Inside The Navy's Indoor Ocean

I'm here at the Navy's Indoor Ocean at Carderock. This is the biggest wave pool in the world, and they can make all kinds of different waves so they can test scale ships and make them better before they actually go out on the open ocean. I came in and I'd seen some pictures, but I just walked in here, and it's just, it's insane. 'Cause they say indoor ocean, but it's exactly what it is. The water even looks ocean colored. (laughing) It doesn't look like a swimming pool; this looks like an ocean, looks like a test facility. It is huge.

It is 360 feet long in this dimension, 240 feet long in that dimension. It's 20 feet deep. Just about the size of a football field out there. The dome above us was the largest free standing dome for a while.

• [Derek] Largest free standing dome in the world?

Yep.

• What? (Miguel laughs)

In this pool, they can make waves of all shapes and sizes using huge paddles that line two walls of the pool.

• We have 216 individual wave makers. We can make waves from -45 degrees up to 135 degrees, which is kind of coming right back at it.

We are now behind the big paddles that make the waves. These 216 paddles are programmed to move in incredibly well choreographed ways so that they can produce reproducible, perfect sized, perfect frequency waves that go across the entire pool.

• You can see these air bellows that are what's making the angular motion. That vertical piece is the force transducer. The other force transducer's right up on the top.

• [Derek] There are lots of wave pools in the world, but what makes this one different is control. You can create waves of a specific amplitude and frequency and do so repeatedly.

Can we try a one Hertz?

• [Miguel] Yeah. Do me a favor and dial up one Hertz.

• [Operator] Amplitude will be 0.078 at one Hertz.

Okay, go ahead and send it from zero please. And so this is the largest wave we can make, at one Hertz. That's based on the motion and power requirement for the wave maker.

• [Derek] There's something a bit surreal about watching this, 'cause it almost looks like an ocean except you never see waves this regular out there.

• [Miguel] Yeah, correct.

One of the fundamental characteristics of a wave is its wavelength, the distance from one crest to the next. The first thing most people learn about waves is they transmit energy rather than material from one place to another. In this case, as the wave travels to the right, the water molecules themselves basically move along circular paths. And the deeper the water, the smaller this motion. All motion stops at a depth equal to half the wavelength. This is known as the wave base. But even in an ideal water wave, the molecules do drift a bit in the direction of wave motion. And this is because the molecules travel faster the higher up they are. So they move farther at the top of their loop than they move backwards at the bottom, creating a spiral path.

This place is perfect for observing properties of different waves. I asked Miguel to show me some waves with different frequencies but the same amplitude.

• So what I'll have him do now is I'll have him stop this wave and just change the frequency. 'Cause we're at .6, we'll go to .5, so it'll be a two second wave.

Here I'm split screening waves with frequencies of .67, .5, and .33 Hertz, all with the same amplitude. So two things to notice. Even though they all have the same amplitude, the ones with higher frequency look like they have a greater amplitude because the slope of the waves is steeper. And second, the frequency of a wave affects its speed. High frequency waves travel slower than low frequency waves. In fact, as long as the water is deeper than the wave base, wave speed is inversely proportional to its frequency.

They have a really cool demo that takes advantage of the different speeds of different frequency waves. You can see it starting here. They send out high frequency waves first, followed by lower and lower frequency waves. And because the high frequency waves travel slower, the lower frequency waves gradually catch up.

Whoa. And they've timed it so that all the waves meet up at exactly the same time and place in the pool, and this causes the wave to break. The ocean engineers can do this again and again, in exactly the same way, thanks to their precise control over the waves. This demo also nicely illustrates the principle of superposition, that when waves meet, they add together. The height of the water is equal to the sum of the heights of the individual waves meeting at that point.

You can see how much bigger the amplitude is. Those individual waves weren't that big, but when you add them all together, you can make this big breaking wave. They can also take advantage of the superposition principle to create standing waves.

• So what's coming up next are two regular waves coming at each other. What we call the quilt wave. So we're gonna have a wave coming this way and a wave going this way, and it's gonna create standing waves. So there's two regular waves coming out and if you look at the wave, it looks like a big quilt pattern out there.

At some places in the pool, the waves always cancel out to zero amplitude, and at other places, the waves add up for maximum amplitude. They can even send waves from all directions, so they form circular wave fronts and then all the wave energy is channeled into one spot they call the bullseye.

• And so now we're gonna run the bullseye wave which is essentially the same thing, but instead of having a line of waves, we're having it all coalesce at one individual point. So you can start seeing the waves are coming from the long bank here, and you can see they're making a spherical wave. And then you have another spherical wave coming from the short bank. And this is breaking due to the coalescing waves, and the wave height being more than one seventh of the wavelength.

We tried throwing some toys into the wave to see what would happen to them. Would they get pushed into the breaking wave? Even though there's not much net movement of the water, the ducky drifts with the waves, and pretty quickly is pushed into the bullseye.

How's the ducky doing? (Miguel laughs)

• He's getting to the danger zone right now. It's starting to funnel him right into that breaking wave. Ooh, it's getting up, getting up.

• [Miguel] Oh! (laughs) It swamped it.

• [Derek] That's amazing.

That was right where we wanted it.

• [Derek] Now the real purpose of this facility is not to play with toys or make perfect unnatural waves. It is to replicate on a small scale the types of waves Navy ships will encounter in the oceans of the world. Research engineers place ships modeled after billion dollar vessels in the water to see how different designs actually behave in real world conditions.

• [Miguel] Right now this is coming from 45 degrees. It's gonna be about a five inch significant wave height, which if we were to scale it up for this model would be 20 foot waves. When we're doing a free running model like this, we usually run a race track, like a big circle or a figure eight track, so we know the headings that we're running in, so that we can correlate that to the full scale vessel.

• [Derek] For the model to provide an accurate representation of the real world, a lot of things must be taken into account. Is the water fresh?

• Fresh water.

• [Derek] Okay, not salty.

• Nope. Fresh water. So when you're in salty water, you're gonna have a lot more buoyancy. So when we're balasting our models, we have to make sure that they take into account that buoyancy difference. So when we go full scale you're in the same conditions.

• [Derek] For fluid mechanics, I always expect that you have to keep the Reynolds number the same as in the real world phenomena. But actually to get the right wave dynamics, you have to use a different scaling which is based on the Froude number.

So the Froude number is a measure of the ratio of inertial to gravitational forces. It's equal to the flow velocity divided by the square root of the acceleration due to gravity times the characteristic length, like the length of the ship. In this case, the model ship's hull is 46 times smaller than the real thing, which means to get accurate data, it should be traveling at one over the square root of 46 times its real world speed. And to make the footage from the model look the same as that from the full size ship, you have to slow it down by a factor of the square root of 46. So roughly 6.8 times slower.

I'm amazed at just how well these shots match, but of course, that's the idea. Scale the model and the waves, so the physics are identical to a real ship out on the open ocean. Naturally, I asked if I could go swimming in the pool, but they said, very kindly, "No way." The closest I could get would be on a little dingy.

This is our boat.

With a catch. It's pretty smooth sailing out here right now.

• (laughs) Yep. No waves while we're out here.

• [Derek] So I'm assuming no one's ever been out here in waves?

• Nope. That's one of the no-nos they don't want us to do. I guess it's a risk thing, so...

This place seems like a... I don't know Like a massive playground kind of. (Miguel laughs)

• It kind of is for engineers like us where we kind of dork out on the science and what we're doing here. It's a huge volume. Like I guess I never understood how deep 20 feet was, until they emptied it to put in the new wave makers. It's a large volume that's taken up by this water.

• [Derek] Yeah.

So as we come by, these are our sensors right here. We have a big array here. These are ultrasonic sensors, and that's how we measure wave height and period and direction in the basin. So we wanna measure that to make sure that what we test in is what we think we have.

• [Derek] In this pool, they can create all sorts of different wave conditions you might encounter in different parts of the world. Most ocean waves are created by wind and the strongest winds occur in and around storms.

Five factors affect the size and shape of waves created. These are the wind speed, the wind duration, the distance over which the wind is acting, which is known as the fetch, the width of the fetch, and the depth of the water. As waves travel out from a storm, the higher frequency waves dissipate their energy more quickly. So the waves that travel a long way are the fast moving low frequency waves, which are called swell.

• When those waves end up becoming hundreds of miles away, like if you have it in the Pacific, eventually you'll get long period swell from them. So you're no longer near the storm, but it created enough energy to make long waves, and that's where you get your open ocean swell.

• Tell me if this is a good analogy. I feel like with sound, a lot of the high frequencies will die off quickly away from a source.

• Yep.

• [Derek] But the low frequencies will travel much further.

• Correct.

• So is it the same thing with the waves? It's like you're walking away from a concert, and you can still hear the bass, but you can't see any of the high frequencies.

• That's a great analogy. Yep.

What's the deal with rogue waves?

• People like to think it's a rogue wave where it just came outta nowhere and just came up. No, it's usually multiple waves that are meeting up and creating an amplitude that's much larger than what the self-standing wave would be. So when it meets it's gonna break, because you have this large wave creating this huge amplitude that it just can't hold it and it breaks.

• [Derek] On a calm day, when you see waves crashing at the beach around 10 seconds apart, that is swell. But because of its long wavelength, swell isn't really a concern for ships out in the open ocean.

• You know, if you're in a long period swell, your ship's probably just gonna heave a little bit. You're more worried about the steep waves and the windy waves that are really moving you around.

• [Derek] Wind waves are formed in three steps. First, as wind blows across the surface of perfectly still water, the turbulent motion of the air creates regions of slightly higher and slightly lower pressure. And this makes tiny ripples with wavelengths of around a centimeter.

But now the wind can act on these ripples, creating larger pressure differences between the front and the top of the wave crest, pulling them up into bigger waves. And the interaction of the wind with these waves then creates even larger pressure differences and even larger waves.

The waves are mostly uniform at this point, but as they interact with each other, they create a range of different wavelength waves. And as the wind keeps blowing, these waves begin breaking, transferring their kinetic energy into swirling eddies that dissipate their energy as heat. Once the energy dissipation matches the energy input from the wind, the waves have reached their maximum size, and this is known as a fully developed sea.

• [Miguel] So this is gonna be an irregular wave.

• [Derek] This is irregular?

Irregular wave, so what you saw earlier with the regular waves were one frequency, one amplitude. This is what we call a spectra, or multiple frequencies and multiple amplitudes. You can see there's higher frequency with the waves that kind of go travel slower than the low frequency waves. Those low frequency waves will travel fast and overcome 'em, and that's what's making 'em look peaky or kind of dulling it out.

• [Derek] What surprised me is that the different oceans of the world have different mixtures of wave frequencies or different spectra, depending on their geography and the types of storms that occur there.

For example, the North Sea and other small bodies of water have a peakier spectrum, and this is due to the limited fetch of storms that occur there. In the mid-Atlantic, a broader spectrum best describes the developing or decaying open ocean waves that you'd find there. And in the North Atlantic, the steady wind across an open ocean produces the broadest spectrum of wind waves.

So when testing, engineers first have to figure out where the ship will be deployed, and which spectra best match these locations before creating them in the pool.

For most people I think, an ocean is an ocean. But you're saying that there's sort of different conditions depending on where you are?

• The destroyer when I was in command, we did an operation off the coast of South Korea in the spring. Very rough sea keeping conditions. But then, when you're crossing the Pacific, a lot of that is a lot calmer. So again, you know from there to the coast of South Korea to the Arabian Gulf, all those very different conditions.

• Were there any conditions that were particularly rough for you?

• So my bed was actually in the middle of a room and the seas were so bad, and this was either South or East China Sea. The seas were so bad that one night, I woke up in the middle of the night and my whole mattress with me on it was sliding off of my bed frame, and that's a pretty significantly sized mattress. So you can imagine the seas we were in that night. Much bigger than this would terrify me. I know it probably looks benign, but... (laughs) Much bigger than this, I think that model will take a lot of water.

• Why do you care about how much water goes on the deck?

• So on the back of this DDG is a helicopter landing pad. They don't want any water on the deck when a helicopter's about to land. That's a big problem. You know, that's one of the tests that we do here is we'll put cameras to look at the deck and understand how much water washes on.

• [Derek] Since I knew they wouldn't wanna risk their fancy model in rough conditions, we brought along a little remote controlled boat to test.

• Yeah, I wouldn't be happy on that boat. A lot of people would be getting seasick.

• Whoa! (Miguel laughs)

• Oh no.

• [Miguel] Is it gone?

• [Derek] It's gone.

• [Miguel] No, it's right there. It came up. It's upside down. (laughs)

It was totally gone. It was in the air, then it went under.

Now, not all the models tested here can be remote controlled.

• So on the carriage is where we're gonna do captain model tests where you can tether, put power and instrumentation onto a model that can't hold it itself. So usually the model go in this moon bay right here.

• [Derek] The models are hooked up here, and then the whole lab speeds over the waves towing the model underneath.

(Waves crashing)

People have been making ships for thousands of years.

• Mm-hm.

Is there actually any innovation today?

• Most definitely. So sometimes, you know, people say that's the way we've always done it. And then when you look into it, there's some validity to some hair-brained ideas. And when we test them, that's why you cut your cost of doing a model test versus building the full thing and saying, "Oh that didn't work." Every ship that's in the Navy's fleet has gone through here, has gone through either our purview, or has been tested peripherally with us. But all of the Navy-owned ships have been tested in this facility, and there is a ship out there with a tumble home design where if you look at this ship behind you, it flares out.

So this flare is usually what helps protect you from, when you start rolling, it gives you a reaction force or helps push you back. A tumble home is shaped, you know, the opposite direction. And if you have a ship shaped in that direction, it doesn't have as much of a restoring force when you roll.

• But what is the idea with making a ship like that?

There's a lot of different reasons why you want to change a hull design. Some of it is the above water signatures. It's all about the shape and radar sections, and there's a lot that goes into that. You always wanna be stealthier; you always want to be faster; you always wanna have more power. And that's always what the innovations come.

• [Derek] So most of the sailors aren't aware of the work that's going on in the background to support what they do.

• When I was in the fleet, and I've been in the Navy 27 years, I never had any idea, certainly not the magnitude of what they do. I'm not exaggerating when I say it's impacted every ship and submarine in the fleet.

(Waves crashing)

(Electronic zoom)

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