Swimming Sea Lions Teach Engineers About Fluid Dynamics

17:09 minutes

A sea lion swimming in open water, its back curved as it turns to swim in an opposite direction.
Sea lions are extremely agile under water. Credit: Shutterstock

The next time you go to the zoo, take a few minutes by the sea lion habitat to watch the way they swim. While most high-performance swimmers use powerful kicks from hind appendages to power through the water, sea lions instead use their front flippers, moving with a pulling motion. With their propulsion source close to their center of gravity and their flexible bodies, sea lions are extremely agile under water, able to weave in and out among the stalks of an undersea kelp forest.

Researchers are studying the movements of these exceptional swimmers to try to design improved underwater vehicles. Mimicking some of the sea lion’s tricks could allow more maneuverable, quieter vehicles that produce less turbulence in the water.

SciFri’s Charles Bergquist talks with Dr. Megan Leftwich of George Washington University about her work with sea lions, and other research into fluids and biomechanics, including the fluid mechanics of human birth.

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Segment Guests

Megan Leftwich

Dr. Megan Leftwich is a professor of Mechanical and Aerospace Engineering at The George Washington University in Washington, DC.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. We are on the road this week in Washington, DC. And, you know, one of my favorite tourist attractions here is the Smithsonian’s National Zoo. It is a real oasis among the politics.

But as amazing as the zoo is as a center for biodiversity and conservation research, you wouldn’t think of it as a center for research into fluid dynamics. Yes SciFri’s Charles Bergquist is here with more. Hey, Charles.

CHARLES BERGQUIST: Hey, Ira. You’re right, most people going to the zoo are probably hoping for a glimpse of the red panda or a Komodo dragon or something. But if you stop by the sea lion habitat, you might encounter some folks observing a little more closely than you might expect. Dr. Megan Leftwich is a professor of Mechanical and Aerospace Engineering at The George Washington University in Washington, DC. And among her research interests, she’s studying sea lion locomotion with hopes of designing better undersea vehicles. Welcome to Science Friday, Dr. Leftwich.

MEGAN LEFTWICH: Thanks. Thanks so much for having me.

CHARLES BERGQUIST: So there are lots of good swimmers out there. What’s special about sea lions?

MEGAN LEFTWICH: Actually, it is the difference in how sea lions swim that really got me interested in them. I was actually like at the zoo with my kids here in Washington, DC. And what I noticed is that most things– animals, mammals, fish– that are good at swimming swim with their tail or their fluke or whatever’s at the back of their body. So dolphins, tuna, they have this rear appendage that produces thrust.

But sea lions actually pull themselves through the water with their arms, so their front flippers, which are very analogous to the human arm in terms of bone structure. They even have five little fingers in there. They take them, and they pull themselves through the water with this big clapping motion. And it’s really very different than anything we see in the high-performance swimming regime.

So I thought, huh, that’s really different. And we could do a lot with that because you’re producing forces at a different location in the body. So you might have a way to exploit just a different sort of parameter set by looking at this forward limb appendage as opposed to the rear limb appendage.

CHARLES BERGQUIST: So if they’re pulling themselves through the water with their front flippers, is their hind end doing anything? Or is it just hanging out there?

MEGAN LEFTWICH: Their hind flippers are essentially like a rudder. So they’ll steer with their feet and pull with their arms. They very rarely kick with the exception of maybe if they’re jumping or doing some sort of other maneuver. They just really streamline those hind legs and pull themselves through the water, so really just steering.

CHARLES BERGQUIST: OK. And just to clear up any confusion here, we’re talking sea lions as opposed to seals?

MEGAN LEFTWICH: Yeah, it’s actually a really important distinction. And I always have to feel so pedantic when acquaintances are like, how’s your seal research? I’m like, oh, it’s sea lions, because seals actually swim with their feet. They have their hind flippers, and they kick them back and forth in this rowing left-right, left-right to produce thrust. And so they actually don’t swim in this mechanism.

So it’s a real important difference for my research. I’m sure there’s lots of research where seals and sea lions are similar. But in this case, seals also swim with their feet.

CHARLES BERGQUIST: Huh. Are there other animals that have a similar stroke? Or where did sea lions get this from?

MEGAN LEFTWICH: So in terms of nonevolutionary similarity but swimming similarity, penguins will also use their wings in a somewhat similar manner. And sea turtles will also use their front limbs to generate thrust. But those are very evolutionarily different. I mean, that’s a mammal, a bird, and a reptile.

Where sea lions got this from, so sea lions are pinnipeds. They’re mammals. They evolved on land and then moved back into the water. And the best theories are from an evolutionary pressure perspective, is that the areas they were, there was a lot of pressure to be quite amphibious.

And so by having these very large foreflippers, they are actually quite maneuverable on the land as well. They can actually run in a tripod gait. And we’ve done some work on sea lion running. And so the pressure is to have this big front foreflipper that can function both in land and in water.

CHARLES BERGQUIST: Wow. So, I mean, those flippers, they must be pretty strong, but they kind look floppy at first glance. What’s one feel like?

MEGAN LEFTWICH: So sea lions feel different than you’d expect. They’re actually quite furry. They look smooth, like neoprene, like a dolphin, but they actually have short hair, kind of like a Doberman Pinscher.

And then the flipper itself, it is relatively less hair, kind of like a human arm. But it’s completely boned. So inside, they have, just like us, a set of bones from the shoulder to the elbow and then a strong but short set of what we would call the radius and the ulna, the elbow to the wrist.

And then when they splay out, what they’re actually walking on is their hands. And so they have five long fingers. And their bones are kind of like fettuccine. And that gives them a little bit more structure only in the direction that they have to support in while allowing them to be flexible in the direction that they move in.

CHARLES BERGQUIST: You mentioned the skin itself and the hairs and things. There’s controversy in the swimming world over those high-tech suits that are modeled after shark skin or whatever. Does sea lion skin have anything special going on that gives it a performance boost?

MEGAN LEFTWICH: We’ve actually done that research. We have put sea lion skin into a test facility to see whether it reduces drag. It turns out it doesn’t. Sea lion skin, at best, is neutral, and in some cases actually will slow the animal down.

However, I said before that sea lions are furry. They’re covered in fur, like a short-haired dog. Sea lions do not have blubber. They just have the same sort of fat as you and I do. And so they need that fur for insulation to keep them from freezing to death.

So they actually pay a penalty. Unlike shark skin or maybe there’s evidence that tuna fins will do this as well, we don’t think that their skin really has much of a hydrodynamic performance. But it does keep them from freezing to death.

I think as engineers, we often feel like nature and evolution has to optimize. But it only has to optimize for keeping them alive. We have to remember, the animals do so much more than just swim. And as engineers, we’re always like, well, it must provide some benefit. And it does provide benefit to the general staying alive but not to the swimming. So I wouldn’t recommend covering a swimming tech suit in hair.

CHARLES BERGQUIST: OK. So you’re modeling these flippers and building replicas of them in the lab. What’s the goal? Why are you doing this?

MEGAN LEFTWICH: For one thing, it’s basic science, fundamentally understanding different modes of unsteady propulsion. And in the long term, we could think about underwater vehicles that have different task options. So one thing sea lions are really good at is maneuvering. They’re very, very flexible. They can touch their nose to their toes by bending either forward or backward.

And because they’re generating thrust right near their center of gravity, they’re able to turn in almost any direction at almost any time. And part of the reason, at least for the California sea lion, is they live in kelp forests, where they’re maneuvering in and out of the kelp.

Currently, our tool bag of underwater vehicles is basically a torpedo with some sort of thrust production at the back, which is great. And we can design that to be efficient. We can design it to be fast. But it’s really hard to design that to be maneuverable. So if at some point we wanted an underwater vehicle that could maneuver through crowded spaces, that could do so very quietly, not leaving a big signal, this idea of generating this unsteady propulsion near your center of gravity could really provide an extra tool.

I never envision a full sea lion replica, but can we take the fundamental physics that allow a sea lion to operate in this really chaotic environment and just extract those principles for our engineering tools? That’s how I think about my process.

CHARLES BERGQUIST: Less turbulence, quieter vehicles makes me think of some submarine movie or The Hunt for Red October. Is the Navy interested in this work?

MEGAN LEFTWICH: Yeah, that is exactly– I mean, there’s a big gulf between that and what we do. But the idea is yes, that any sort of signal in the water of a vehicle is really persistent. It lasts for a really long time. And it’s very predictable. And so the question is, is there a less predictable way that we can move through the water for particular goals?

CHARLES BERGQUIST: I’m thinking of the early days of human flight experiments, where everyone was like, oh, we got to do what the birds do, and we’re going to make flappy wings on our flying devices. Is there any risk here of falling into that same trap of saying, oh, we can just make a flipper-finned submarine?

MEGAN LEFTWICH: For sure. And there’s a reason why we have propellers now. They work really well. But there are some things that we know are very hard to do with a propeller.

I mean, for one thing, propellers are just very– they leave a very known signal. We know exactly what that signal is. And masking that signal is a big whole field of study.

You also have issues with cavitation, right? So you have cases where you’re forming air bubbles underwater because your pressures get so low, whereas if you have this unsteady mechanism, you have the ability to produce those forces in a less regular pattern while still generating regular motion, which can be beneficial. And again, with the propeller-based, the maneuverability is always going to be tricky.

I’m not going to say that the bioinspired vehicle is going to replace our submarine, but it does give options for smaller vehicles that have limited power supply. So the unsteady propulsion can also be a big power savings. You get significant efficiency addition by having an unsteady or intermittent thrust production.

CHARLES BERGQUIST: So what does your research look like in practice? How do you go about figuring out the dynamics of a sea lion’s movements?

MEGAN LEFTWICH: The basic method is that we start with the animal. We do our field work– and you can’t see me, but there’s air quotes there– at the National Zoo. They’re wonderful partners. And we have a population of sea lions at the zoo.

And what they allow us to do is basically just watch them. We set up cameras, and the animals go about their life. And we just get hours and hours of video watching the animals do whatever they want to do. They turn. They swim steadily. They produce thrust.

Sometimes they hang out and watch us. We’ve actually gotten some great data of the animal just paying attention to us. But they hover and move in these really beautiful ways.

We then take all of that– it’s all just video– all of that video and digitize it, put it into computers. We’re able to track. And then from that, we extract what we call the kinematics, a mathematical expression of how the animal moves.

And then we take that to make our models. So then, as you mentioned before, we have essentially robotic sea lion platforms or robotic flipper. We have robotic hind flippers. And we put them into the water channel.

And the reason we do that is we can do whatever we want now. We can change the shape. We know what it actually does, but we say, OK, well, what if we accelerate it a little bit more? What if we slow it down at first and then speed it up? And we’re able to really probe into the fundamental physics that make this work.

CHARLES BERGQUIST: I’m talking with fluid dynamicist Dr. Megan Leftwich of The George Washington University on Science Friday from WNYC Studios. Now, I know your research lab has another project that seems very different. In addition to the sea lion work, you’re looking at some of the fluid dynamics of the birthing process in humans. Tell me about that.

MEGAN LEFTWICH: Sure. And I’ll say, it doesn’t seem all that different to me. Both projects, what unites them to me is understanding these natural processes that just happen in the world but have to be underpinned by science and physics and particularly fluid mechanics.

Now we’re just looking at a different system. But to be honest, we do it in a lot of that same way that I just described for the sea lion research. I wish more people would want to talk about my work on human birth because it’s just such an important problem. And female reproduction is severely understudied in the scientific community.

What I’m interested in is the underlying physics and forces during pregnancy and specifically delivery. So we take model systems of the human birthing process. And fortunately for us, there’s lots and lots of ultrasound data. So we can get images of fetuses in different positions, different sizes, different gestational ages, and then design models to look at the forces required during active labor and delivery.

CHARLES BERGQUIST: This feels like it would be more difficult to study than the sea lion flippers. But you’re saying it’s not necessarily because you have all this data.

MEGAN LEFTWICH: Yes. So there are some things that are quite difficult to study because there is some data lacking. So, for example, we just did a project not on labor and delivery, but at a condition that happens somewhere around the middle of pregnancy, which is called premature cervical remodeling.

So the cervix is hard and firm and at the bottom of the uterus. It’s kind of what keeps the baby in. And towards the end of pregnancy, it has to get soft and open up. Like when a woman’s in labor and she’s 7 centimeters dilated, that means that the cervix is open 7 centimeters. But there is a small group of women that about halfway through pregnancy, the cervix will start to soften prematurely. And this makes pregnancy really hard to maintain.

But what’s really tricky about studying this is there’s no easy way to get data on the properties of that tissue. How soft is it? How elastic is it? Because, obviously, you can’t take a sample of a pregnant woman’s cervix. And it’s actively changing. So there is data on the cervical properties, but it’s almost all from hysterectomy patients.

So the consistency of that cervix is going to be very different from a pregnant woman. And so just getting that data is really tricky. But what we’ve done is developed a wonderful partnership with obstetricians. And the way that they diagnose this condition is they touch the cervix, and they determine if it’s soft, medium, or hard.

And so what we did is we made our models and had the obstetricians diagnose our models, our different tissues that we make in the lab, and then we tested those tissues to get a sense of what is the actual consistency based on the physician’s touch?

CHARLES BERGQUIST: The term “fluid dynamics” makes me think of liquids and amniotic fluid, but your interest in the birthing process is more general than that.

MEGAN LEFTWICH: Yeah, so that’s where I started, too. I thought, let’s look at the role of amniotic fluid in labor and delivery. And babies come out covered in goo. That goo has a name. It’s called vernix caseosa. But it’s kind of a lubricant. And so that’s a fluid dynamics lubrication problem.

It has turned out that there is no way to study this problem without also looking at soft materials and tissue dynamics. And so it has become more than just the fluid mechanics of human birth. And now we say the biomechanics of human birth. But fortunately, in Mechanical Engineering Department, we also have material scientists and lots of people I can call on to fill in those gaps.

CHARLES BERGQUIST: What’s the goal here? What do you hope to achieve by studying this?

MEGAN LEFTWICH: So with the human birth work and with delivering babies, we’re really good at getting babies out. But if there is any problem, we go to a surgical-assisted delivery. And so my underlying hypothesis is that if we understand the fundamental mechanics of this problem better, we could develop better mechanical solutions to difficult births.

And this would prevent in the developed world a lot of surgeries. And we know that a surgical delivery is not the best start for a baby. It’s also more expensive, has a longer recovery time. But we are lucky in the developed world that that’s even an option. So if we could develop better mechanical solutions to difficult births, this could also be something that could help the developing world, where maternal deaths are still quite high because surgery is not always an option or if you’re very far from the nearest surgical center.

CHARLES BERGQUIST: Well, we wish you best of luck with that goal. And thanks so much for taking the time to talk with me today.

MEGAN LEFTWICH: Thank you so much.

CHARLES BERGQUIST: Dr. Megan Leftwich is a professor of Mechanical and Aerospace Engineering at The George Washington University in Washington, DC

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