# How evolution builds the most efficient airfoils

Swimming and flying animals are optimally adapted for cruising through their environments, producing thrust via propulsors—wings for birds and caudal fins for fish. Over millions of years, the morphology of these animals evolved for maximally efficient cruising, and about 30 years ago, researchers proposed that most swimmers cruise within a narrow range of Strouhal numbers—these are dimensionless numbers describing oscillating flow. And more recently, researchers determined that flying animals cruise in the same range of Strouhal numbers.

Specifically, for swimming and flying , the Strouhal number is defined as St=f / U*A, where f is the oscillation frequency, U is the flow rate, and A is the oscillation amplitude. And the narrow range of Strouhal numbers in which swimming and flying animals cruise is 0.2 < St < 0.4.

OK. So why is this narrow range of numbers the most efficient? Daniel Floryan, Tyler Van Buren and Alexander J. Smits, mechanical engineers at Princeton University, conducted a study into the efficiency of swimming and flying animals in order to answer that question.

They used an experimental setup derived from earlier studies, which consisted of an airfoil in a water tunnel. The airfoil made biologically relevant heaving and pitching motions that were measured by encoders. The researchers performed experiments over all possible combinations of kinetic parameters and analyzed the resulting data mathematically.

Since drag can never be completely eliminated, perfect efficiency is not possible. But the animals selected as the fittest have evolved to a narrow range of highly efficient parameters.
A swimming animal at a constant velocity creates thrust with its caudal fin, and experiences drag produced by two sources: its body, and the "offset" drag caused by the propulsor frontal area projected over its range of motion.

For flying animals, the physics are somewhat different since their propulsors need to resist gravity in addition to providing thrust. However, the need to produce lift doesn't affect the physics of propulsion and drag when the animal is at a steady cruising speed.

In previous studies, researchers suggested that large-amplitude motions set the Strouhal number for efficient cruising. The authors of the current study argue that the amplitude sets the total efficiency without dictating the optimal Strouhal number. Instead, the authors write, "the offset drag is crucial in determining the low Strouhal behavior and setting the particular Strouhal at which occurs."

Drag turns out to be key. Based on their analysis, the researchers conclude that the range of Strouhal numbers that defines highly efficient cruising for swimming and flying animals is largely determined by the fluid drag on fins and wings. "In other words," the authors conclude, "energetic considerations set the kinematics of the propulsor to the most efficient one, and the net thrust of the propulsor at peak efficiency balances the of the body to set the cruising speed."

More information: Daniel Floryan et al. Efficient cruising for swimming and flying animals is dictated by fluid drag, Proceedings of the National Academy of Sciences (2018). DOI: 10.1073/pnas.1805941115

Abstract
Many swimming and flying animals are observed to cruise in a narrow range of Strouhal numbers, where the Strouhal number St=2fA/USt=2fA/U is a dimensionless parameter that relates stroke frequency f, amplitude A, and forward speed U. Dolphins, sharks, bony fish, birds, bats, and insects typically cruise in the range 0.2 Journal information: Proceedings of the National Academy of Sciences

Feedback to editors

Jun 30, 2018
This is fake news. There is nothing in this article that supports what the headline screams that Evolution is responsible for intelligent life. A true scientist would observe the amazing characteristics of fliers and swimmers and take note. But only fakes would then attribute this without question to a totally unproven theory.

Jun 30, 2018
This is a clickbait title. While I agree that the airfoil and aerodynamic structures are highly efficient, I was expecting some actual work done to verify or simulate that this was via some kind of evolution mechanism. This is just bad science and discounting the actual findings which I think by themselves are cool.

Jun 30, 2018
NATURE is Funny ! People take so many steps to preserve Animals for future generations. Remember, Nature is NOT so kind to us. Our lives are limited to a few decades. Meteorites/Asteroids could strike ! A house-sized meteorite can explode in Earth's atmosphere with a force greater than the nuclear weapon the US dropped on Hiroshima in 1945. An asteroid more than half a mile wide would have global implications, the aftermath that follows killing most life on Earth. Same size killed the dinosaurs.
http://www.thisis...t-2018-6

Jun 30, 2018
NATURE is Funny ! People take so many steps to preserve Animals for future generations.

https://www.youtu...8KlJ7qrU

Jun 30, 2018
NATURE is Funny ! People take so many steps to preserve Animals for future generations.
Why do we need seals, sea lions, dolphins and whales (unless there are some people out there ready to eat'em)? TO SEE AND ENJOY ? What is there? We have recorded so many videos of them on youtube already !
Fish population control is beyond us. Just keep the air and water clean. That's all.
http://www.latime...ory.html

Jun 30, 2018
B_A & HHH, the clickbait headline worked. They baited, you clicked, the advertisers paying for this site smiled at your gullibility.

By the way, if you wanted evidence for the process of evolution? Hahaha!! -giggle- I just can't say that with a straight face...

If either of you were competent to understand the process of evolution? -chuckle- okay! okay! Let me get myself under control....

-ahem- i

If either of you were paying any attention in your sixth grade biology class? You would be learning to ask your questions of first, archeologists.

Cause the slow bird got eaten. The remains become a museum exhibit. As natural selection methodically propels biological organisms up or down the ladder of survival to reproduce descendants.

Or not....

Second, you would ask geneticists about the subtle & randomly influenced biological processes tweaking this & adjusting that. If it works? You're a daddy!

It often fails. You're dinner.

Jul 02, 2018
. Over millions of years, the morphology of these animals evolved for maximally efficient cruising,

This is simple,straight forward, pure and fact free non-science.
Compare that to the rest of the article and see all the measurements made and that can be done by other scientists to confirm the classification that was then deduced.

@rrwillsj who thinks that the first two commentators do not understand what the evolutionary theory says, please, get your thinking straight!

(Darwinian) "evolution" cannot design ANYTHING, least of all all the magnificent requirements for getting birds and insects into the air. Random mutations and Natural selection are not able to produce the required coding and decoding that builds new systems and integrates new functionality into existing ones. You cannot have purely random material/physical/chemical processes produce abstract entities. How does it generate LOGIC????
Explain that first before insulting other people's noting of nonscience

Jul 02, 2018
Hey there fred

So if god is such an efficient and perfect designer, how do you then explain the eyes on a flatfish?
https://www.youtu...8B0aMB6I
http://scienceblo...the-w-1/

-Heres where you evoke 1 Corinthians 2:11
least of all all the magnificent requirements for getting birds and insects into the air
"God wiggled his nose and poof! it happened' -is not an explanation.

Jul 02, 2018
Well FJ, you are notorious for preaching anti-science based on your belief in fairytales and other creationist googoowoo.

Logic hasn't a damn thing to do with the reality of survival. In your infantile creationism, your mayfly lifespan is conflated with eternity. A six thousand year old eternity. You are a prime example for my Theory of Stupid Design.

The random processes of Natural Selection are self-inflicted experiments, spanning billions of years. Mindlessly, ruthlessly testing to destruction. A constant process that, not only never stops... It never even slows down! Biology is always careening along the narrow-edged precipice above the chasm of extinction.

If you keep flagellating yourself with childish imaginings of Intelligent Design by a creator-deity or super alien or cookie monster?

Explain the failures. Five small worlds available and only one success, the Earth? An 80% failure rate?

"Fire that Coach and bring in someone competent!"