Bret Tobalske – Fluid Motion in Flight


[Music] Linda: Welcome, everybody, to the second in our series about birds, inside and out. I think we’re off to a good start last week with Professor Ken Dial. And I’ve very, very excited about
tonight’s lecture as well. First, my usual task, is just to ask you to make certain you’ve
turned off your cellphones so they don’t embarrass you. Um… I usually have to run down
and turn mine off just after – just after I say that, so… Um, also to remind you that all of the proceedings are being
taped by MCAT and will be available for viewing
on television after the series is over and
we’ll be able to let you know the series – uh – the schedule as always,
as the series moves on. We will be having our usual festivity at the end of the last night’s lecture,
so please show up. I don’t know what it’s
going to have to do with birds, but at least we’ll have
wine and cheese, I think. So, that’s always a favorite
no matter what the subject is. So, thank you all for being here. It’s really a wonderful, uh, thing
to see such a turnout. I’m going to introduce my dear colleague
Eduardo Chirinos, who is going to come and read
from his – from his book “39 Zoological Lessons and Other Didactic Poems” and I’ll ask Eduardo,
would you come on up, please? [Applause] Eduardo –
[Applause] Eduardo couldn’t be here last week because he was in Mexico to celebrate the celebration of the Spanish
edition of his book. Which has just come out.
Yay! It’s a beautiful book. And soon we’ll be able to get our hands on a bilingual edition.
Correct, Eduardo? The bilingual edition will come soon. Ok, and I also want to
introduce tonight David Miles Lusk.
David, would you stand up? This is our artist.
Let’s give him a big round of applause. [Applause] David couldn’t be here
last week either, I think, but I’m very, very glad to meet him. And he’s a, uh, a lively artist
here in Missoula. He graduated from the
University of Montana in 2014. So, David, thanks for being here and
thanks for your work. It’s really quite splendid. So, I’m going to ask Eduardo, now,
to read, uh, one of his poems in Spanish, and then, just like last week, Jeff Badenoch
will be reading the English translation. Then I’ll come up and introduce
Bret and we’ll be on our way. Ok, Eduardo. Eduardo: Good evening, everybody. I – tonight I will read a poem whose speaker is nine feet tall. An enormous bird called
“Dinornis robustus” in Latin. Well, these birds are extinct and
live in New Zealand as a little kiwi. This is a poem. “Dinornis robustis” (Speaking Spanish) [Clears throat] (Speaking Spanish) [Applause] Jeff: We came to this island over land. Long before it split into two. Long before it was called “Aotearoa”. We can’t remember when we lost our wings, but don’t even display their vestiges anymore. Why would we? Our legs were strong and sturdy. Able to bear our weight and even support
a height of almost ten feet. We arrived here walking, from Africa or India.
From Madagascar or Antarctica. It’s all the same now. They formed one continent at the time,
and that landmass was ours. It didn’t matter when we first realized we were alone. Our new home had plenty of food.
The climate was good and there were no predators. Our path to extinction began in the middle
of the year 1250. When the Maoris arrived
from the islands to the north. Within a couple of centuries,
they’d eaten all of us. [Applause] Linda: Thanks Jeff and Eduardo. Um, next I have the joy of being able
to present to you our lecturer for tonight, Bret Tobalske. It’s really sort of interesting to me because I know very few
of these bird people. So I’ve asked them just to
tell me something about themselves interesting or not, everything they’ve said
is interesting so I think it’s great. So, that I wouldn’t have to read to you
what’s printed on the page because I think you can all read
what’s printed on the page. And what I found out from Bret
is that some time ago he was working as a carpenter
in Virginia, I believe, and he began to see wood
coming through his shop which was stamped,
“The Stimson Lumber Company.” And he was thinking about
woodpeckers at the time. Woodpeckers sort of fascinated him, and he’s become, in the meantime,
interested in the way the fly, which maybe isn’t too fluid
from your description, but we’ll see. Um, and so he decided to make
his way out here to Montana. So, it’s one of the blessings that
the Stimson Lumber Company has given to us, I think we can be thankful to them for that. Um, Bret is one of our
newer faculty members here. He’s only been here since 2008.
He’s an associate professor? An associate professor, um, and
his topic for tonight is, “Fluid Motion in Flight,”
so I’m not sure if we’re going to hear anything
about woodpeckers tonight. Bret (O.S.): A little bit. Linda: Un peu, just a little bit about woodpeckers.
[Bret laughs] But I’m really excited to hear your talk.
Bret, come on up. [Applause] Bret: Thank you and it’s a pleasure
to be here. I’m going to start in bit of
and aesthetic challenge with this. There’s a – there’s a dualism here. We’re going to go on a little bit of a voyage
that invokes physics, for sure, and I’m assuming you’re interested in birds,
so it’s the confluence of physics and birds with just a little sprinkle of mathematics. But before we get there,
and dive into that, because that can potentially be
intimidating and it shouldn’t be, I just want to focus on some aesthetics. Flight, itself, is a charismatic event
that animals accomplish. And this image is the type of work that I use
to interpret what going on inside and animal, which is the theme of this lecture series,
inside and out, “Birds: Inside and Out.” We’re looking at a sequence of, uh, flight where the record of air motion behind the animal
gives insight into what it’s doing as it’s flying. These images, there will be a few more here.
This is another, this is and image of a zebra finch. Just look and expose the notion of –
people have been fascinated with flight forever. Since we’ve been conscious. And in terms of the techniques
that allow us to investigate the mechanisms that birds use, we’re lucky. We have lasers, we have computers. Other features that allow us to decompose
the structure of what they’re leaving in their wake, and then interpret what’s going on inside
them and that’s the theme of tonight. These images are obtained using
a laser based system that I’m going to explain in a little more detail, but I want to give a nod
to an individual I collaborated with who’s a graduate of UM.
A Master’s in Fine Arts 1984, Fernanda d’Agostino. She lives in Portland, where I
used to be before coming here. And she approached me, interested in
the confluence of art and science, and took these types of images and created
art exhibits, where we collaborated. And, uh, these exhibits went all over the world,
from China to France and Spain and so forth. My point is, there’s a beauty in flight and what my goal is tonight
it to simply, uh – hope to expose you to a
different way of knowing about it. Again assuming you’re interested
in birds and that’s why you’re here, I just want you to see something
different in the air that they move in, primarily. There’s penguins and so forth that swim
and fly through – but that’s not the subject tonight. It’s mostly through air. So just seeing the world
in a different way is my goal. If I can accomplish that, and there will be
some complications and wrinkles and so forth, but hopefully we’ll have fun
with that, that given theme. I want to talk about Lift, which is with a capital “L”
because that’s the magic thing. Anybody who’s flown –
that’s the magic going on. Origins and costs of lift. Unsteady, and the magic, if you will, of hummingbirds
is what I’m going to emphasize. Because for the past 12 years or so
I’ve been focusing on hummingbirds as a model subject for flight. And then drag.
So with every positive, there’s some kind of cost, and I’m implying some cost
embedded earlier, but drag is an obvious thing
that birds must deal with. So, lift and drag, the composites of flight,
is the subject tonight. The important thing is to have a sense, intuitive,
of the differences between us and birds. And I could be substituting insects and bats
for this, but we’ll stick with birds tonight. For you or I, we walk
and we walk faster, and we might jog and run,
and if you ask a horse to do that same thing, or a deer, or what have you, it might
walk and trot and gallop. And the cost is linear
with increasing speed in terms of the energy being bur –
burned per unit of time, that’s power. So again, the power required
to walk is kind of low. To jog is a little bit higher.
To run, higher still and it’s linear. And that’s our world, being confined to the
terrestrial environment, such as we are. So we’re seeing a walk, trot and gallop
cost as a function of speed. It’s not the same thing for things that fly. And this works for everything from a mosquito
to a finch to an eagle to a 747. It’s very costly to move slowly. And then we’ll explore the mechanisms
for why that’s so costly. It becomes less expensive
at an intermediate speed, and let’s see if I have a pointer here. And it becomes more expensive
again as costs rise… Barely visible.
So, very expensive to fly slowly. There’s some minimum speed
where it becomes cheaper, then it goes up again. And this is a product of the cost
of doing business as a flying thing. Again, this will work for helicopters to planes.
It works for hummingbirds to eagles. It works for bats and insects as well. There’s a cost associated with producing lift
that’s called the “induced cost.” The cost of doing business. There’s a cost with drag, and I think
we all have a general sense of what drag is, on the wings and the body of a bird. So the cost of producing lift
goes down with speed. Going up. And the costs with drag on the wings and body
goes up exponentially with increasing speed. And that gives rise to a “U” shaped
general curve with flight speed. So, again, sticking with what we know, we run, there’s an equal and opposite force
everytime we impact the Earth, but the Earth is so big
it doesn’t move out of the way. So that process of our locomotion – the fascinating thing with birds, it turns out
with fish as well, they’re living in a fluid environment. Air is about 800 times less dense
than water but it’s behaving very much like fluid and that in and of itself
is one of the major themes tonight. That air is a fluid and the dynamics of that
determine how birds behave in the air. So what I’m going to show here,
I think, we’ll find out… Nope, I’m going to have to go
to my computer. We’ll make this work this way. These are data from a scientist
at Harvard University, George Lauder. Just looking at the reaction of a
sea start in a fish. My point is that when fish move
and bend their body and undulate they’re imparting forces to water.
And the way water responds is to form vortices. This is a sea start or escape mechanism. So the Earth, it reacts, but it doesn’t move
anywhere because it’s so huge. And in contrast, water, and by extension
where we’re going, air, moves out of the way
when something imparts force to it. This is a type of information
we obtain from birds at the field station across town. The Flight Laboratory – this is a pigeon with video showing its wing beat cycle
during a flight between two perches. Everyday pigeon, Columba livia. And over here we look
at the flow pattern behind, and I want to explore a little bit more
about how we obtain that type of flow pattern. So before we do this,
I’m going to jump over to play around. I brought lots of props tonight.
Let’s see… [Console beeps] One more time… [Console beeps again] There we go. Is that me?
Yes! Alright, so again, you know we’re going to laminate
some physics and math in there, but I think the more important thing is to have fun
and I actually would welcome anybody to just raise their hand if something
goes off in left field too far. This is water, right? And the thing about water is we see it. We don’t actually see air.
We see trees with their leaves moving in response to wind, and that’s how
we recognize wind more so than we recognize that
something’s going on with air. So my point here – I’m just going to
drop a little bit of food coloring in here. And start to point out that even with this
you get patterns of vortices. Looks a little bit like what you see on
The Weather Channel with the hurricane in the gulf coast. Actually, I don’t even think I can top that. [Audience laughs]
The beauty of what I do, is that it’s charismatic, right? So what did I do? I smacked this with a little
flat plate and I created a vortex and I created a pattern of flow
that’s complicated. And that’s the way fluids react. And we’re a little bit – as humans –
we’re a little bit more evident – uh – our interpretation of water
because we see it. We see it behind a rock, you see vortices. So I’m going to – these are sprinkled
tea leaves from my wife’s stash of tea. And this is just to help visualize
what I want to point out here. The point then is, again,
we push on something solid, we get an equal and opposite reaction
that allows us to run. Water, and we’ll get to air in a moment here,
behaves by forming vortices in the wake. If I stopped here, I probably would’ve done my job.
(Chuckles) Right? But we gotta get to birds. And I – you know, apologies to penguins
because they – and other species that fly through the air.
Fly through the water, excuse me. But in this, I’m just going to create
some movement patterns here. And point out that there’s a reaction.
That if we can read the tea leaves, literally, we can interpret what sorts of forces
I’m putting in with my little paddle, ok? And again, just one more time, to show –
we’ll maybe come back to this. I’m going to take a plate here
and move this through. Works quite nicely. Alright, so extremely dramatic.
Water’s a little easier than air is. Ok, so in air we require
a little bit fancier Hmmm, the water won’t go away.
Let’s try again. There we go.
Close? Nope, nope.
I gotta wait. Patience is virtue. We use a process called
“Particle Image Velocimetry.” It’s a laser-based system.
This show a wind tunnel which is in my lab. The portion of the wind tunnel here
that is visible is where birds fly within. And the overall pattern here
is one in which you have a suction fan that pulls air through
we use lasers to illuminate the airflow. And the laser flash – I’m going to
give you a demo over here in a moment – is something that tracks small particles
of olive oil, so this is a fancy vaporizer. And again, if there’s a point behind
what I’m about to demonstrate it’s that we’re not aware
of what I’m doing with my hand, until we visualize it.
We’re incredibly visual as humans. [Vaporizer hisses]
until we visualize it.
We’re incredibly visual as humans. [Vaporizer hisses]
This is a vapor cloud of micron-sized
particles of olive oil. [Vaporizer hisses] And if I zap this with a laser,
or even just, you know, a bicycle headlight you get a sense of the mag –
there’s some wind in this room. Now what I’m going to take is this wing from a goose,
which I want to point out, by the way, is from the Phil Wright Museum.
An important resource in this context. To have access to dried spread wings to demonstrate
this or actually study aspects of wing design. So let’s just give a little bit
of air here and I’m going to move it around. Now, I’m not a bird, obviously.
But I’m inducing an incredible amount of flow. Let’s just let this accumulate a little bit. And my point behind this is
once we see it, we start to believe it’s real. I’m simply going to move this wing
like a bird would, and you see the reaction, hopefully. Hopefully the people in the back see
there’s an incredible downwash of air, just from this simple wing. And my action, which is nowhere near
as sophisticated as a bird. So there’s an equal and opposite reaction
that we’re leadaing towards, the wing pushes the air down, and in response,
the air pushes the bird up and that is lift. We reveal that using fancy lasers. And… A systems – a close collaborator, a friend of mine,
Doug Warrick, looking at a chamber, we’re looking down on a hummingbird
in this image. And there’s a laser,
a thing sheet of laser light much fancier than this little red laser
that I’m holding, but it illuminates that kind of fog field,
if you will, and zaps twice, like a strobe. And from that we can obtain information
on local velocities. This is a cartoon from NASA
demonstrating this principle. You take a couple laser flashes
with a bunch of particles moving by, use a computer, and reveal the flow field. And from that calculate things such as
the velocities of the air and other things of interest.
Such as the forces that an organism or anything, in the case of NASA, they might be interested
in a spacecraft, but whatever it’s producing in terms of the forces. So again, a cloud of those sort of particles,
just like I showed here. This olive oil illuminated with a laser, tracked,
and if they move from Time 1 to Time 2, you know the velocity
was in that direction. That’s called Particle,
because it’s particles of olive oil, Laser, is used there, but it’s
Particle Imaging Velocimetry. Alright, so then I have anoth –
I’m full of toys tonight. This is a device you can get science and
outreach-type – science and industry museums. It’s a vortex generator. If I pull this off correctly,
hopefully, especially the people in the front will be able to see this.
I’m going to try… just the light here. This is a vortex generator,
is that working? Yep, fair enough.
Let me try it right behind the light. I’m smacking the little trigger,
and you can keep yourself occupied [Audience laughs]
for long periods of time. You can shoot vortices into
other vortices. Alright, so, the beauty of this is
as a human you have a sense of flicking this little trigger and a rubber drum,
and playing with the drums, but again, my point is we don’t have
as much of a sense of what goes on with the air. So as we create a bolus
of acceleration, it creates a vortex. This type of system, the way I analyze this,
is involved with taking that kind of pattern, shooting a laser plane through it,
and you get the computer to generate a flow field. So that a downward flow would be this way
and blue is showing upward. And then there’s – again – there’s an aesthetic
component here, there’s a little techie component, it’s called “vorticity,” it’s local spin.
All it means is the rotation in the fluid, rotation in the air. It’s shown in yellow, because it’s going this way.
It’s shown in blue, because it’s going that way. And it’s the core,
it’s the eye of the hurricane if you will, or the eye of the tornado. And from a biologist’s perspective
wanting to understand what the animal is doing, that is of interest, it turns out. So we might take everything above a certain threshold
and sum that up and calculate things such as forces. It would be like dissecting
the little donut of vortex and then using that, reading the tea leaves
to get at what’s going on inside the organism. So just to give this a feel, if a bird was taking off
you’d have these boluses of (imitates sharp wingbeats), every time it engaged in a downstroke.
You’d see an equal and opposite reaction. Another (imitates sharp wingbeat),
bolus of downwash, equal and opposite, and so forth.
That enables the bird to take off. So now, we ramp up into
a little more formal stuff. Alright, Lift, the capital “L”.
What’s going on with lift? The first thing I want to share with you is that
when air moves over a surface, right at the surface, it’s stuck
to that surface. There’s no speed locally, there. That’s called a “no slip condition.” There’s no velocity right next to the surface,
if air is blowing this tabletop, ok? There’s something, like a fan
blowing this way, it’s not got any speed
right next to the surface, and then it gradually gains in speed
til it reaches free stream speed above. So what we’re after is just a reasonable interpretation
of how birds generate lift and are able to fly. That’s one part of the equation.
Another component is that fluids have viscosity. And when they encounter a surface that’s curved,
the fluid wants to stick to itself, little blobs of fluid, if you will, little particles
or patches of fluid want to stick to themselves, and that creates a fascinating feature
you can demonstrate in your own sink. And I would challenge you to go home
today or tomorrow – tonight or tomorrow, whatever, and play around with the coanda effect, which is simply
pointing out that since fluid wants to stick to itself, as it moves around the curve,
it’s going to continue to follow that curve, unless the angle is too great
and then it’ll fall off. Remember, my being water and air
behave very similarly. Water moving over a spoon does this.
Air moving over the wing of bird follows the curve. Ok, so that’s step one
in understanding lift. Lift is hyper-complicated and intimidating
but that’s a reasonable start. Ok, next then, invoking Bernoulli. And there are many different ways
to approach this idea. I brought a golf ball
to approach this idea. Alright, hopefully everybody hears that.
Now, this golf ball, if it’s moving, has a certain amount of momentum. And it emanates from
Newton’s laws. I’m not going to get too heavy here but it emanates
that there’s a conservation of momentum. The way the world is, our reality is that
momentum is conserved. So pressure, that’s exerted on you and I
and all the walls in this room, is static. It’s not moving to any great extent.
The pressure is pushing in on us due to molecules bombarding us
from the atmosphere, that’s static pressure. That’s not everything, but there’s no wind in this room,
so that’s mostly what’s defining this room right now. If there were a wind in here,
we’d have dynamic pressure. And that’s pressure associated with movement,
or if we will, loosely speaking, forward momentum. So this golf ball, pretend it’s an air molecule
and it’s beating against the table like all its other friends, it’s air molecules
that’s exerting a pressure because of the change in velocity that
it’s experiencing as it bounces against the surface. The way to think about Bernoulli’s theorem
as a first pass, is that if there is momentum in this direction, right?
This way. Then if momentum is conserved in the whole system,
there’s less momentum to beat against the wall. Ok? So let me restate that one more time.
If something is causing – whatever it is – wind, flapping wings, jet propulsion engine, something
is causing wind to move air molecules this way, there’s momentum this way
and in a fixed system, there’s less momentum available
to beat against this surface. So going back to, now, an airfoil,
air coming in, It turns out that it follows
the upper surface. It may even follow a flat surface
inclined appropriately, and we’ll see that in a moment. It doesn’t have to be asymmetrical like this,
but this airfoil, it does follow this surface. That’s that coanda effect. And because it’s changing direction
as it moves along the surface, technically, it’s accelerating. Speeding up, there’s a higher velocity.
That’s the dynamic pressure. It goes faster.
If it’s going faster over the surface, less chance for the air molecules to bounce
against that surface, so there’s less pressure above, there’s greater pressure below
and that’s, again, the magic we call lift. There’s an equal and opposite reaction and that’s called “induced velocity,”
that pushes the air down. So the air gets pushed down, it gets
accelerated downwards, the bird gets accelerated up, and flight is accomplished. So… This just summarizes the same thing. If you were, let’s say, a bird – sometimes they have little mice – not mice.
Mites (chuckles) or lice, some tiny thing that could be sitting on a feather,
going for the ride and observing what’s going on. You’ve got free stream velocity coming in, you’ve got this fast velocity above,
you have this slower velocity below, in black, and then what would you see
from your perspective just sitting there?
It just feels like there’s this pattern of wind coming at you. Then you drift down to the bottom side
of the wing and it seems like little bit different velocity. What’s shown in red here are
the relative terms that a little mite or a louse or what have you,
would observe back and forth. If you took off the average velocity and just
looked at what’s happening in relative terms, in relative terms, the air is moving fast above,
it’s moving slow below, so if you subtract off this free stream, it looks like, to a mite or to the wing itself
that air is moving this way. And that is a term called “bound circulation”
meaning that it’s bound on the wing. It’s not a reality.
All the air is moving past the wing. So, I’ll pull my wing back here, again,
orient it the proper direction, which is this way,. So, wing moving this way,
all the air is moving over it and below it. But if you subtract off the rate
at which it’s going and just say, ok, relatively speaking it’s moving
fast up here, this way, it’s moving slow down here, this way
so in relative terms it creates a pattern of relative airflow like this. So why that becomes important is
that that is shed. And so we’re gonna look at lift production
producing this circulation. As it happens, it’s always
perpendicular to the wing. This process of producing lift gives rise
to a force that’s like this yellow stick. Ok? So the major first point
I want to share with you is that if a bird is gliding as it moves along, right? That is supporting its weight
and it’s going to inevitably slow down or decrease in altitude
because of drag slowing it down. What we have in this complicated diagram
is simply a… there’s a simpler message in all the
little vectors, and so on and so forth, and that is, when we have lift oriented
this way in a glide, an animal gliding will
slowly decline in altitude. Trading off altitude to maintain speed. What’s shown down here, in contrast,
is the miracle of flapping, alright? Take the stick away and just say,
ok, a flapping motion moves in this sort of fashion. So an airplane would have a wing
to provide weight support and it would have motors somewhere,
a propeller or a jet engine to provide thrust. A bird is using its wing for both
weight support and for thrust. And I’m going to demonstrate that this way. Again the lift coming about from that
distribution of flow around the wing, is perpendicular to the wing’s surface. Gliding would be like this, tilted a little bit,
and the animal would just slowly decline. In a flapping mechanism, it’s flapping its wing,
which way is that yellow stick oriented? If I make it extreme, it’s oriented this way,
like a propeller that’s providing thrust. So birds’ wings, unlike an airplane’s wing,
because the wing is fixed. All an airplane wing does
is provide weight support. A bird’s wing is providing weight support,
and it’s providing thrust. And it’s doing that by flapping.
Point #1. Ok, so it accomplishes both weight support
and thrust by flapping its wing. Now a problem arises from that,
it turns out, enigmatically. How many people would count themselves –
a little show if hands – as a birder-type? A reasonable – that’s at least half, ok. So going back to my yellow stick –
maybe some people just not admitting it. In a downstroke, the way this stick is oriented
there’s an upward component that’s providing weight support and forward component
that’s providing thrust, the bird moves forward. On upstroke, if nothing changes,
and it pulls its wing up, That provides weight support, no problem,
but which way is the lift oriented? Remember this is lift –
the mechanism is lift. Here’s it’s providing weight support and thrust, here it’s providing weight support, but the animal wants to fly this way,
it’s pointed the wrong way, right? So it’s a problem. And birds have a very straightforward way
of dealing with that, which is shown in the lower figure here, and that’s
flexing their wings, getting them out of the way So the bulk – I can stick my –
I hope that red little dot is visible enough. This bird is flying forward, it’s in
upstroke right here. Downstroke, upstroke. Here’s an upstroke.
You really get a sense of how much the wing is flexed.
This is a great tit from over in Europe. And the point is, behind this image, that
on downstroke, they provide weight support and thrust. On the upstroke, they’ve got to do something
with their wings to avoid this negative thrust. Many species flex them completely. Almost, whatever, 9 or 10,000 species
of birds out there. Every one of them, pretty well,
does a downstroke the exact same way. But upstroke is where all
the interesting variation occurs. Some of them flex completely, like this. Some of them might turn their wing around
and flex them and turn. Like we’ll see in a pigeon, earlier. You’ll see a variation in the upstroke,
but it’s all mechanism – the primary flight force is coming from the downstroke,
regardless of what circumstance. And then depending on how fast
they want to fly or whether they want to take off
or accelerate or maneuver, the may use their upstrokes
in different ways. So that’s lift.
Let’s go back to these vortices. The cost, I wanted to explain lift itself
as a product of what’s necessary to fly. There is an associated cost
with producing lift. And that’s that left-hand side
of the curve. On a wing, if we have negative pressure,
relatively speaking, lower pressure above. Not negative but lower pressure, higher pressure, there’s going to be a tendency for
the higher pressure air below to move over the top of the wing. That creates what are called “tip vortices.” And this is by no means
unique to birds. So it’s manifest in large jets.
It’s a dangerous prospect in airports where airplanes have to be spaced out, these large ones
have so much energy in the wake of these tip vortices that other planes, particularly small planes,
could be tipped over and crashed as function of the energy in the wake. You see these manifest as the contrails. So the vortices coming off the wings of jets
high up in the air, with enough humidity, a little bit of exhaust from the plane,
all mixing up and leaving these twin contrails. That is the product of this vortex
production at the tips. The more strength there is in this downwash,
the more costly it is to fly. And we’ll get back to that in a second,
but again, my point is it’s not unique to birds. These planes do this
and can be enormous. These are guidelines for small plane pilots to avoid
getting mixed up in the mish-mosh of that vortex. Some examples, just from additional plane –
they’re aesthetically pleasing, And we’re going to see some things in birds
as well – a crop duster there, and a Cessna jet. And it so happens that’s exactly the type
of tea leaves we read from birds. So an animal in motion, just a simple gliding,
it’s giving rise to these tip vortices. Wherever it started, if it would’ve just started
it would’ve left this in the wake. This greenish, sort of appearance is intended to
represent a laser plane that we might use to visualize a flow with that
olive oil vapor. And what you see then is
this core of circulation left So as this imaginary animal
just started moving, it would leave this vortex
in its wake. And we could slice that
with a laser, sample twice in quick succession and look at a couple of terms
to determine lift. It happens to have this term called “circulation”
which is the sum of the area involved in that core. And then how intensely it’s spinning,
how rapidly it’s spinning. But from that, we can get at
that magical component of lift. And that’s exactly what we do
across town at the Flight Lab and Fort Missoula. So again, to give you an example,
just to walk through this a little bit, you have flexion of the wings. This is a pigeon viewed from this way,
and high speed video of a collared dove flying in a wind tunnel
and you get a sense of that flexion. What’s happening is this animal moves at about,
almost 20 mph, about 10 meter per second, is that it generating lift,
it’s generating a downwash that the wake – we’d be able to
observe if we flashed it with laser light. It’s flexing its wings
to cut down on this negative thrust, and all the while, you might be looking
at this bird, or you might go outside and look at a bird and not realize any of this
is going on simply because it’s invisible, alright? We don’t see it,
therefore it isn’t real. We just have an incredible bias,
as humans, towards what we see. Let’s go back to this curve this “U” shaped curve,
and let’s now focus on why is it so costly
to move slowly? I’m going to focus on hummingbirds, but every animal that flies spends a key figure
taking off from a surface and landing on a surface. And those phases are very slow as well,
so the things I’ll talk about with hovering slow flight correspond to take off, the very few wingbeats
right before a bird gets cruising along, and the last few wingbeats
as it comes in to land. Over here then, again, is the power required, which is
the most expensive of any form of animal locomotion. Much more expensive in terms of power
than running, or by far, swimming. It is very high under hovering conditions,
this curve doesn’t even go to the limit here. So we’re going to focus, right now,
on this cost and it’s this induced cost which I want to share with you. And the magic of how
animals deal with that. Helicopters do this, right?
Because we’ve engineered helicopters with a wheel. It’s a pretty good design, right?
Hats off to the people who do that. You actually see the effect of the wake downwash
being blasted down on the surface of this water. And the master, by far I would argue,
of all animals that fly, and I’m a little biased because
I study them, but, hummingbirds. This is a Calliope hummingbird
from here, Missoula County. Tend to bring them into the lab in May,
keep them for a couple weeks, work with them,
and let them back on their way. But it is a master of the air and it’s generating
a downwash, even though you don’t see it. It’s generating a downwash just like that helicopter.
I’ll prove that in a moment. Turns out that a helicopter, in some ways,
is the better design. In terms of energy use, it’s useful because
there’s a wheel there, and we’re pretty smart
for having invented the wheel. That it’s constantly circling around and around
and pushing down the air all the time. And hummingbirds, because they’ve inherited
forelimbs like you and I have, They oscillate.
So they don’t move around and around and around in a perpetual circle. And that means they have to
stop, turn around, and come back. So the motion that a hummingbird generates –
I’m going to actually move back one slide and show you this again with the… …with the smoke. It’s actually olive oil vapor.
People up front notice it smells a little bit like a pizza parlor. But the type of motion a hummingbird generates
with its wings as it flies is a figure 8. Like what we use when we – a goose can’t
move its wings this way, but if I were a hummingbird – moving like – if you do tread water, if you swim
and tread water – a figure 8 pattern. That’s what a hummingbird is doing. It’s turning its hand wing over and generating
lift of both halves of the cycle. Ok, very different from the bulk of birds, by the way,
all the other 10,000 species out there tend to flap their wings and then fold them up
and pull them around or maybe pull them in, turn them over, and swing them
around like this, a pigeon or a duck does that. Hummingbird, again, keeps things out
and does this figure 8 pattern, alright? And that’s a partial explanation.
It’s small, it’s got a lot of relative power. Ken Dial talked a lot about relative mass specific
or the proportional power, it’s got a lot. But that active upstroke is a key
component of its ability to hover. It’s still not as good as a helicopter
in that domain because it’s not a wheel. So that means it has to stop, turn around,
and come back. That notwithstanding, hummingbirds
are good at what they do. And they’re charismatic and cool. And the… …thing that’s shown here is this amount
of air they have to push down, say, relative to a helicopter with the same
wingspan or rotor span. They would have to push down more,
it’s going to cost them a bit more than a helicopter simply because they have to stop, turn around,
go back, turn around, go back, and so forth. And that direct cost is manifest with this
particle image velocimetry like this. And again, sometimes it’s nice to stop
and just realize it’s kind of cool looking. This bird has about a five centimeter wingspan,
ten centimeter, max, right? It’s like, this big. (Chuckles)
Like the end of my finger in terms of size. And it’s generating downwash
that’s about 10 mph right below it, alright? And we see the hint of these vortices here,
these are from each downstroke. So to just interpret this, imagine a laser plane
and I’m the bird and I’m doing this, treading water motion, and there’s a plane
just coming up this way. I move my wings in and out of the plane
and you’re seeing what’s left in the wake as the wings pass in and out of this
thin sheet of laser light. And the computer does the calculations.
Downwash here, and these vortices
being shed from the wing tips. This is another view of exactly
the same thing, in the red box with dashed lines just shows
one full downstroke. So anything that happens –
it’s exactly like footprints, right? Footprints in sand, you can go back and look at
them and say, oh, I think that person was running. Here you can say, ah,
I think this hummingbird was doing a downstroke because that’s the time interval
over which a downstroke occured. These boluses of vorticity, or spin… …and the upstroke is a little less significant
but it’s most definitely there. And then another downstroke
in formation right now. So again, generating this downwash. It’s costly because that air
has to be accelerated. And there’s a big acceleration.
You go from nothing, still air, to something like 10 mph,
is an enormous cost. And that’s why the left-hand side
of that curve is costly. The power curve is costly. And we may see kingfishers or
kestrels or ospreys hovering. “Hovering” in quotes. But they’re not really.
They have to have incoming wind. They are staying in place
relative to the ground, but there’s no way any bird
bigger than an hummingbird can sustain hovering unless it’s got wind
coming at it, helping. This, we’ll maybe save for questions
if people find interesting, but the thing is, from a hovering bird’s perspective,
in this room, hovering, if I’m the bird I’m having to generate all this downwash myself.
There’s no free ride. So it’s an enormous amount of energy required
from the musculature, it’s incredible. If wind is coming at me,
that’s already helping my wings function. It’s – another way of looking at it is
if there’s a mass of air coming along, so I can give it a more gentle push than what I would have to do
to the mass of air that my wings can intercept. So I’ll leave it at that.
It’s very costly to just do it all on your own. But if you have some in-current wind,
such as a typical wind coming left to right on this frame, the kingfisher can accomplish
hovering that way. It would not be able to sustain it more than
10-20 seconds otherwise. Hummingbirds can do it for hours. As long as they have access to food. It’s still very costly. And what I want to now turn towards
is some of the ways they’ve converged with insects in order to accomplish
this costly form of flight. This is an airfoil behaving very much like
and airplane wing would behave. We’re looking at streamlines,
we’re looking at an increasing angle of attack as it tilts up relative
to the incoming air, and then we see stall behind,
and I’ll play this video at least twice. And it’s fully stalled and an airplane
under those circumstances would drop. And with unfortunate consequences, right? Sudden loss of lift with increasing angle of attack. So let’s look at this video
one more time. Again, streamlines – this is in a wind tunnel.
An airfoil, common engineered airfoil And we have a pattern
at which… Increasing angle of attack.
When it does increase, first of all, notice the induced velocity,
the air is being pushed down. That’s the equal and opposite reaction.
Over 5 or 10 degrees, all is good. Little bit higher, still being pushed down.
You see some turbulence back here. A hint that things might
be going awry. And a slightly higher angle of attack
is going to be kicked in, and it goes into full stall.
You can just see the flow separate completely. So again, a plane would crash
under those circumstances. Another major point
I want to share with you: I’ll stick my neck out a little bit
because I’ve studied hummingbirds and a few other species now
with students in my lab. But all birds in slow flight generate lift
under conditions that would cause stall in airplanes. Which is cool as a general observation. They have the capacity to generate what are known as,
and this is not just birds, it turns out, insects and bats, it gotta tip the hat
to them as well And, for that matter, we’ll see in a moment,
samaras, like the maple seeds that spin. They do it too.
And I’m going to come back to the water bin and show you that practically
any flat plate moved the right way does this. But, that notwithstanding, the key
to this performance is called the “leading edge vortex.” So airfoil at a low angle of attack or
a low angle relative to incoming air, generates lift and drag. Everybody’s ok with that. You turn it completely perpendicular and
behaves very much like a paddle. Pure drag.
And you can see this. And I’ll demonstrate it again
in a second here, which is vortices shed behind. But in an appropriate high angle,
under some circumstances, especially with rotation and short
duration of swing, like this, You can see a standing wave of
vorticity forming on the leading edge of the wing. And the function that this has
is to increase lift. It increases force in general,
it increases drag as well. But this is exactly where hummingbirds,
nectar feeding bats, and in every other bird out there
that takes off in the first few wingbeats of its flight, or the last few wingbeats as it lands,
it’s using its wings in this way. What it’s doing is it’s
boosting force production for its circumstance. Let’s go back to the water here. Looks gross. Alright, so, I’m going to demonstrate
with this flat plate, this is a flat piece of plastic, and I just move it
in the correct fashion and you’ll see (chuckles)… It’s almost too easy.
There’s a leading edge vortex forming. You follow? You see it? It can’t persist under these circumstances. And by the way, if you catch this in the back
we’ll also see, over here, the shed vortext. But again, as I move this forward
there’s the leading edge vortex forming. Under translation, this would be
linear motion like this, it naturally forms so it’s not
that mysterious. It turns out that can’t be sustained.
If I had a bigger I could move it a little further and eventually that would separate
completely and stall. So part of what’s going on
in animals that fly is that their cuing their subtleties
of wing motion in such a fashion, and they stop and turn around
and start over again, before that thing is shed off
and it goes into stall. So it’s called “dynamic stall.” Again, I can’t really demonstrate it here.
I can demonstrate the leading edge vortex, but I can’t demonstrate any more distance to show
that it would separate from the wing and go into stall
with a simple plate. By virtue of rotating
and turning around, they avoid that stall. “They” being the birds, the insects… …and the bats. And the samaras. An individual named David Lentink,
he’s now at Stanford University. At the time he was a post-doc
working with these maple seeds but, the point is any thin, small, suitably shaped,
flat object spun in the right way can generate a leading edge vortex. And a hummingbird wing is
most definitely in that category. Small, thin, and flat. And we’ll show how these motions
converge on each other in these videos. A friend and collaborator of mine, Ty Hedrick
at the University of North Carolina Chapel Hill, studies hawk moths and noticed
the pattern of motion in these hawk moths. They’re about the same mass,
two or three grams, less than an ounce. A hummingbird, the Calliope,
and the samara moving around. If you look closely at the little field
around this maple seed, right on the surface you’ll get a sense of
the leading edge vortex there in that pattern. How many hundreds of millions of years,
evolutionary-wise, you have to go back to realize that these things converged
on the same aerodynamic mechanism. And these are images, then, showing the… …leading edge vortices, the wave, if you will,
standing on top of this flat wing that allows it to boost force production. It is a key component
for a hummingbird’s ability to hover. It’s a key element in insect flight
and so that’s one mechanism, and it’s not unique, I don’t have a slide here,
but it’s been shown in bats as well. A group working in Sweden
have shown this in the nectar-feeding,
hovering fruit bats and these samaras. Now I’ve been interacting in – a number of angles
of the work that I do, fascinate engineers and mutually to develop flying autonomous vehicles,
or “bird bots” if you will. These two individuals – we’re looking
at a tern that’s been modeled – and I’m going to show you some images
to give a bit of better insight, but again these data were obtained
right across town in the Flight Lab. From a Calliope hummingbird.
They show up in early May or maybe very late April and
stick around for a couple months. This is a very careful modeling
of its motion and its body form. These are my collaborators
on this type of work. And computational fluid dynamics accomplishes… …a phenomenal application of computers
to model what the flow would be. So we’re going to look at,
during this downstroke, starting right now, on the right-hand side,
these leading edge vortices forming. And this seems fast, we’ll just let it go
for awhile, but it seems fast. Remember, the hummingbird is beating
its wings almost 50 times per second. So it’s just a blur, but it’s slowed down.
We take this information and use that to model the flow characteristics
using and iterative process on computers, very fancy iterative process. So those leading edge vortices are key
and the text down below is supplemental, but the point is
they are actually modifying the – by the position of the wing,
they are modifying the intensity of circulation in those leading edge vortices
to accomplish that turn. Another view of the same thing.
Just giving you a feel for this. That maneuvering can accomplish small changes,
subtle changes in the angle of the wing relative to how it’s being
presented to the air. That modifies that wave
sitting on top of the wing. And then that’s used to accomplish this turn,
this yaw type of motion. Certainly, when I see things like this I feel very lucky
to have access to these technologies. Going back to the start and stop problem. There’s a nice solution, in spite of the fact that
a hummingbird or a moth or a bat moves its wings back and forth, back and forth,
stops, turns them around and comes back it turns out that during this phase of rotation,
they can actually entrain air and produce some lift. And that’s what this
slide shows. I mean it’s not mysterious in terms of intuition
once you see what’s happening. Again, I can show it again. It’s as simple as that. Notice how much air is getting whipped downwards
as I swing this goose wing around, right? If it’s moving down, that gives the potential
for the bird to move up. That’s called rotational circulation. And they absolutely capitalize on this. In terms of thinking of… …an insect moving its wing this way
as the figure is showing. The blueish line shows what kinds
of forces it would produce if it were just moving back and forth,
back and forth. But it times its rotation appropriately,
the red line shows the peaks in force that kick in. If it times the rotation appropriately And lo and behold, hummingbirds with this
figure 8 pattern and rotation timed with reversal, absolutely do that, the trajectories shown here are
the tips of the wings shown in black dots and circles. They’re indicating the trajectory
if the bird was holding still. And when it’s moving through this arc, from above you would see this pattern
for the wrist – fort he wingtip. And from the side, you get a bit
of a figure 8 pattern. About like what we do
when we tread water, roughly. And this just showing it flips
around from 0 degrees. So as a function of the wingbeat,
during downstroke, it’s going back and forth above and below 0 degrees.
Just a fancy way of showing that it inverts its wing. But during that phase, then,
what we’re looking at right now is right over the shoulder
of this hummingbird from above. And this laser light is
illuminating this flow field and we’re looking at the air
entrained by the wing rotation. So to summarize, we have leading edge vortices
boosting force in a hummingbird. And at the rotation ends of its downstroke
to upstroke transition or up- to downstroke transition,
it’s entraining air and helping to produce lift. So in spite of the fact that it doesn’t
have a wheel, it has to turn its wing around, it’s maximizing the full extent
of its excursion across the way. And again to emphasize that, the fact that these are
vectors, black arrows, that just shows air velocity. This wing has just turned around and is starting
downstroke and so all of that motion is exactly what I showed you
early on as circulation. So that’s the left-hand side of the curve. And that’s some ways in which nature has dealt
with dealing the expense, the power costs of slow flight and hummingbirds are the extreme
performers in this regard, but much of what I’ve just shared with you
applies to other birds when they take off. Let’s now look at the other side,
the fast flight side. Alright, this curve, this “U” shaped curve,
we’ve been dealing with induced costs, the cost of pushing down the air. But these two curves here
are for the wings and for the body, and drag goes up as a function
of increasing speed. And what I want to share with you
are tail effects, the tail of this green woodpecker. Somebody asked, Linda I guess it was,
and sure enough, there it is. And the body shape itself
engaged in a bound with the wings tucked in. Alright, so let’s think now about a sphere. An orange sphere, such as, for example,
this, big enough to see, basketball. Let’s think about air coming at it. Alright, invoking Bernoulli again. Total pressure in this room,
there’s static pressure on the basketball. Just like on your head
and my body and so forth. But now, if I move this forward,
there’s airflow relative to the basketball. Or if I had a fan over here
blowing air this way, or some in current wind,
there’d be wind coming at it. Alright, so this would have dynamic pressure,
there’s static pressure on the balloon, air molecules come along
and slam into the front. Zero velocity right at the stagnation point. All the pressure possible, the dynamic,
due to whatever force. You know, something was propelling it,
or wind was coming at it. Everything arrests and there’s
a lot of pressure, then, on the front end. Now, d’Alembert, a French scientist,
elucidated that if you think about this this way, the air has to move around,
or the fluid has to move around the object, but then it ought to separate
at the back side, and when it does separate,
so it’s going to speed up as it moves around. That coanda effect. It’s going to come – slow down as it moves
to the back side, slow down to zero and separate off. Real fluids never behave that way. It’s paradoxical because they don’t
in any water flume or running air past this, it doesn’t behave – it slams in and you get
an increase in velocity around the curve. Back down to zero here,
and separation. In contrast, the way the real world behaves
is like this, below. All you have to do is go out to the
Clark Fork – the bridge, the pedestrian bridge, look for one of the rocks, look behind it
and you’ll see this exact pattern. Or play around with some water
in a coffee cup tomorrow. The way the real water and air
of our world behaves, it’s got viscosity. Which in this sense is like the friends –
little molecules want to stick to each other, they want to hang out together. So as this air slams
into the front and moves around, it’s heading this way and it’s saying,
“Hey friends, hang with me.” Right? That’s viscosity. It’s come along for the ride.
So it’s coming over the surface, it’s trying to go around but actually
it’s got its own momentum in this direction, and it pulls some of its friends along
and that causes vortex separation. A turbulent wake
right behind anything. So an object in a stream, you will absolutely see
a vortex generation zone right behind it. The problem is, right, high pressure. All this dynamic pressure plus
the static pressure that’s already there, boom! On the back side, you’ve still got velocity
and that’s dynamic pressure. And as a consequence
the static pressure is less. And the net result is, wind coming this way,
whoooaaa, we got drag on this basketball. That’s drag. An imbalance, there’s pressure on the front end
that’s not balanced by pressure on the back. Because of flow separation. So that absolutely works, and that’s a separate –
I’ll go into the questions of why golf balls have dimples but I’ll leave that,
if you’re curious. I do have an answer for that. But it’s the wake that is evidence,
again, that there is a cost – a drag cost. One way to accomplish a reduction in drag
is to be suitably shaped. And lo and behold birds bodies, as are trout,
as are submarines, roughly cigar shaped, or fusiform. And they’re shaped in such a way
that the tapir of that body helps to recover
some of that velocity. Recover some of that static pressure. And so it reduces the wake profile,
if you will, the width of the wake. As the air moves around, you see an acceleration,
the streamlines get closer together, but it doesn’t separate until the very end here,
we have a minimal flow separation zone. Where the separation of these streamlines
indicates the velocity slowing down, and that’s minimizing drag. Because where velocity is lower,
the streamlines are further apart, the static pressure
is greater. And if you’re following me, that means
the static pressure is helping to reduce the drag because it’s pushing somewhat
back that way. So a streamlined, bluff body like that
and lo and behold, yet again a woodpecker, pileated woodpecker,
they are shaped that way. But another major point I want to share with you,
among many functions of the avian tail being a splitter, helping to keep flow attached
off the back side is absolutely one of them. This is a starling, a carcass that’s been frozen.
This is work from Mayberry and Rayner in England in 2001 taking, in a wind tunnel,
with flow – you’re see an intact tail, and the lack of a tail
with a much wider wake. So lacking the tail, the flow
separates earlier, there’s more drag. Again, the tail is used
for many things, many things. But among them, a splitter to help
keep flow attached is a function of the avian tail. Alright? The last thing I want to share with you
is something that actually got me started When I came out here after
having been a carpenter and was working under the guidance
of Dick Hutto, who will be talking in a few weeks, studying woodpeckers in clearcut areas where
the Forest Service had left snags for them to nest in, and watching this intermittent flight style
that’s very characteristic of woodpeckers. Most small birds do it,
and that was the first inkling that drew me towards flight,
is this fascinating flight style called “flapping and bounding” where the bounding
is tucking their wings in and bulletting through the air. Literally, about like a human ski jumper. There’s a flapping phase – flap, flap, flap, flap –
gain altitude and bound. We’re looking at a female zebra finch
doing this in a wind tunnel at about 8 meters per second.
About 16 miles per hour or so. High speed video just demonstrating that
the bird folds its wings completely against its body like it’s perched and bullets through the air.
This is so common. So, just look at any small bird out there and, other than the swallows
that aren’t around this season, everything else out there
is doing this if they’re small. Up to, say, the pileated woodpecker in size,
maybe 200-300 grams. Why? Before I explain the absolute “why,” I want to share with you that this
can be readily observed outside, and I would challenge you
to do exactly that. This video does kick in,
this is up in the Lower Rattlesnake at a good friend’s house
on Missoula Avenue. We’re looking at a black-capped chickadee
taking off from a feeder, engaging in flapping and bounding. So you can see this
in your own backyard. And it’s pretty…
[Audience laughs, amazed] [Audience laughs, amazed] …freaking insane, right?
[Audience laughs, amazed] I said, all the math all the physics notwithstanding,
it’s just plain cool what they can accomplish. It’s just aesthetically pleasing. Amazing, that’s a black-capped chickadee,
just a mile from here, up in the Rattlesnake. Here’s what’s going on. In that posture, they’re organizing flow with their body
and with their tail to support some of their weight, and they’re reducing drag
on their wings for a period of time. So the short answer is, it’s a great strategy
if you want to fly particularly fast. And a little bird like this is a great prey item for virtually anything from a cat to an accipitor, or what have you. The rapid take-off that they engage in –
you would think, on one hand, they’d just flap their wings as fast as possible
and getting going would make the most sense. It certainly makes sense
from an energetic standpoint. You could never make a case for hovering
and every once and a while closing your wings. Because gravity is the enemy. But what’s happening is that these birds
have relatively round wings and they generate a pattern
of flow over their body that is pushing down,
and they’re getting body lift from that, and they’re getting a reduction in drag
that helps them accelerate horizontally, more quickly. These figures are PIVs,
you should be familiar with this kind of pattern. This is a prepared, taxidermy mounted bird
that died for other reasons than my study. And taxidermically mounted in a wind tunnel,
looking from behind, and you’re seeing downwash, and you’re seeing these
vortices over the tail. This is a cartoon of the same thing,
with blue being up, red being down, and more evidence
of the same thing, you sample right in the middle, it’s red.
That means down. You move out towards the edge,
20 millimeters out, that’s upwash. It’s not unique to birds, and in fact
suitably designed cars actually generate the exact same kind of pattern. Being appropriately shaped,
a bit like a cigar, fusiform shaped, and oriented properly
into the wind. With one major addition,
and that is the tail. Even without the tail
they can generate this downwash. This is the taxidermically mounted
specimen with no tail. But with the tail, the lift and the lift relative
to the drag is much better. So the tail is functioning on the one hand
to reduce drag during fast flight, that’s that splitter effect. And then the other is that it’s
assisting and generating body lift during these bounding intervals. Back to a major foundation for the work I did
for my PhD many years back, Jeremy Rayner modeled this mathematically and
this shows that “U” shaped curve with flight velocity as
a function of power cost. And this flapping and bounding
never, energetically, makes sense in slow flight. But once you get going faster,
it absolutely makes sense. Cutting down on drag from the wings momentarily,
translating that into forward velocity. And just as an observation, say for example
off Cape Cod, radar stations will observe masses of migrants leaving, heading south
for example, and seeing signatures of this flapping and bounding
interval on the radar images. So there’s pulses you can actually identify
according to roughly the size of the species. So this is a, no pun intended, bit of whirlwind tour
of fluid dynamics and bird flight. And, you know, to get a couple
of main ideas across, I hope. But part of it is, the process of producing
forces in the air, that generates vortices, and those are like the
tea leaves in the water. We can read them and get real insight
into the biology of that kind of locomotion. That hummingbirds have converged on insect
mechanisms and it turns out bats and samaras also do some of that. But it’s not unique, it’s very costly
to fly slowly and couple of key features: that leading edge vortex and the circulation
that’s accomplished at the edge of the stroke help hummingbirds accomplish what they do. Tails, again, among many other things
like maneuvering, just generating lift, being used to advertise towards potential mates,
on and on and on, but one of the features of tails is that it helps keep flow
attached on the back side of that bird’s body. And reduce drag, if didn’t have a tail
it would have more drag. And then flapping and bounding. Extremely widespread in small birds
and medium sized birds. But it is accomplished, in part,
by producing body lift. And reducing drag on those wings. So I have so many people to honestly thank
that I just put “many” and “all” here instead. I got a great lab of students and many positive interactions
with post-doc’s and so forth. Wanted to give a special nod
to my mentors When I say, “Standing on the
shoulders of giants,” when I came out here 20 – many –
20+ years ago, 25 years back, the first individual I met on this campus
was Dick Hutto and his wife Sue Reel. They welcomed me into their home and went up and started a project
involving woodpeckers that essentially, full-circle, brought me
right back to this podium today. So major nod to him.
Andy B. Winter, a huge influence on my life as a post-doctoral advisor. And Ken Dial, you saw him talk last week,
he’s a force unto his own. [Audience laughs] I think I’ll leave it at that. (Laughs)
Much appreciated though. And among many, many, many, many
collaborators a special nod, a Most-Valuable-Player nod,
even if I’m not good at sports allusions, Doug Warrick, you saw a picture of him
and Don Powers, another collaborator. This is the field station across town,
and I guess I would extend a welcome. I mean, a major component of my research
is, actually, outreach and I’m pleased to be talking to you tonight, that way.
But if people want to tour some of these facilities, not everybody at once,
but I’m certainly open to hosting and allowing you to see some of the these
laser light shows in action, and so forth. Thank you. [Applause] Linda: And do you have a mic?
O.S.: Yep. Linda: And Susan, do you have a mic?
Ok, so as usual, thanks a lot, Bret. Bret: Absolutely.
Linda: And we’ll have time for question and answer I wonder if it’s possible
to bring the lights up Maybe, if not, maybe we’ll be able to see,
let’s give them a try. [Unintelligible] Ok, so questions?
Here’s one right here. Susan? Audience member: I’ve got a loud voice.
Can you explain golf balls? Bret: Sure. Sure.
[Audience laughs] Absolutely. I’m going to explain it
with a basketball, but it was noticed back, historically, that old golf balls would
fly further than new golf balls when they were smooth. And that absolutely led
to the dimpling. And the phenomenon – ok, so pretend
that this is now a golf ball. And if it’s very smooth, what’s going on
is that flow is occurring around the surface And that bend, the Coanda Effect,
air is attempting to follow, but it can’t,
eventually it separates. And the momentum that that air has
is a product, in part, of it’s velocity. Remember that no-slip condition
attached right to the surface is very low velocity. The dimpling creates turbulence
right on the boundary. So, using my hand on this basketball
to represent the boundary layer, what the dimpling causes is turbulence
which energizes that boundary layer, and helps keep
the flow attached. So a dimpled golf ball,
the flow will stay attached and you’ll have a smaller wake on the back side, just because there’s more energy right on that boundary layer Linda: How many golfers are there
in the room? [Laughter] Linda: Who else has a question? [Questioner mutters something]
Susan: We need it for the recording. Questioner: Ok, a question on vortexes.
I probably just missed some of this, but are they always a negative,
always a negative? Bret: That’s a great – no, no, no, no,
I was nodding because that’s an awesome question and no, they’re not always negative.
There’s been incredible work with insects. Michael Dickinson, who’s now at Caltech,
was in various other places, but insects, it’s been shown,
that they actually recapture. So they generate this vortex
and there’s this thing spinning in the air, a bit, not being trivial about it,
a bit like a surfer would ride a wave. If you move your wing appropriately
relative to this circulating air, you can harvest some
of that energy. That’s never been shown in birds or bats
but it has been shown for insects. But there’s one other cool feature,
I don’t have the slides to pull up and show you, but I’d encourage you to go look at it,
these ibises in Europe. There’s a phenomenal study
just published last year. Picture a “V” formation of geese,
now this was worked out in ibises, not geese, but this is the basic idea. I’m a goose, I’m the lead goose in the “V”,
and I’m generating these vortices behind me, and then there’s a bird
off and behind to the back. Alright? Again, this was work out of the
Royal Veterinary College in the U.K., Alan Wilson’s group, Jim Usherwood and others,
with ibises, they had an ultralight plane, they had these birds imprinted and they monitored
and showed that the birds sit there in the wake of the bird in front of them
and move their wings appropriately to harvest the air coming off
the other bird’s wing. That’s long been hypothesized to be the case
in geese, it remains to be shown for geese, but indications are
it’s likely. Questioner: Ok, I’ve noticed driving
and getting too close to a semi there’s a point in there where your car
begins shaking around, and if you get up real close, which is not the thing to do.
[Bret and audience laugh] Questioner: But it suddenly smooths out
when you’re close. Bret: Sure, absolutely, yep.
Again with the basketball there’s – Questioner: Those are vortexes? Bret: It is and it’s a wake.
So the vortices, looking at air coming this way, imagining that’s the truck, right over the back,
there are these vortices forming. You get right in the quiet zone,
and in a way, you’re actually being pulled along. And in fact, for people that race bicycles, for example,
motorcycles, that’s called, “sucking the wheel.” You suck onto the person’s wheel in front of you.
You’re literally in the wake. And that’s what’s happening behind the semi. Questioner: And one other thing,
airplanes recently, the manufacturers have added these, what they call “winglets”.
What effect does that have on – [Bret laughs]
I think the flight is smoother, maybe less vortexes? Questioner: I don’t know.
Bret: Ye – I owe him money for all these great questions. That’s a wonderful convergent pattern
between engineering and birds. If you look at picture of a vulture,
or if you look at a vulture in real life, you see the upturned primary feathers. That isn’t – you know – crows,
many other birds have primaries out, gliding and the feathers
of the tips are turned up. So going back to that inevitable
vortex production. It’s a cost.
I’m going to actually use this goose wing. This is something that a student in my lab
also, first name of Bret, Bret Clausen is studying. These feathers are not separated,
and they’re not bent up. But again if this is a wing flying towards the screen,
and you have this tip vortex being formed, and that represents a cost
because this portion is essentially lost as far as lift production
is concerned. So the governing hypothesis is that that flow
bending up and around gets that vortex off the wing. And it is absolutely known, for airplanes,
that it reduces fuel cost by about 5-7%. Which is not trivial,
and here’s the funny thing. And this gives you real insight into
the way natural selection and evolution works, going back to the planes, the same effects would be accomplished just by having longer wings. It doesn’t quite look as cool, and I’m not
making fun of that, it’s true. You could just have a longer wing
and get the same effect, but there’s design constraints in terms of
hangar size and ignoring the “cool factor” of the upturned wing,
it does require more space. So there’s a message there,
and again, I don’t have lots of figures to show you, but picture an albatross
with a very long, skinny wing, which has low induced power requirements,
it’s very efficient at flying, and in part because that long and skinny wing
out here, there are tip losses, but 90% of the wing
is not suffering those. Where you see those upturned primaries
and the emarginate primaries are in birds that soar a lot,
but they’re on land. So a red tailed hawk or
a turkey vulture. And that tells you something that there’s
probably a selective pressure associated with something, you know.
Other things, not bumping into trees or surfaces or what have you.
To have a short wing but to make the best of it. Relatively shorter than an albatross,
which is not suffering that constraint. So I guess the hypothesis we’re working with is,
yeah, that they’ve – the ideal design is probably an albatross wing, but if you’re not going
to have it long like that for some other reason such as you don’t want that slamming into a
Ponderosa pine tree when you take off, the upturned primaries and
emarginate primaries are a solution. [Audience member talks unintelligibly] [Audience laughs] Bret: The gas went down?
No. No. (Laughs) It’s a one way – right, right, right.
It’s a one way ticket, yeah. Do I select, or – go ahead. Questioner: I understand that owls have
especially quiet wings. I suppose that has something to do with vortices.
How is that connected to this? Bret: Yeah, that’s a great point.
If you have – I’ll make a little plug for the Phil Wright Museum,
if you want to see some owl feathers, or if you’ve ever had an owl in your hands,
the feathers that are on the specimens have a velvety almost, I guess velvet is the right term, rougly.
Kind of a fuzzy appearance, relative to, say, this goose wing. So, it’s not that it’s been fully studied,
but it does have something to do with vortex production, absolutely. And the simplest way to convince yourself of that,
I would encourage you go on a bicycle and go downhill. Like Duncan.
Drive over to the Rattlesnake or something. That noise you hear is turbulence
being shed behind your head (laughs) literally. The roar of wind is turbulence.
So there’s most likely a component of that fuzzy, velvety surface
that is keeping the boundary layer from generating too much
turbulence in owl wings. But that’s an open hypothesis.
Nobody’s ever demonstrated it. You can absolutely put a microphone on an owl
and see that it’s extremely quiet. You can see that the wings are fuzzy
but the link between the two, in terms of the types of – that remains to be done, it may – Not joking, it’d be an interesting
dissertation project for somebody. Linda: Are you a pilot, or do you hang glide?
Bret: No, no. I have too much respect for gravity. [Audience laughs] Yeah. Yep, I don’t have any urge that way.
And oddly enough, in truth, I think for humans the closest you can get to experiencing,
if you want to experience what birds truly experience is to ride a bicycle. You figure, on a mountain bike or road bike or whatever,
you’re using your leg muscles, not your pectoralis, but still, you’re using your
own muscles to crank along, you get up to the same
speeds they do. Even the big birds,
like eagles or whatever. You know, roughly, easily 20 mph
if you’re working at it, or certainly if you’re going downhill,
and I think that feeling is close enough for me. [Bret and audience laugh] Questioner: Just wondering, does the
hummingbird create lift on its backstroke? When you say it moves its wing backward,
it’s doing the same thing right? Bret: Yeah, yeah. Right.
So this was long thought to be the case. I had the good fortune to actually be the
person, with my collaborators Doug Warrick and Don Powers,
to demonstrate it for sure. It does, yes.
So it turns its wing over and when it does that inversion,
it generates lift. It doesn’t generate as much lift
as the downstroke, the downstroke does about – If 100% is the total lift generated,
the downstroke does about 75% of it. The upstroke 25% but still, under those extreme conditions, every little bit counts. Questioner: Right, because the shape of wing,
instead of going like this, is tucked under a little bit. Bret: You know, this goose wing is nice to show you.
This is called “camber”, it’s curved. The hummingbird wing is remarkably
flat relative to this. And it may have been, it may have passed quickly
but let me show you that it’s – Any suitably, any suitable – this is just
a pure, flat piece of plastic. If you hold it just right, you can generate-
and you move appropriately you can generate some – it might be very costly, but it’s flat and you move it the right way, you can generate some downwash. So that causes nausea. (Laughs)
[Audience laughs] But all perpetual downwash
was just from this simple plate. And so, yeah, it doesn’t
have to be cambered. This is not a good design for
generating lift this way. This is terrible explains, partially,
why birds don’t do that. But a flat wing can, especially
if it’s held the right way. Questioner: And a bird that’s flapping – right here –
a bird that’s flapping, is there any affect on the angle of the feathers as it goes up,
so there’s less resistance and there’s more airflow
through the wing and on the downstroke? Bret: Yes. Let’s see, I’m not sure
I’m understanding correctly, like the… Questioner: When the wing is going down,
they’re fixed so that there’s no wind. Bret: Yeah, ok.
Questioner: When they’re going up, Questioner: do the feathers change? Bret: Ah, right.
Yes, you know, they do. In various ways, if I can pull this off
with – let’s see – show you this pigeon again. It’s kind of an extreme example
of that very feature. Early on, I showed a
pigeon in this talk. If I can find my cursor here.
Let’s see… There it is. Let’s go way back to the beginning somewhere. Where are we at here? So, yeah, you see that separation?
Kind of a Venetian blind – [Questioner speaks unintelligibly]
Yeah, so there’s a lot going on in that pigeon in addition if you just listen
to a pigeon take off, you’ll here that [pats chest rapidly] sound.
That’s called a “clap” it does clap its wings and peel them, which is on the back side,
over the back, I can’t do it with my arms. Behind, its wings clap together
and peel apart and that generates a little pulse of air. But, yeah, they manipulate their wings.
So this is related to what the hummingbird is doing. The hummingbird is taking it to the extreme
and turning the whole wing over. Pigeons, I don’t have videos, unfortunately tonight,
but ducks, mallards taking off do this routinely. Ospreys, you know, just go on YouTube
or wherever on the web and just type “osprey take off,” and you’ll see this incredible
Venetian blind apperance. So what’s really going on is they’re
bringing their wrists over their head somewhat, turning the hand-wing over
and spinning it this way. And that’s very different from birds –
relatively speaking the pigeon, like this goose, has a relatively pointy wing. Relatively. Whereas something like a finch
or a magpie has a relatively rounded wing. And those birds with rounded wings,
many of them out there, instead of doing that in slow flight,they literally just bring their wings completely close to the body. And then start over again,
get them out of the way. Questioner: Given the question on the tail
and reducing drag, why the variety and different shapes of tails?
Bret: Yeah, that’s a great question. I mean a major component in that,
and a huge aspect of bird biology research has involved sexual selection. So one way to think of it
is that the ideal from an aerodynamic standpoint is a bit like a triangle. So it a bird, an imaginary bird here,
and its tail, and you’re looking at the tail flared, and it formed a triangle,
that would be the optimum for producing lift,
relative to drag costs. And then you start to look at birds, like a magpie,
that has this tail, that if you spread it out and looked at it, it would go down like this,
and then it would have this long middle part. All that long middle part, all available aerodynamic
evidence is that that just creates drag. Now both male and female magpies have that,
so let’s leave that alone for a second. But let’s look towards
barn swallows in Europe. That have very long streamers
on the outside. And it’s been quite a vigorous area of research
to tease apart how much of that streamer length is due to female choice as a handicapping,
an index, a true signal of male health. Like, “Hey, I can catch insects all day,
in spite of having these streamers that cost a lot.” [Audience laughs] Right? And for the European barn swallows
that seems to be – beyond a certain length these tail streamers are costly. So the interpretation is that it’s
due to female choice. That’s at least one – Questioner: Yes, you suggested early on that only small birds could hover,
at least in level flight or calm wind. Now is that because of their – of larger birds’
inability to to generate enough energy to support their weight, or is it also
in part to their not having evolved, anatomically, the ability to flip their wings over
like a hummingbird, or is it a combination? Bret: Like almost everything in biology,
I’ll take the fifth and say it’s a combination. (Laughs) But not jokingly, a major component is the size. Last week, Ken Dial presented a number of
features that varied with size. Smaller birds can literally fly circles
around bigger birds. Not a hummingbird, but a chickadee
can flight right around the trunk, all the way to the top. And go mob a crow and fly circles around it,
and crows can mob eagles. So the relative power available compared to
what they need to fly is greater in small things. And one way to think about that
is that they’re small and they can move their wings
extremely fast by virtue of that. So that’s a portion of the ability
to generate lots of power. Another metaphor, if you will, is like
comparing a Porsche, it’s got a small engine with high RPM’s and it can just zip
and zip right up a hill, whereas a larger, just by virtue of the way
the engine works and the pistons have to move back and forth,
and they’re bigger and they move slower a bigger vehicle can’t
generate as much. It can generate more absolute power. Absolute terms, but it’s relative to its mass. So that’s the major feature, and
it’s only the smallest bats that can hover. But any small bird, a finch, a chickadee,
a nuthatch, whatever – warbler, can certainly hover
for 20 or 30 seconds. But I had a Rufus hummingbird when I was
in Portland working, we brought one into the lab,
to get him acclimated we put it in the wind tunnel with plenty of sugar water to feed on.
It was a female. And the female would fly around and look
for a way to get out and she did that for an hour and 15 minutes
without stopping. That’s 40 wingbeats per second
for an hour and 15 minutes Every once and while stopping to feed. And eventually, she just calmly said,
“Ok, I guess (laughs with audience),” “I guess I’ll sit now.” So, yes it’s size and yes it is that they haven’t
evolved with the ability to invert their wings. Questioner: Um… Bret: In the corner, uh-huh? Questioner: Yeah, hi.
So, thinking about the geese, working together behind each other, what about the birds that swarm together
or school like fish and move in unison? Questioner: What is the study…
Bret: That’s a fascinating question – Right, I mean, it may be disturbance even in certain –
you know, so many animals like that you may have issues of
turbulence causing perturbations. In a fascinating study, it turns out, from the
exact same group that did the work with the ibises, they but physiological, heart rate monitoring devices,
a person named Jim Usherwood, with pigeons and measured them
flying in a flock around and around in circles.
You can get pigeons to do that and come back to roost and you can
get your recording apparati. And showed that that flocking
behavior cost energy. Above, you know, what it would
be to just simply fly by yourself. So the addition of all the birds around,
creating kind of a random array of eddies an vortices, in that study was indicated
to be a cost. [Inaudible question} Yeah, I, well I would say that
remains to be studied, honestly. If everybody’s behaving in tight unison,
and sometimes you can see in high speed video that they seem like they’re timed down to
the microsecond or millisecond in incredible unison and that hasn’t been studied,
and that’s a great possibility – a nich hypothesis that aspects of that
may facilitate performance. Linda (O.S.): If somebody asked me question
I can’t answer I just say, “Why don’t you go home, do some research,”
(Bret laughs) Linda (O.S.): “write a paper and come back to me.”
Bret: And come back to me, sure. Linda: (O.S.): And then they never ask a question again.
(Bret laughs) Linda (O.S.): So it’s a technique I recommend.
Bret: Ok, that sounds like a plan. Linda (O.S.): I have about 8:30, if you have
further questions, let’s give our professor a hand. [Applause] [Music]

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