a bit more about why these are so special?

Dave -   If you're a conventional engineer building something like a bridge, or a car, or an aircraft carrier, you want it to do things which are very, very predictable.  So you tend to use big lumps of solid materials which stay in one shape and you might have joints in between them but the actual materials themselves are always the same shape.  And so, what we're looking at today is structures in which the whole structure can actually reconfigure itself and change shape.

Meera -   And one person working with these structures is Dr. Keith Seffen who's from the Advanced Structures group here.  Now Keith, you've got something most people are familiar in your hand, it's a snap bracelet...

Keith -   Indeed it is, Meera.  As you can see, it's got a little bit of plastic covering on it but it's predominantly straight or it is straight in one configuration, and if you flick it against your arm, it'll wrap around it and adopt a circular shape.

Dave -   This has got two configurations.  One where it looks straight but if you look carefully at it, its curved in the other direction just very slightly.  The second one is when you wrap it around your wrist and it's essentially just a straight coil in one direction.

Keith -   That's right, Dave.  If we take it apart, if we remove the plastic sheath, it looks like a tape measure.  Straight in one configuration, but curved gently across its width and in the other configuration, wrapped up a bit like a tape measure, sitting inside a cassette spool.

Meera -   So, this is something many of us have seen, but how does it actually work?  How is it possible for it to have these two configurations?

Keith -   Well your ordinary tape measure won't have this.  It will just want to be straight.  What you have to do is manipulate it.  Create or embed the second shape; the desire to be curved.

Dave -   So you're basically just distorting the metal itself, plastically deforming the tape.

Keith -   What you can do if you can get hold of a length of the tape is to cut a length, wrap it into a coil, wrap it around a pencil perhaps, or your finger, and try and pull it as tightly as possible.  What that does is to permanently deform the tape measure into the circular position.  It likes to be in that shape, but if you then pull it straight, it will prefer to be in that shape as well, and that creates this bistable property, where it desires to be in both configurations at once.

Meera -   So you've now rolled up this tape measure quite tightly around your finger and opened it up back up.  So now, with just certain pressures or just pushing on certain points, it should just flick.

Keith -   That's right.  If you just take it and push in the middle, as you can see and hear, it sort of pops into the cylindrical configuration or the wrap-around configuration.  And then you have to physically unravel it to create the straight structure again.  What you need however is two very different shapes for this to work and in the case of a tape measure, it's long across its length but curved across its width, and that changes when you wrap it around your finger. When you wrap it around a pencil, it becomes flat across its width and coiled along its length, and it's that antagonism that permits the properties that you see.  The shapes are so different that to physically move from one to the next, you have to come along and break it with your finger or push it in the right place.

Meera -   Can this also be used in other types of materials as well?

Keith -   Sure thing.  We have different engineering materials; a popular lightweight structural material is carbon fibre, and we work closely with a company that make tubes from carbon fibre that have the same bistable property.  I've got an example here of one where we've actually tuned the properties so that it prefers both to be straight and to be coiled at the same time whilst you're holding it, and we call that a neutrally stable structure.

Dave -   So, it's not snapping between one to the other.  It's just moving very gently.

Keith -   That's right.  In the previous structures, there is a desire to be in both shapes, at the same time, they can only occupy one shape at a given time.  Whereas this one can occupy both and it occupies both by having part of it straight and part of it coiled and a funny transition region in between.

Dave -   I guess this is a really neat way of storing a very, very long tube.  If you want a long tube or something, you can just roll it up the other way and it turns into a short, fat, easily storable object.

Keith -   And in addition, because it's quite happy to be extended to any length, you can have a tube of any length and for that reason, imagine you wanted to have some kind of device that would enable you to look into buildings at particular height - so what you can do is extend the tube, have a camera on the top of it and then look into say, the window of a burning building if you're a fireman or whatever, but you don't want to be carrying this around in the extended configuration.  So you'd roll it up and stick in your backpack, it'll quite happily fit there because it's coiled and neatly packed.

Meera -   Is there anything else we can also use these structures for?

Keith -   What these little demonstrations have shown you is that, crucial to the performance of these is what you start off with as a basic shape.  With the snap bracelet, what we thought of doing was taking several of them and linking them together to produce something which was a bit wider, rather than having just a single strip, several strips next to one another, then we had the idea of alternating those strips.  What you end up with then is something that looks a bit like a corrugated sheet.  Like a flick bracelet where we're able to give it pre-stress, we can make the corrugated sheet which is nominally flat, coil up into a cylinder.

Dave -   This is actually a really quite nice design.  Basically, it's a whole series of these tape measures next to each other, but one upside down and one in the right way up.  But instead of separate tape measures, it's all just made out of a single sheet of metal which has been bent.  And this means, that because it's actually got some depth, it's actually reasonably rigid as a flat sheet, but if you bend them beyond that point, it just starts to roll up into a tube, about 2 inches in diameter.  And now, it's still quite a solid structure, but it's rolled up and takes up so much less space.

Keith -   What this demonstrates is a very simple mock up of an application of our technology where you might want to give some support to a very flexible, thin electronic display.  There are people out there in the electronics industry, trying to make displays as thin as possible to give them this nice, flexible feature.  Electronic ink, electronic newspaper, but delicate structures nonetheless.  What we're thinking is we'll put one of our sheets on the back of it to give it some strength and stiffness, but at the same time, not removing that rollability, that foldability, of the display or what it's offering, and at the same time, offering protection to the structure.

Dave -   You could basically have a computer screen which is the size of laptop screen but protected because it's got this nice strong back that sits nice and flat, but when you click it, it will then roll up into something 2 inches across, you can put in your pocket. But it's still quite protecting. It's got this big metal sheet on the back and you can't crease it.

Meera -   And so, with applications like this Keith, so eBook readers and electronic tablets, they're all very popular at the moment, and so in the future, we could just have thinner, more portable ones that would fit quite neatly into our bags and be protected.

Keith -   I think that's  where the industry might be heading.  The thinner the display, the lighter it is, the less power it uses, but the more delicate it becomes, so you need some kind of protective backing in these and one that enables or imparts the shape changing capability that makes it easier to put in your pocket or wherever is something that would help.

Sarah -   That would certainly make working on the move a lot better if you could just roll up your laptop and put it in your pocket.  That was Dr. Keith Seffen from the University of Cambridge, talking to Meera Senthilingam and Dave Ansell for this week's Naked Engineering. 

Meera -   Cambridge University's Hugh Hunt is a multi-talented engineer and he's recently been involved in a project that looked into the engineering behind the famous dam busters bouncing bomb used during World War II.

Dave -   We joined him along by the River Cam in Cambridge to find out what's involved in designing something on a large scale to bounce on water.

Hugh -   Barnes Wallis, in 1943, came up with this great scheme for blowing up German dams - having a bouncing bomb, bouncing on the water, nestling up against the dam, dropping down beside the dam to a depth of about 20 feet and then blowing up just at the right height.  It was successful, although there were quite a lot of lives lost, but the technology is really quite fascinating and hasn't been reproduced since 1943.

Meera -   Well, to find out more about the workings of this bouncing bomb, we've come along to the River Cam.  It's a beautiful sunny day.  Dave, why are we here?

Dave -   One of the things which inspired Barnes Wallis to design the bouncing bomb was essentially just skipping stones on a river.  Normally, when skipping stones, you get a nice flat stone, get very, very low towards the water, and you tend to throw it as hard as you can on a very, very flat trajectory, and with any luck, when it hits the water, it will skip.

Meera -   Yep!  So yours has just skipped twice over, which it's actually quite hard to accomplish.

Dave -   So what's happening is the stone hits the water at a slight angle.  When it hits the water, it gains lots and lots of lift, and it bounces off, and it skips a few times.

Meera -   Hugh, is this basically the principle behind the bouncing bomb?

Hugh -   Barnes Wallis was sort of inspired by skipping stones, but actually, he figured out with his kids in his backyard and that he could actually get marbles - ordinary round glass marbles - to bounce.  The great thing about something round is that it can contain a reasonable volume.  It is predictable, but actually, there is an even better shape than being spherical.  If you have something which is cylindrical then you can fit more explosive power into the given space in the bomb bay of your Lancaster bomber.

Meera -   This bouncing bomb is something that you set about recreating recently.  Firstly, what you even have to consider when designing or engineering a bouncing bomb?

Hugh -   First thing I suppose you have to do is to decide what size you're going to do it.  Barnes Wallis had one which was about a metre and a bit in diameter.  We went for half the size.  Then we had to design how to fit it underneath the plane.  We had to design how to make it spin and that was one of the things that Barnes Wallis found that's really important - that this thing had to spin.  We had to figure out how to release it at the right time, at the right height, and actually, we had to make ourselves a target, and that was quite interesting because Barnes Wallis had German Nazi dams as a target.  We built our own dams!

Meera -   To see how you set about coming up with your design, we've got a variety of bits and bobs around us.  We've got cricket balls here, we've got cylinders representing different scales of your bouncing bombs.  So firstly, these cricket balls, what have you used these for?

Hugh -   We started off on a small scale just with cricket balls.  We used a cricket ball bowling machine in the Jesus Green swimming pool and we wanted to figure out how we could make cricket balls bounce, and we worked out the height and the angle and the speed, and that was great.

Dave -   So how fast are you actually having to fire these cricket balls to get them to bounce satisfactorily?

Hugh -   Well the ordinary lightweight ones, we could get them to bounce at maybe 30 or 40 miles an hour, but the heavier ones, once we started putting weights in them, we had to get maybe up to 50 or 60 miles an hour, except if you put backspin on them.  And this was a eureka moment for Barnes Wallis back in the 1940s.

Meera -   Why is backspin so important and how does it have that beneficial effect to the movement?

Hugh -   Well, backspin is really important when the bomb goes through the air.  Now if you think of Roger Federer and his topspin, topspin makes the ball dip over the net really fast.  So you can imagine if you put topspin on a spinning bomb, it would just dive into the water and just sink.  But backspin sort of hovers over the net and so, you can make it skim at a much shallower angle onto the water.

Dave -   So the important thing is the angle that which the bomb hits the water and if that's too steep, it just digs straight in, but if you can get that shallow enough, it skips.

Hugh -   That's absolutely right Dave, and if you have this thing called 'magnus lift' - the magnus effect is what it called - you can get a really nice sort of shallow angle under the water and we've got a really good demo how that works.

Meera -   So here to actually show us just how important this is is Hillary Costello, one of Hugh's PhD students.   Now Hillary, you've got two polystyrene cups there that have both been taped together at their bottom ends and then you have an elastic band which has been cut open.

Hillary -   Well basically, this our cylinder, or bomb.  It's a cylindrical polystyrene cup thing that we stuck together and then if you just wrap the string around, you can give it some either backspin or topspin, depending on how you throw it.  So if I just wrap the elastic band around the cylinder or the polystyrene cups...

Meera -   Okay , so you've wrapped that round now and so there's not much elastic band left...

Hillary -   Yup!  And then if I just pull the elastic band with my thumb so that the cups go forward when I release it, it gives it some backspin and you could see that it sort of hovers in the air before coming down.

Meera -   So that actually hovered about a good meter away from you.

Hillary -   Yeah.

Meera -   Now you're just wrapping the elastic around again but in the opposite direction so that you'll have topspin.

Hillary -   I actually just hold it from the top so that it has top spin and then when I release it...

Meera -   That pretty much went straight down to the floor.

Hillary -   Yeah, exactly.  So, the backspin gives it a little bit of lift and the topspin gives it a downwards force.

Meera -   And Hugh, so why do we need this backspin and for it to hover, I guess at a quite constant height?

Hugh -   Well the idea really is to get the angle as shallow as possible.  When you're skimming stones, we all know that you need to get it down as low as you can close to the water, but one of the problems that flying at night with gunfire, and all sorts of this stuff, you really don't want to be flying very, very close to the water.  So, Barnes Wallis and his pilots figured they couldn't fly lower than 60 feet, the backspin enabled them to get a shallow angle of bounce on to the water.

Dave -   So this basically stabilises it because otherwise, after the first bounce, it would start to tumble and it would go anywhere and stop skipping.

Meera -   You've got a scale model here.  It's a cylinder with a 180 millimetres or 18-centimetre diameter.

Hugh -   This is what we used to do some tests.  In fact, it was in the Cotswolds.  We got a gas cannon to fire this cylinder at a high speed across the lake and we had to make it spin.  And you can sort of see that - if I toss this up in the air with no spin...

Meera -   It's turned over.  It's rotated.

Hugh -   It turned over very quickly, but once I put spin onto it...

Meera -   It smoothly just come straight back down to your hand.

Hugh -   The spin stabilised and that was very important.

Meera -   So what scales were you working to when you recreated your bouncing bomb?

Hugh -   We've got the cricket balls here.  We've got our 20-centimetre diameter scale over here.  We've also got our 60-centimetre diameter bomb.  It's actually a plastic replica because the real one is really heavy.

Dave -   So, you were working with lots of different scales.  It's not entirely obvious that you can transfer things you've learned at the size of a marble to something of a size of a person.  Was it easy to take information you learned on the small scale to the largest scale?

Hugh -   It's very interesting doing scale model tests.  People designing aircraft do this all the time.  You put a small scale model of an airplane in a wind tunnel and they'll learn how the aircraft works at that scale and then they'll scale it up to the big Airbus or Boeing full size aircraft.  We did the same thing.  We were able to use what's called dimensional analysis to scale up the effects we saw at the small scale cricket balls up to the bigger scale.

Meera -   So we've established that scaling is important and backspin is crucial, but what about delivery?  There are planes delivering these bombs so there must be something about the speed they're moving, the height that the bomb has actually dropped at, and so on?

Hugh -   The critical parameters are speed; speed of the plane, height; the height that it's flying at, and how much you spin the bomb.  And so, for a given diameter, it turns out that if you do dimensional analysis, the speed squared divided by the diameter and G - the acceleration of gravity - is dimensionless.  That means that if you were to have a bomb which was twice as big, you would have to fly 40% faster  - the square root of 2.  It's important to know that scaling factor.

Meera -   And so, moving back to you Hillary, just to conclude, having done all these tests with you, what were your final parameters to actually drop the bomb and explode the dam?

Hillary -   From our testing, we determined that we needed to drop it 350 metres away from the dam, going 180 miles per hour, flying at 60 feet, and spinning the bomb at 800 rpm.  It turned out that on our final run, the pilot decided to fly a little bit lower at 30 feet, so the bomb did hit a bit harder than anticipated, but it was a successful run.  So, like Barnes Wallis, we bounced a bomb and blew up a dam.

Dave -   Hillary Costello and before her, Hugh Hunt from the University of Cambridge, taking us through the process of designing a large scale bouncing bomb.