(Inside Science) -- How many years have we been coming to the shore? How many trips? Why do we keep coming back… the air in the sky? The sand? The water?
So familiar and yet constantly changing. We feel the same excitement every time we come. Fluid flows under us, around us, over us -- constantly blurring, constantly refreshing.
The field of fluid dynamics is very, very old -- Isaac Newton, Daniel Bernoulli, and yet it keeps reinventing itself as technology emerges. Multi-material 3-D printers, high-speed cameras, new underwater vehicle and aircraft design. The questions stay constant but with new answers and new applications: cargo delivery, ice-resistance, airborne disease spread.
Inspiration comes in the simplest of forms: fish, feathers, fluid, phlegm and freaks.
How a Surfer’s Secret May Help Commercial Shipping
Most people just surf in the morning before going to work, but Eline Dehandschoewercker from École Polytechnique in France studies surfing all day long in her laboratory. She is trying to understand how catching the perfect wave may lead to new ways of delivering cargo by sea.
“When you are talking with a surfer, they can tell you something like: ‘OK, I’m waiting for the good wave, with the good velocity, with the good position in the wave; and I'm having fun by riding the waves.’ What we are trying to understand is: what is this good wave, this good velocity, this good position and everything,” said Dehandschoewercker.
In her lab’s tank, Dehandschoewercker and her team generate both non-breaking waves, which are waves moving at a faster velocity than the water inside them, and breaking waves, a wall of water surging forward at one speed.
She fashions boards out of balsa wood to control their length, width and thickness. Floating these boards in her tank led her to two conclusions.
“For non-breaking waves, if you don’t have an initial velocity you cannot surf. For the breaking wave, you can surf without an initial velocity. And so the main conclusion that we had in our work is that to surf on breaking wave the length of your board has to be not too long, to catch the wave. If it’s too long, you cannot catch the wave,” stated Dehandschoewercker.
She hopes that her research can lead to innovations in cargo delivery for locations without proper docks. By floating cargo on long boards with their own wave generators, ships might be able to surf their deliveries to shore.
Preening Penguins Inspire Ice Resistance
While surfing may unlock new means of freight delivery, UCLA professor Pirouz Kavehpour wants to eliminate ice build-up on planes by studying the cold-weather adaptations of penguins.
“In general I’m an observer of nature; and when I was watching a documentary on PBS, there were a lot of penguins jumping in and out of water and splashing, there were a lot of droplets around. I was very curious that none of these droplets stick to their feathers, and it’s minus 40 degrees C outside but you don’t see any ice form on their feathers,” Kavehpour said.
Kavehpour’s lab found that the waterproofing preen oil found in many birds was especially water-repelling, or hydrophobic, in Antarctic penguins. Moreover, the penguin feathers featured nanostructures that have trapped air inside which help form water droplets on the surface of the penguin’s feathers rather than the feathers becoming saturated. Kavehpour realized that this was due to a geometric reason similar to that of the science behind heat transfer. These features delay freezing long enough that penguins can simply shake off excess droplets from their coats.
“We are hoping to take this, to further study this, look at the different species as well as try to have a bio-mimicked, bio-inspired surfaces. Ice formation changes the profile of a surface. So for example, on airplane that can affect the drag and lift and actually may cause a lot of accident, a lot of money spent at the airports in winter time trying to de-ice the airplane, cause a lot of delays.
"Well obviously we’re not going to put preen oil on the surface of airplane, but there will be a combination of some kind of chemicals, and these chemicals won’t wear off, that’s a big challenge. The true direction is to further push the science to understand these systems better, also try to see if we can come up with the right manufacturing part for the larger scale surfaces,” Kavehpour said.
3D Printed Scales from a Shark
From the ice floes above the Antarctic breakers to the dark deep of the waters below, fluids also inspired professor George Lauder of Harvard University, who is studying the speed adaptations of shark scales to further underwater vehicle design.
“One of the key aspects to understanding fish propulsions is to understand how the fluid moves over the body, how does water move over the body. There’s a lot of friction that develops when water moves over a surface, and so the goal is to understand, are sharks doing something clever with their surface that enables them to reduce that friction, to enhance thrust, and to propel themselves more effectively through the water. Well one of the things we’ve found was that there’s a specific pattern of arrangement of the individual scales that are on the surface of a shark, and that that pattern enhances swimming speed by up to about 20 percent,” Lauder said.
Lauder’s close study of the nature of sharks’ rough skin revealed how vortices of water are created as sharks swim, creating small low-pressure pockets that pull their bodies forward.
“We began our studies by using real shark’s skin, but there are real limitations to that; you can’t alter it, you can’t change spacing, you can’t get a very good smooth control. So we were able, after actually several years of work to 3-D print a satisfactory biomimetic shark skin-like structure, and we did that by printing very tiny rigid denticles on a flexible membrane, so a multi material 3-D printer, and then being able to alter the spacing so we could just print every other one, we could print them in a kind of a diamond configuration, we printed them in linear or rays like a matrix.
"We were able to test the alternative swimming performance of those different designs and we found actually that the diamond-staggered pattern produced by far the fastest swimming performance.
"I think the motivation for working on fish swimming is derived from just our typical observations that fish are fantastic at moving through the fluid, at maneuvering; it’s almost effortless, really, to them. And in comparison to human-made devices, with propellers, rudders, sails, submarines, and sailboats, which look very cumbersome in comparison to a swimming fish,” said Lauder.
A Fish Moving in and Out of Water
At the Massachusetts Institute of Technology, professor Alexandra Techet is also studying fish behavior with an eye toward underwater vehicle applications.
“There is a fish species called archer fish. They have this great evolutionary process where they can capture prey that is hanging on branches above the water by both spitting and by jumping out of the water. And there’s been a lot for work done on their spitting jets and the physics there, and that’s not really what we’re focusing on. We’ve been focusing on how they jump, and they actually start from almost at a dead stop. Hovering under the water, they sit, they sight their prey, they even give a nice, standard ‘S’ shape to their tail, and they go right out of the water.
"My initial motivation in coming from an ocean engineering background is underwater vehicles. And can we come up with an understanding of the water exit problem, can we translate that to larger vehicles, or to smaller autonomous vehicles that maybe can be underwater or water exit vehicles. And operate in two phases and make that transition gracefully instead of maybe just coming to the surface, waiting a second, and then setting the rotors up and like, helicoptering up? Can we do a smooth, graceful exit out of the water?” asked Techet.
To study exactly how the fish move, Techet’s team drew on advancements in computer vision to develop a new kind of 3-D imaging that allows for camera repositioning.
"A lot of the work we’re doing, we're using a technique based off of light field imaging that comes out of the computer vision community. And the way we work is we set up an array of cameras, and we take that and we take all the information from those cameras and use a calibration matrix to basically re-project all the information into a 3-D volume. So our eyes look at things we see stereo, we know depth and we have depth of field because we have two eyes. In our case the 9 cameras inform everything, so when we have a particle hiding behind another particle, or a droplet behind another droplet.
"Some camera will be able to see that droplet in the array and will be able to come up with all the particles, even those that are partially occluded. And what’s very exciting is that we’re able to do not only tracking of gross groups of particles, but we’re able to do things like Lagrangian tracking, where we track an individual particle in time and space and see how it moves. Which is excellent for things like sneezes or multiphase flows where we’re interested in following individual droplets and seeing how they evolve over their life,” Techet said.
The Secret Life of Sneezes
At the same Massachusetts Institute of Technology, professor Lydia Bourouiba is heading the fluid dynamics of Disease Transmission Laboratory to focus on droplets, bubbles and multiphase flows that shape disease transmission.
“Both coughs and sneees in terms of violent expirations have never really been looked at from a fluid dynamics point of view. I was thinking about influenza and when I wanted to identify, but wait a second, what is the root of transmission? Is it actually air or surfaces? What are the distances involved here?
"I stumbled into the research of [William and Mildred] Wells from the 1930s, who basically were the first to introduce this notion of small drops versus large drops that was discussed and debated during the Ebola outbreak. And it’s surprising to see that the classification of the WHO, or the World Health Organization, today is still based on the notions introduced by Wells from the 1930s,” Bourouiba said.
Her experiments involving high speed imaging in collaboration with the Edgerton Center at MIT produced these highly detailed, infinitely manipulative videos of sneezes and coughs.
“So what you see in that video is typically a white cloud coming out. And you see also that that cloud is traveling, and as it’s traveling, it’s growing. And its growth is due to the fact that this is a turbulent cloud, it’s entraining ambient air within. And as it is entraining ambient air within, this ambient air is actually quiescent, and therefore the cloud slows down.
"This whole dynamic of entrainment and mixing is something that we applied for that context to basically describe fully the trajectory of the cloud, its size, and where the drops would fall,” Bourouiba said.
The images showed that, contrary to earlier theories, small drop expulsions are not trapped near the host by drag but are pulled into the advancing cloud which can travel an entire room-length from the host. Now, with a basic understanding of the physics established, Bourouiba wants to begin application in the world of public health.
“We’re really trying to, for the sneeze problem, tackle the question on what is the heat map, or the contamination map, for given individuals and for given environments? And with that, what is the risk? If we were in the time of outbreak and we needed to have a number in terms of ranges between individuals or contamination heat maps for intervention or decon, what should you account for?
"So I started that discussion with the CDC this past year , and we’re starting the discussions about continuing the conversation and seeing how this could be incorporated in the future, for fundamental research, but also fundamental research done now that would then be used in times of outbreaks for risk assessment,” said Bourouiba.
Interconnections -- Like Waves to the Shore
From the shores of Antarctica to the coast of France to here in Florida, from fish to fluid to phlegm, scientists around the world are applying fluid dynamics in concert with technology, engineering, medicine, and most importantly their colleagues. It’s these intersections and interconnections that drive progress as we seek to better understand, relate to and be inspired by the natural world.
This video was produced using interviews from the 2015 Annual Meeting of the American Physical Society (APS) Division of Fluid Dynamics. APS is an underwriter of the Inside Science program.