The Joy of Why - What Can Jellyfish Teach Us About Fluid Dynamics?

Episode Date: June 28, 2023

Jellyfish and other aquatic creatures embody solutions to diverse problems in engineering, medicine and mathematics. John Dabiri, a fluid dynamics expert, talks with Steven Strogatz about what jellyfish can teach us about going with the flow. The post What Can Jellyfish Teach Us About Fluid Dynamics? first appeared on Quanta Magazine

Hello, I'm Brian Cox. I'm Robin Ince, and this is the Infinite Monkey Cage trailer for our brand new series. We've got mummies, we've got magic, we've got asteroids. Mummies, magic and asteroids. What's the link? That it was an asteroid that magically went over the world that led to Imhotep the mummy coming back to life? That's correct. I thought it would be weird, scientific as ever. But the most important thing to know is that we are going to deal with the biggest scientific question we finally ask. What is better, cats or dogs?

Listen wherever you get your podcasts. I'm Steve Strogatz, and this is The Joy of Why, a podcast from Quantum Magazine that takes you into some of the biggest unanswered questions in math and science today. People say that biology is a great teacher for engineers. Just think about all that a soaring eagle can teach us about aerodynamics. My guest today thought a jellyfish would be an instructive thing to study for a summer internship in engineering. And years later, he's still studying jellyfish for the wealth of information they have to offer about fluid dynamics. The subject of this episode, what can the movement of jellyfish and schools of fish

teach us about the motion of air, water, and even blood? By studying the math of how schools of fish move in unison, our guest today has been able to figure out how to place wind turbines to generate clean energy more efficiently. But that's not all. It turns out that the way a jellyfish swims can even inform us about the health of a human heart. And jellyfish have taught us new tricks about underwater propulsion, which might be helpful to a new generation of submarine design. But let's let our guest John DeBury tell us more. He's a professor of mechanical and aerospace engineering at Caltech.

He won the Waterman Award in 2020, the nation's highest honor for early career scientists and engineers. He's also a member of President Biden's Council of Advisors on Science and Technology. Welcome, Professor John DeBury. Thanks, Steve. It's great to be here. It's really a great pleasure to have you here. We've known each other for a little while, but I don't think we've had a chance to talk shop before, so I'm excited about this. You know, I have to confess, although we're going to be talking with you a lot about jellyfish, I have never held a jellyfish, never been stung by a jellyfish. You're missing out. I've done both.

How so? What was your close encounter with jellyfish like involving stinging? Well, you know, it was actually a photo shoot that I was doing for a magazine and the photographer thought it would be nice for me to get up close and personal with my subjects. And so he got me into the water and told me to hold on to the jelly. And meanwhile, its tentacles started dripping all over my legs. And so it was a very painful photo shoot, but we got the shot. Are you grimacing in the picture? You know, somehow they managed to make it look like I'm smiling and enjoying the whole thing, even though it was pretty miserable.

Well, I'm sorry. We won't be subjecting you to any of that today. Thank you. But so, you know, when I see like on David Attenborough TV shows or other nature shows, jellyfish swimming around, they look almost like a bag, like a cellophane bag kind of just being pushed around by the water. But I know that can't be right. They're not just passive swimmers. So can you tell us a little, how do they move? Do they have muscles? They do. And in fact, jellyfish are the first animals we know of to be able to move around in the ocean.

That swimming that you see in those documentaries is powered by a single cell layer. Think of a very thin layer of muscle that is able to contract and expand with a rhythm almost like the beating of your heart. And that allows them to propel through the ocean. So when you speak of the rhythm, that makes me think then they must also have a nervous system controlling the muscles. In fact, jellyfish don't have a central nervous system at all. They don't have a brain either. All they have are these little clusters of cells around their body that tell them when

to fire their muscles, when to contract. And so they use those muscles to coordinate their swimming motion in a way that's very different from how you and I move around. Uh-huh. So it's, I mean, there's a bell, right? They talk about the bell, that sort of, what is meant by the bell? That's right. So if you look at a jellyfish in an aquarium, it kind of looks like an umbrella or a bag, as you said. And around the bottom edge of that umbrella, there are a couple of clusters, usually about eight of them. And those are the places where the body sends the signals to swim,

to contract the muscle. And so by coordinating those contracting signals, they're able to swim through the water with very low energy consumed in the process. Yeah, I can definitely not relate to that. When I think of my own swimming, which is so awkward and expends a lot of and wastes a lot of energy. So what is it that you're saying here? You say they're very efficient swimmers. What do you mean? We know that jellyfish were some of the first animals to swim more than 200 million years ago. They've survived mass extinction events. And so for a long time, it's been thought that there must be something about their ability to move efficiently that's allowed them to survive for so long in the oceans, to survive even in the face of more

exotic swimmers like dolphins and sharks, the ones that you might think of when you think of an excellent swimmer. Well, it turns out that the very simple body shape of these jellies, that simple umbrella, it creates what are called vortex rings. Think of a donut of swirling water. So each time the animal contracts its muscles, it creates this donut of water, and it almost pushes off of that donut of swirling water to move through the water without having to use a lot of energy in the process.

So it's a very different swimming stroke than what you or I would try to accomplish in the ocean, but it's quite effective. So suddenly an image is coming to my mind. Tell me if I'm on the wrong track with this or not. But as a kid at summer camp, I remember doing canoeing and they would have us put our paddle in the water. And I was told to make a J stroke where you push back with the paddle and then curl it back. And you could see little eddies, little whirlpools of water coming off of that. That's right. That

stroke. Is that relevant to what you're talking about with vortices? It is. So throughout the ocean, and in fact, even now, as I'm speaking to you, my mouth is pushing the air around me and creating these swirling currents that we call vortices. So when you're swimming, you're creating those vortices. That canoe paddle creates these swirling vortices. What's different about the jellyfish and their vortex rings is that they have this almost perfect circular shape. And that circular shape allows them to swim with an efficiency that's better than what you or I are able to generate by stroking our arms or a

canoe paddle. So it's really the shape of those vortices, those swirling currents, that's the key to their very efficient swimming. And that's what for a long time we tried to understand in unlocking the mystery of how these animals have survived for so long in the ocean. It's really those circular vortex rings that's the key. So let's see if I have the picture right in my head. When you speak of a circular vortex ring, now the other image that's coming to mind is those – not people don't smoke as much as they used to, but you know where I'm going, right? Like there are guys that will smoke cigars or people who blow smoke rings. Is that the kind of circle I'm supposed to picture coming off of someone's rounded lips? Absolutely. When I used to teach,

this was the example I used classically, but now we're trying to discourage, you know, smoking or vaping. But if you imagine a non-toxic version of that example, you're exactly right. It's those smoke rings people would blow that look like a donut of air and it's swirling. And it keeps that circular shape for long distances away from the person who blew it. Maybe another version of this is sometimes you'll see dolphins doing this in the ocean, playing with bubble rings that have a similar kind of a shape to them. It's a donut of water with air trapped in the center. And the way that the dolphins are able to maintain those rings in that case is because of the stability of that particular type of swirling current.

It's really unique in fluid dynamics. All right. So it's as fun as it is to be talking about jellyfish, and they are admittedly very cool and efficient. But for those folks out there listening who may be wondering why are we spending so much effort on them, help us understand more broadly what is fluid dynamics about? Where does it apply in the rest of science or technology? Yeah, so fluid dynamics is all around us. In fact, for me, one of the really exciting

application areas growing up as an aspiring mechanical engineer was in thinking about more effective rockets and helicopters, propulsion systems in general. Now, we know this field of fluid dynamics, the study of how air and water move, is really complicated in terms of the motion that the water or the air makes, in terms of how we try to describe it using physics. And so there was a movement that emerged now a couple of decades ago to say, why don't we study some animal systems that have already figured it out, have figured out how to swim efficiently or how to fly efficiently? You can actually go back centuries to Leonardo da Vinci and trying to understand how to develop human

powered flight by looking to birds. So there is actually a long legacy of studying natural systems to gain inspiration into how we can develop more effective technologies. That's kind of how I entered the field, but the jellyfish came about really out of my desire to come up with a convenient summer project. I was here at Caltech for a summer research project, and my advisor here said, let's go to the aquarium and try to find an animal system to study in the same way that I had in my college years been studying helicopters and rockets. To be honest, I wasn't thrilled about that at the time. I thought I was coming to Caltech to study rockets and propulsion. You know, Caltech has the Jet Propulsion Laboratory, which it's famous for. But we got to the aquarium and

I thought, well, I've got a 10-week project here. Let me pick the simplest animal I can find. You know, it should be easier to come up with a simple model for it. And so the jellyfish seemed like an easy out. And of course, here we are 20 years later and I'm still trying to figure out how they work. It turns out that even a very simple animal like the jellyfish has a lot to teach us because of how they interact with the water in such an elegant way. And that's really what has driven us to study jellyfish in particular, in this broader field of what's sometimes called biomimetics or bio-inspired engineering, looking at biology to find solutions for engineering challenges. I have to say, as a mathematician, I was always drawn to fluid

dynamics because it's so difficult. Some of the most difficult math problems that we have faced in the area I'm interested in, in differential equations, first arose in connection with problems in fluid dynamics. So you mentioned, okay, so rockets, jet propulsion for, we could think about airplanes. There's medical applications. Absolutely. We just came out of COVID. I mean, to give you a very present example, questions about the transmission of COVID really were fluid dynamics questions.

How do the aerosols form? How are they transmitted? How are they collected on other people? If I want to design a mask, what's an effective way to do that? In climate change, modeling the Earth's climate is in large part a fluid dynamics problem. Fluid dynamics shows up in all aspects of our life. What I think is really exciting about this study of animal systems is that from my perspective, if you're building an airplane, it's a human that sits down at a computer and tries to solve those very complex equations that you described to figure out what's the ideal shape of the wing, what's the ideal shape of the rest of the aircraft. In some ways, jellyfish are

solving partial differential equations every day as they swim through the water. And so we just have to figure out exactly what it is about their swimming that allows them to come to that particular solution to those differential equations. And then the hope is we can apply that to our own design problems where we don't have the same constraints as jellyfish had in evolution. We have a brain, a central nervous system, and more than a single cell layer of muscle to work with. We have engineered materials we can work with. Now we have AI to work with. And so if we combine what we know about jellyfish with all the tools at our disposal as engineers, really the sky's the limit as to what we can develop.

Well, so then let's get into the question of how the jellyfish are doing it. What kinds of experiments did you do to figure out how they're using the vortex rings that they generate when they contract their bell? Yeah, so the first challenge to tackle is the fact that water and air are transparent. So even as we're sitting here talking to one another, the air around us is in constant motion due to our breathing. We can't really perceive that. The same thing is true in the water. the air around us is in constant motion due to our breathing, we can't really perceive that. The same thing is true in the water. If you go to an aquarium, for you, the main attraction is probably the animals. But for me, it's the water surrounding them. The issue is you can't easily see that water motion just staring at the tank. And so what we did was to develop some new technologies to help us measure that water surrounding the animals. The first thing you

could do is think of putting dye in the water, like a food coloring, because that'll show how the water locally is being carried. It's a qualitative picture. It gives you sort of a general description, but not something you can easily put numbers on to say the water's moving this fast in this direction. But what we can do is use some techniques

that are common in engineering using lasers for example so in the water there are tiny suspended particulates think of the the the sand or silt that's suspended in the water we can illuminate that with laser sheets i take a laser pointer you might have at home and shine it through a glass rod and it'll spread that beam into a thin sheet of light so we put that sheet of light through the water. It reflects off of all of those suspended particulates that are in the water. And now we can track each of those little particles, almost like a moving starry night. That's kind of what the videos look like. And each of those stars, those particles of

sediments in the water, tell us something about how the water is moving locally around the animal. So we developed these techniques in the laboratory. The big challenge then is to go and find jellyfish in the field and actually measure this. I was fortunate to find students who were game to go swim with jellyfish and take lasers with them. But so let me get this, you can take the laser pointer or whatever underwater and there's no problem? Well, so that was part of the student, Kakani was her name. Her PhD thesis was to develop the technology to allow us to do this so that a scuba diver could go into the ocean, sidle up very carefully next to these jellyfish, and then be able to turn on a laser and measure the water around them. And it turns out she was able to be quite successful in capturing for the first time

these swirling currents in really exquisite detail. And is there also some video camera setup? There is. In fact, that imaging technology is largely video-based. So you're getting a video of that moving water, the sediment particles reflecting the laser light. of that moving water, the sediment particles reflecting the laser light. And so by looking at how, as time evolves, the water around the animal moves, we can figure out in some cases the animals are not putting that much energy into the water in order to move. We call that efficient motion, when they can move forward without having to churn up a

lot of the water around them. Interestingly, some species of jellyfish will rarely swim, but when they do, it's in a survival mode. It's to escape a predator or to catch their prey. In those cases, they will put actually a lot of energy into the water. Our thought on that is that it's a question of survival. You're not so worried about efficiency when it's either kill or be killed. And so in those cases, we're able to also see a difference in the water around the animals, all captured by this laser technique. Okay. Maybe my whole cellophane bag picture is just so wrong and I need to get that out of my

head. But it feels to me like that would encounter so much drag, even if it's got a nice coordinated motion. There must be some trick to the way that these vortex rings are behaving to help the motion be as efficient as it is. Did your measurements reveal something surprising or tricky that the jellyfish are doing? Yeah, it's a great question. And there's a couple of ways to think about this. First of all, I should back up and say, in terms of the behavior of the jellyfish, one of the differences between what they do naturally and what we might think about in our own submarines, the jellyfish are using those same currents to feed. So as they create these vortex rings, that swirling current actually pulls in prey toward their tentacles where it gets captured and eaten.

And so it's very plausible that, in fact, the motion we see, them moving from point A to point B, is not actually the desired outcome. It's just the inevitable consequence of Newton's laws of action and reaction. In some cases, the animals are creating these vortex rings just to draw in prey. But because they're pushing that water, the reaction is they move in the process. water, the reaction is they move in the process. And so for them, that efficient motion is not necessarily trying to get somewhere in a hurry, where what we've been able to do is to say, let's take that same idea, the vortex ring formation. Our submarine doesn't need to feed in the same way as the jellyfish do. And so we can go faster, for example, using that same propulsion technique, even though the real animals themselves don't.

This is really the distinction between a rote copying of biology, you know, going back to the days of people trying to achieve human-powered flight by flapping wings really hard. Eventually, we found success by using fixed wings and sticking a jet engine on the thing, and that was the trick. taking a jet engine on the thing. And that was the trick. So here we want to be careful about not simply blindly copying what the jellyfish do, but asking what aspects of its behavior lead to efficient propulsion. And then when we want to design a submarine that's fast and efficient, we can deviate from the blueprint that the animals gave us. So with regard to design of futuristic submarines, is there some principle or observation that we have drawn from the jellyfish that could suggest some kind of crazy new design? Yeah, we've explored this question. And the key, again, is these vortex rings, these swirling circular donut-shaped currents.

If we can come up with a submarine design that could create those, but that doesn't require the back that instead of having a smooth, continuous jet flow propelled in the back, it creates a choppier flow. So think of a pulsing of the flow behind the vehicle. We were able to show that that vehicle could be 30 or even 40% more energy efficient than the same type of vehicle without that pulsation in the flow. Now, the tricky part here is coming up with a mechanical design that isn't overly complex. If you make that part too complex, you're going to be replacing those components. And in fact, those mechanical components themselves can suck energy from the vehicle. And so we haven't been able to come up with a design that achieves the fluid dynamics inspired by the jellyfish without overly complex mechanical components. And that's been the unsolved mystery there. Well, before we leave jellyfish and their propulsion for, I want to get into wind turbines in a minute. I would like to just talk a little more about vortex rings across the animal kingdom because I've heard from some of my colleagues who study insect flight or hummingbird flight or, you know, dragonflies, hawks.

There's just a lot of creatures that make use of vortices in various ways. Although all the examples I just mentioned are in the air, not the water. mention are in the air, not the water. Can you tell us a little about differences or similarities between the airborne creatures and the, well, I don't say waterborne, but you know what I mean, if I'm in the water or the air? Yeah, absolutely. The aquatic ones. Yeah. And we can take that a step further to blood because in the human heart, the same sort of vortices end up forming in your left ventricle, that oxygenated blood as it passes from the left atrium to the left ventricle. This is before it goes through the rest of your body. There's a point in which it passes through a valve and you get vortex rings that are strikingly similar to what a

jellyfish creates or what a squid creates. So you're absolutely right. This vortex loop or ring motif, sometimes they're more complex chain structures. But in each of these different animal systems, we see this reoccurring. And so a lot of our research, in fact, has been trying to understand whether there are some underlying principles that we can learn about the design of these vortex rings. And it turns out that there are. So all vortex rings are not created the same in the sense that there are certain vortex rings that are great for efficient propulsion, like the jellyfish example we've just talked about. But there's different types of vortex rings that are created in the case of just trying to generate a lot of force. If I just want to move

really fast, for example, the jellyfish that want to escape a predator create a vortex ring that's different from the very efficient vortex rings we've been talking about a moment ago. So what we thought, and this is maybe a couple decades ago now, is perhaps we could use that insight to understand the vortex rings in a very different system, the human heart. So like I said, during the filling of the left ventricle, you get this vortex ring that forms. It turns out that in a healthy patient versus a patient who has certain diseases, one called dilated cardiomyopathy in a large chart, for example, their vortex rings look very different from the vortex rings that form in a healthy patient. What we found was an interesting correlation where the change that we see between a healthy patient and some of these patients with these pathologies

is very similar to the difference between an efficiently swimming jellyfish and one that's escaping a predator or trying to catch its prey. And so one of the key benefits of looking at these fluid dynamic signatures of efficiency versus dysfunction is that those changes can sometimes occur before the structural changes in the heart or before some of the systemic, you know, body-wide changes that would say something's wrong with you. And so we saw this as an opportunity for a more sensitive and earlier diagnostic or a flag for disease and dysfunction in the human body. Subsequently, there have been other labs to show that, in fact, these changes in the flow within the heart can, in fact,

be an effective marker of disease in humans. Wow, John, that is exciting. Yeah, very neat and unexpected connection. But Steve, it goes back to your earlier point about the recurrence of this vortex ring motif in fluid dynamics, whether that's air, water, blood, whether it's swimming, whether it's flying organisms, or whether it's us sitting here talking to one another with our hearts pumping blood. Well, this is great. I'm really bowled over by this last medical example because, I mean, especially that it could be an early warning system, an early diagnostic. But I'm wondering, what is the imaging technology that allows, you know, you're not going to put sediment in the heart. What are we doing? Does it show up on ultrasound or MRI? How would you look? Exactly. Yeah. So the early work was done in MRI, more recently ultrasound techniques. What current labs are also working on are potentially even

acoustic detection, so that the blood flow in certain types of vortex formation would have a sound that's detectable by effectively an electronic stethoscope. The goal here is to come up with the simplest technology that would allow you to detect this, because not everyone's going to have an MRI machine at their disposal or an ultrasound machine at their disposal. But you could imagine a $10, $20 acoustic measurement, sound measuring device that you could buy at Walmart and be able to detect these types of changes and have that at home. So that's the goal. We're not there yet by any means. But what the jellyfish have done is given us an initial target for what to look for in terms of the changes in the flow that occur in those healthy versus sick patients. Well, all right. So let us now step out of the water and start talking a little bit about some of the work that you've done with your colleagues about wind turbines in California, in Alaska, to help make them more efficient. So first of all, if I say wind turbine,

the first image that comes to my mind is one of those giant white propellers standing way up tall in some field somewhere. Is that the right image or should I have a different picture in my head? Yeah. So these turbines are a different type of turbine, although our work was largely motivated by some of the challenges with those large turbines. The biggest challenge is that the individual turbines are very efficient in terms of how well they're able to convert the motion of the wind into electricity. The challenge is that downwind of each of those turbines, they create a lot of choppy air or turbulence. That choppy air would reduce the performance

of any turbine that was downwind of the first one. And so that's why if you see one of these wind farms out there, the turbines are all spread very far apart because they're trying to make sure that the choppy air between the turbines doesn't reduce the performance of the group. It always struck me as sort of ironic that if you look in nature, think of schooling fish in the ocean, they're flapping their tails, they're creating their own wakes, as we call them. So that choppy air behind the wind turbine we call a wake. The fish create these wakes as well. They swim in groups and they don't spread out as far apart as possible, but instead they coordinate their positions one with the other and in fact can take advantage of the flow that's created so that the whole is greater than the sum of its parts, meaning

that group of fish can swim more efficiently together than they would separated from one another. We see this in cycling, the Tour de France, you'll see the cyclists taking advantage of the aerodynamics of their neighbors. And so the question here was whether we could come up with an analogy to those fish schools that would work to sight wind turbines. Now, here's a place where almost by coincidence, I teach a class at Caltech on the fluid dynamics of swimming and flying. And in my lectures on fish schooling, I write on the board the equations for how you would predict that beneficial interaction between the wind turbines. One of the key features of those models are these vortices that we've been talking about so far, these swirling currents that the fish would create. currents that the fish would create. The mathematical model for one of those vortices is almost identical to how you would represent what are called vertical axis wind turbines.

So I'll pause there for a second and say the wind turbines you're used to seeing, the propeller style turbines we talked about, are called horizontal axis wind turbines because the blades rotate around an axis that's horizontal. A vertical axis wind turbine, the blades rotate around an axis that's sticking out of the ground vertically. So like a merry-go-round, for example, would be an example of a vertical axis type system. Those systems mathematically can be represented almost identically to fish schools. And so that was the connection where I said, well, let's try to think about designing wind farms that would have that fish school type of orientation to them. So I had a couple of students in the lab for one of their projects do a back of the envelope for

how that would improve the performance of wind farms in terms of the energy you could produce on a given plot of land. Let's say I give you, Steve, 10 acres and I say, I want you to generate as much electricity as you can using the conventional wind turbines. For the propeller style ones, you could probably only fit one of those turbines on that plot of land. For these smaller vertical axis wind turbines, it turns out on pencil and paper calculation, you could get 10 times more energy out of the same plot of land by taking advantage of these effects. Now, that's a pencil paper calculation. And so you could say, well, that's a great theoretical idea. But we were lucky to be here at Caltech, where I went to the department and said, I'd like to buy some land and try this. And so this was around the time of the 08,

09 market crash. And so you could get land pretty cheap. So we bought a couple of acres of land here in the northern part of LA County for, I think, just $10,000 or $15,000. And we made a deal with one of the companies that builds these vertical axis wind turbines that they would give us the turbines for free in exchange for the data, because it's really expensive to test a new turbine if you're a startup. And so we put a set of these turbines out in that field. We got up to about two dozen of them, in fact, at our field site. And we were able to show in the real world that, in fact, you could get 10 times more energy out of a plot of land using this fish-inspired type of design. So it was really exciting,

a really exciting find, and one that we're still continuing to pursue today. really exciting find and one that we're still continuing to pursue today. Very, very, very exciting. I had never heard about this. I mean, I had some vague notion that you've worked on fish school-inspired placement of wind turbines. But just to hear you tell the story and the buying the land, I mean, I don't know. It's just a personal aside. So I'm a mathematician who doesn't ever buy land and test my ideas.

I'm wondering if – when people think of the normal criticisms of the big, tall, propeller-looking wind turbines and – is this sort of more appealing do you think aesthetically or less appealing? I would imagine it's like it would seem they don't have to be as tall or block people's view or... Exactly. In fact, we studied this scientifically while I was at Stanford University working with Bruce Kane, a social scientist. We were able to study in California attitudes about these different types of turbines. And you're exactly right. It's the lower visual impact as an important feature. But one that even is more significant is the potentially lower impact on birds and bats, which is for the large turbines, an ongoing challenge, the potential for birds to

run into the blades or bats and other areas. These vertical axis wind turbines, they're lower, as you said, to the ground, but they also have a different visual signature. So, frankly, in the large turbine cases, a bird can simply not see the blade before it's too late. In the case of these vertical axis wind turbines, the visual signature is much more apparent because the blades move more slowly than they do for those large turbines. because the blades move more slowly than they do for those large turbines. Now, the reason you don't see them everywhere now, given what I've just told you, is that there's still work to be done to improve their reliability, which in some ways, I like to say, it's not rocket science.

You know, we have people here on the campus putting rovers on Mars. So clearly, we should be able to design a wind turbine that can last through the Alaskan winter, for example. But we're not actually there yet. There just hasn't been a lot of investment in these new types of technologies because it's very expensive to develop a new energy hardware. So it's a work in progress. You mentioned that some of the ideas came from math, like that there was math associated with schools of fish

that could then be adapted to the case of the wind turbines. That's right. I'm trying to imagine that math. Can you say a little more? What is the math that goes into that? Yeah, sure. So what we try to come up with when we're thinking about a vortex, for example, is a simple mathematical description of how a vortex affects the surrounding flow. And so we have in our field something called potential flow theory. It's a simplified representation of these more complex fluid flows we've been describing. The benefit is that on a piece of paper, I can write down an equation that says, if I have a vortex at a given location,

here's what all of the air or water around that vortex will do. We can write that in a single line of math. So the benefit of this potential flow theory is that if I, say, have a vortex on my left and a vortex on my right, I can immediately calculate how they affect one another just by adding those two effects together. We call this a linear superposition, but we're just adding those two effects on top of one another. What that means when I study fish schools is that I can write an equation one time, and if I want to know the effect of 20 fish, I can effectively multiply the answer by 20, give or take, without having to do a lot of more complicated calculating. In the case of the wind turbines, in order to design an optimal wind farm,

once I have the representation mathematically of one of those wind turbines, I can optimize an entire farm of a thousand or, you know, if I wanted to, 10,000 wind turbines without having to develop any new math really. So it's a really convenient way to represent these systems. It turns out that that fundamental mathematical representation of a vortex that a fish sheds is almost identical with a pre-factor difference to the mathematical representations of those vertical axis wind turbines. And so that convenience of mapping one-to-one, the fish school problem to the wind turbine problem, allowed us to borrow a lot of the

same mathematical optimization that was done to come up with optimal fish school configurations and use that almost directly to optimize the wind farms. The only difference is the objective. In the wind farm, rather in the fish school, you might say the optimization is trying to minimize the drag that that group of fish is going to see as it's moving through the water or minimize the energy expended by all of those fish as they swim. In the case of the wind farm, my objective might be, let me maximize the amount of energy I'm collecting from the wind, or let me try to design this system so that for wind coming from particular directions, I get maximal wind depending on the local topography that I have at work. So the underlying mathematical machinery is the same. The objectives that we optimize for could be different. kind of mind it takes to do the work that you're doing, the breadth of interest that you show with,

you know, moving freely between engineering of wind farms, the medical aspects of vortices in the heart, the math needed to understand it. Probably you haven't even mentioned computer science yet, but I'm guessing that would come in. Absolutely. There's the base. I mean. It's a lot of fun. Yeah. Okay. Good attitude. No, it is. You know, I would just say that a lot of times I think students, you know, those in high school or in college, you get the impression that in life you have to pick one thing. I'm going to study biology or I'm going to study chemistry or I'm going to study physics.

And that's the thing. In reality, some of the most interesting research is really at the intersection of these different fields. And so, you know, it's not to say it was an easy path to become comfortable with those different fields. Here at Caltech, in my first year as a graduate student, I took a biology class with Francis Arnold, the Nobel Prize winner. And let's just say I took the class twice because it didn't click the first time for me. At the same time, it's worth it, I think, to struggle to learn these different fields because you can see problems, I think, from new perspectives that way. Well, it's very inspirational.

So let's then shift gears to something that you're busy with these days, which is advising the Biden administration about wind turbines. And can you say anything about the work that you're doing with the government? Yeah, absolutely. You know, it's been an honor to serve in this capacity. And I'll say it really hasn't been directly connected to any particular of our research aims. The group in the President's Council, I think we're all broadly interested in science and its development in this country. One particular area that I'm passionate about is seeing that our research infrastructure, and by that I mean from high school to colleges and universities to the graduate research programs, that we're enabling people to pursue these more unconventional lines of research like what we've been talking about.

So in retrospect, you know, I really appreciate hearing the positive sort of reaction that you have to these ideas. I can tell you that when I first wrote proposals to try to get this work funded, they were rejected one after the other after the other because they sound a bit odd. You know, the idea that anything about jellyfish swimming would inform cardiac diagnostics or that fish schooling would tell us anything about wind turbines. It feels a bit too foreign. And I didn't have examples to point to to say that this would necessarily be a success. And so the reviewers would typically have the initial reaction, well, what if it doesn't work? Where I always think, well, what if it does work? How would that be? What could that unlock? And unfortunately,

we right now don't typically fund work on the basis of what if it does work? It's usually what if it doesn't? And I think that's one of the policy pieces that I hope within the President's Council we can tackle. Well, so you are in California. A big issue, as everyone knows in California, is wildfire. And I think that that should be something that a person interested in fluid dynamics would have thought about. Do you have something to report about that? That's right. In President Biden's science council, I've had the privilege to co-chair a group thinking about how we can use science and technology to better address wildfires. We know that in recent years, they've been becoming more frequent and in some cases,

more severe, particularly here in California. And yet there are technologies we're not currently using, for example, communication for the firefighters, AI to help predict the progression of the wildfires, and even technologies like robotics and drones to help interfere with the fire's path before the first responders can arrive. Our work has identified a host of new and emerging technologies that we believe could help to stem the negative impacts of these wildfire events. And so we're looking forward to both federal and state and local level action on those recommendations. And so fluid dynamics plays into all of that somehow? Yes. Fluid dynamics is, in fact fact one of the most important drivers of the

progression of a wildfire. Think of the winds that carry burning embers and could dictate whether or not they end up crossing a fire break. The winds can determine how fast a fire moves. So when we've had really catastrophic wildfires, in some cases it's been because the winds were in some cases 70 or 80 miles an hour. One of the key challenges then to combat these wildfires is to be able to use fluid dynamics models to predict the future progression of the fire. It requires new types of data on the wind near the ground to complement upper air data. But also what we can do in simulating different locations is to help vulnerable communities prepare in advance for wildfires, to know that based on their topography and vegetation and with these fluid dynamics models, be able to tell them which parts of

the community are likely to see the front of that fire first. That could inform evacuation plans, for example. Well, I suppose no discussion of fluid dynamics would be complete without mentioning turbulence. It's often called the greatest unsolved problem in classical physics. You know what I would like as just a little tutorial, like what even is the problem of turbulence? What is it that people would like to understand? Yeah, you know, the simple way that I sometimes describe it is that in fluid dynamics, we have a set of equations that explain fluid motion in a way that's good enough to design an airplane, but not good enough to tell you when that airplane is going to hit

turbulence. So our fluid dynamics equations haven't been able to predict some of the very common occurrences we see in a fluid flow. If you think of your faucet at home and you turn it on just a little bit, it has that really glassy appearance to it. You turn up the faucet a little bit higher and then spontaneously it becomes much rougher. You get a transition to a turbulent flow. We observe this in all sorts of lab experiments and we don't yet have a clean theoretical explanation for when that type of transition to turbulence occurs. So, so interesting. You know, by coincidence, last night, maybe it's not coincidence, maybe I sort of subconsciously was thinking about our upcoming discussion,

but I just happened to be thinking about Richard Feynman's lecture in his famous Lectures on Physics, right there at Caltech, probably not too far from where you're sitting. That's right. Where he talks about the flow of water and the enduring mystery of turbulence. And he even mentions that on a fan, if you look at a blade of a fan, like up in your attic or something, you'll always find a thin layer of dust, very tiny dust particles, which seems mysterious, Feynman points out, because the fan blade moves at tremendous speed through

the air, and yet it's not blowing off those little dust particles. And so I sort of feel like this is the place we need to end, that you, I wanted to say you're some kind of modern-day Leonardo da Vinci, but now I'm starting to think you're also maybe a modern-day Richard Feynman. But now I'm starting to think you're also maybe a modern-day Richard Feynman. Maybe if one day I'm able to actually solve that turbulence problem, we can entertain that kind of an idea. But for now, yeah, I'm just a kid from Toledo who loves jellyfish. Perfect. Thank you so much, John DeBury, for joining us today.

Thanks for having me. Space travel depends on clever math. Find unexplored solar systems in Quantum Magazine's new daily math game, Hyperjumps. Hyperjumps challenges you to find simple number combinations to get your rocket from one exoplanet to the next. Spoiler alert, there's always more than one way to win. Test your astral arithmetic at hyperjumps.quantummagazine.org. The Joy of Why is a podcast from Quantum Magazine, an editorially independent publication supported by the Simons Foundation. Funding decisions by the Simons Foundation have no influence on the selection of topics,

guests, or other editorial decisions in this podcast or in Quantum Magazine. The Joy of Why is produced by Susan Vallott and Polly Stryker. Our editors are John Rennie and Thomas Lin, with support by Matt Karlstrom, Annie Melchor, and Zach Savitsky. Our theme music was composed by Richie Johnson. Julian Lin came up with the podcast name. The episode art is by Peter Greenwood, and our logo is by Jackie King. Special thanks to Bert Odom-Reed

at the Cornell Broadcast Studios. I'm your host, Steve Strogatz. If you have any questions or comments for us, please email us at quanta at simonsfoundation.org. Thanks for listening.