Making Waves

Making waves in the maths lab at the University of Cambridge with the Naked Scientists and researcher Megan Davies-Wykes… audio available here.

Megan – This is what we call the solitary wave tank. So it’s a very long tank that we use to produce waves.

Tom – I’m not going to lie, it looks like about a 10-metre long fish tank to me.

Megan – It’s pretty accurate.

Tom – It seems to be filled about mid-depth, so maybe 30, 40 centimetres of water. Is this freshwater or salt water?

Megan – This is just normal tap water.

Tom – Okay, great. How are we going to be generating waves? At the moment, it all looks very still and nice, and very tranquil.

Megan – So I’ve got a paddle here. If I move the paddle backwards and forwards, it creates waves in my tank that have various frequencies and wavelengths. The wavelength is the distance from the crest to the next crest.

Tom – So when you say wave here, I imagine almost like a squiggly sort of like sine wave.

Megan – Exactly. Like the traditional example is, imagine a wave along a skipping rope.

Tom – And the frequency I’m guessing is how often these waves appear. So that’s going to be to do with how fast we move the paddle.

Megan – Exactly, yeah.

Tom – So the waves this tank can make, are they a particular type of wave?

Megan – So, there’s a couple of different types of waves. The ones I’m making right now are deep water waves. They’re deep water waves because the depth of the water is quite big compared to the wavelength of the wave. There’s another type of wave which is called shallow water waves. They’re when the wavelength is quite large compared to the depth of the water. They’re ones you’ll see if you’re standing on a beach watching the waves come in to shore. Something that is quite interesting about shallow water waves is the speed of a shallow water wave depends on the depth. That means that as these waves are coming in towards the shore, the bit of the wave that’s over the shallow bit of the shore goes slower and the bit of the wave that’s over the deeper bit will go faster. So, waves will kind of turn in towards the shore and that’s why when you’re standing on a beach looking at the waves washing towards you, they’re always coming like straight towards you.

Tom – The big question is Megan, can I have a go?

Megan – Absolutely. If you move it backwards and forwards, you generate waves. These waves are actually going all the way down the other end of the tank and splashing onto a sort of beach we have at the other end. So we’re going to do a little experiment. So we’re going to make two strokes of this paddle to make a wave. I’m going to do it twice. So the first time, we need to move it to some set distance and do it quite slowly and in the second time, you’re going to move at the same distance but move it a bit faster. And we’ll measure how long it takes to get the wave from one end of the tank to the other.

Tom – So this is two full strokes of the paddle at a slow speed and go… Okay, how long did it take?

Megan – It took 4.5 seconds.

Tom – 4.5 seconds. So it’s 4.5 seconds for our wave to go the full length of the tank. Experiment number two, so I’m doing the same thing. I’m moving the paddle the same distance and you want me to do it faster this time.

Megan – Yeah.

Tom – Right. Ready… Okay, I’d say I moved the paddle almost twice as fast that time, so I’m going to go out on a limb a little bit here and make a guess and say it went maybe twice as fast?

Megan – Nope. It went at the same speed.

Tom – So it’s 4.5 seconds again?

Megan – Yeah.

Tom – Okay, you’re going to have to explain this one, Megan. What’s going on here?

Megan – So what we’re doing in this tank is we’re making deep water waves. And thinking about deep water waves, the speed is set by the wavelength. So because we moved the paddle the same distance, the wavelength is the same so the speed was the same.

Tom – That was quite a fun experiment, I’m not going to lie. I did enjoy making some waves in this pretty much giant fish tank, but of course, you actually do real scientific research here in the Fluids Lab at Cambridge. Can you tell me what kind of things you look at?

Megan – Something we were very interested in is mixing in the ocean and there’s waves inside the ocean, just like there are waves on top of the ocean, and these waves are called internal waves. These internal waves are really important for what goes on inside the ocean and we’re able to – in this tank – run experiments where we’re able to recreate internal waves just like they appear in the ocean.

Tom – So, it’s a case of – we have data from real internal waves in the ocean, but it’s very hard to obtain, and it’s very hard to measure and study them in the ocean. You know, you have to go out on a boat and what not. So, you’re saying, “We take the data that we have and then use it to recreate the real world situation in this lovely controlled environment in the lab where we can really study it, and really understand what’s happening.”

Megan – Yeah, exactly.

Tom – Why are internal waves important and why do we need to know more about ocean mixing?

Megan – So, if you want to make models of the ocean, you may need to understand how it works and internal waves and mixing are really important to that. If we understand how it works then we can build models of it and understand how things like climate change are going to affect the ocean.

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