Components - Tether Dynamics

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Up to the highest heights...

Most Ph.D.s focus on niche topics in a very specific field (see here) and Hilary Costello's research project is no different. Her ongoing Phd looks into the properties of aerodynamic stability of very long tethers for very high balloons or vew deep offshore oil and gas platforms underwater. 

Understanding tether dynamics is extremely complex with numerous factors effecting the behaviour of a cable - from the shape and aerodynamics of the tether to the elasticity, bending and torsion of the materials used not to mention the chaging wind speed and temperature at heights of up to 20km. 

Testing the cable dynamics of new tether designs in the real-world is a crucial part of the research process and by modelling and testing cables in isolated sections, it is possible to better understand and model how these may operate together in a larger system.

With limited access to mile-high balloons, Hilary can often be found in the green pastures of Cambridge University flying kites! Join her as she explains her quest to design an aerodynamic, stable tether that will reduce tension and, ultimately, lessen the risk of large-scale failure. 

This film is part of a project funded by the Royal Academy of Engineering to develop the on camera  communication skills of engineers across the UK.


Engineering, Materials


The Royal Academy of Engineering
Hilary Costello
Cambridge, UK


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cc_by-nc-sa License: Creative Commons




So normally, when you fly a kite, you care about the kite and not so much the string. But in my research, I'm interested in the dynamics of what the line is doing, whether it be a kite or a tethered balloon, offshore oil and gas platform underwater. There's cable dynamics in lots of different engineering systems, anything with cable moving around in the sky. You have changing tension along the line. You can get things like travelling waves. And that's what I'm trying to model.

Even in a simple system like this - this is just a bathroom chain and a shower ring - you can get really complicated things going on. So if I just put the ring on the chain, so it's just free to fall off the end like that - I'm just going to put it back on - you can get complicated things going on. Because if I just drop this ring...

No! Failure is a part of life. gets stuck.

So you see that and you wonder, well how does that work? And its to do with the tether dynamics. So if I have this chain and this ring falls, I get a travelling wave going down the chain like that. So when this ring is falling, it goes down the chain. And there's a wave following it down the chain. And when the wave gets to the end, there's a whiplash and this ring is going to fall into that hole and get stuck.

So you can see that you can get complicated things even just on a short length like that. But when you have something that's longer - a balloon tether out in the environment, you have this long tether - you can still have these travelling waves going down the length of your cable. There you can see that wave travelling down.

You're going to have changing tension. So the tension at the top, because of the self weight, is a lot higher than the tension at the bottom. And not only that, if this is outside, you're going to have wind. So you're going to have drag on this tether.

And you can imagine that normally you have tethers that are circular in cross section, so something like this. And in terms of drag, it's really surprising that the drag on this circular cross section is actually equal to the drag on this air foil. So you can get a huge benefit if you can streamline the section in terms of drag, which will decrease the tension on your overall system.

So my research has to do with if you do change the cross section of the tether, will it be stable? Can you even design something that would work? So I have an aerodynamic tether here, and I'm trying to model whether this would work.

So I have this tether, and I'm going to discortise it into a bunch of tiny sections. So I'm going to break it up into small chunks. And that way I can model the elasticity, and damping, the bending, torsion, all of those things. And I can also model the changing parameters up the length - say this is 20 kilometres long - into the sky.

So I'm going to have changing air density, changing wind speed. So I have a computer model to model that behaviour. But not only is that important, the actual cross section of the tether is also important in terms of stability.

So if I draw this cross section out, I have a bunch of different centers. So on this air foil I'm going to have a centre of mass, a centre of tension, a shear centre, which has to do with bending, and the aerodynamic centre, which is where the lift acts. And depending on where these centres are with respect to each other, is going to dictate whether this cable is stable.

So I have this 10 metre long length of the cable - of aerodynamic tether. And manufacturing this is a project on its own. And you can see that this one this is the very first tether that's been manufactured and it has some imperfections in it. You can see there hasn't quite filled in. So it's just it's just a first attempt, but I think would be quite good fun to string this up on a kite and have a go at flying it and see what it does.


Now pull it tight.

You want to hold it? You want to hold it from here?

Oh, no! GoPro rig has descended.

So now we have the aerodynamic tether section attached to the kite line flying up there. And you can see that it alters the dynamics of the entire kite system quite a bit.

This particular section isn't really manufactured in any way that it should be stable. It's just an extruded piece of something aerodynamically shaped. So it does flutter around in the wind. It flips. And those are all of the problems that you could encounter if you'd had something that wasn't stable. But when it goes under tension, you can see that it sort of straightens out. And ideally, what you want to manufacture is something that you've played around with the cross sectional properties so that it does the align itself with the wind.

So going out and flying kites for a Ph.D. might seem like a bit of a laugh, but this is the part that makes a good film. What doesn't make a good film is me sitting in front of a computer doing a bunch of code, or me writing out a bunch of equations. But that stuff's really interesting for me, and it's those equations that dictate whether something like this, or an aerodynamic tether would be stable. And by the end of my Ph.D. hopefully I'll have a better idea of whether you can make an aerodynamic, stable tether.

I think I might need some help first.

Still recording!

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