h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I have a question: do birds experience turbulence? They fly just like planes so they'd fly into the same air currents. If anyone can enlighten me on this I'd be very grateful.

The short answer is yes. If you want a long answer ask an expert.

I wish I'd done physics, too. And Maths. Or just about anything really.

Don't many of the larger birds (eagles, buzzards etc.) use the 'turbulence' to stay up in the air, in much the same way that gliders do? They ride the thermals, to save energy while they look for prey.

I'll never regret not taking physics. Ever.

I think 'turbulence' might be the wrong word here. That is just swirling air, like the wake of a ship.
Everything that moves through the air, leaves some turbulence behind.
Some birds of prey even seem to use it by braking, sending turbulence ahead what disturbs the prey so the can catch it easier.

They ride on air currents, don't they? And especially on thermals - hot air rising from the land.

Mr L - who hisses and makes the sign of the cross when shown a physics book.

*Eagerly joins the anti-physics thread!*

Funnily enough, I do read a lot of popular physics - that's all you need to argue with people on h2g2

You can see birds experiencing turbulence on windy days, especially the ungainly ones (ducks especially). Watch closely - binoculars may be useful - and you can see them failing to fly in a straight line. Quite amusing, in a You've-Been-Framed type of way.

B

I took physics in high school. I did an entire year's worth of work on the desktop because I consistently forgot my workbooks. But that's beside the point...

Have you seen birds trying to fly into the wind on a very windy day!

Alji, of the Red Dragon (Swynwr y Ddraig Goch) (conducting a sun sign poll @ A712595)(Member of The H2G2 Guild of Wizards @ U197895 looking for wiz kids to join, though you don't have to be a wiz kid just know a bit about some subject that you think will be of interest to others or just bore the pants off them. This is an equal opportunities space open to all sexes, ages and abilities)

It might help to define "turbulence", and its opposite, "laminar flow".

Picture a pipe in which thick liquid, glycerine, say, is flowing, slowly. The pipe is wide and smooth. The glycerine is flowing fastest in the centre, and slowest at the sides. The progression from slow to fast is fairly uniform across the radius of the pipe. A graph of velocity against radius would look a bit like a parabola (although not exactly).

That is laminar flow - it's smooth, and there's not much radial mixing - that is to say, if you introduced a stream of ink into the centre line of the pipe, it would tend to stay in the centre line of the pipe for quite a way downstream before diffusion spread it out.

Now, make the pipe rougher, or narrower, or make the liquid denser, or less viscous - water, say. Eventually, the smooth flow will start to break down. There will be mixing across the radius - your ink will mix throughout the water much quicker, almost instantaneoulsly in fact. You've got turbulent flow, and the profile of velocities across the pipe is almost flat - it's all moving down the pipe at the same speed, except for the bits at the very edges which are slowed by friction on the walls.

There's a thing called Reynolds' number, defined as liquid density, times velocity of flow, times diameter of the pipe, divided by viscosity. The funny thing is, if you do it in consistent units (like SI units) all the units cancel out - it's just a number with no units at all. If the number is less than about 2100, flow is laminar. Above about 4000, it's turbulent. In between, it could be either.

Now, wings only work if the air flow over them is laminar. Too much turbulence, and they just stop working and your plane (or bird) drops through the air like a rock, until the laminar flow over the aerofoils starts again. Birds DO experience turbulence, but their wings are a LOT more adjustable, and they are instinctively able to make minute adjustments to their wing geometry to reestablish laminar flow. Birds being blown off course are not, technically, experiencing turbulence, at least not on the scale of their wing surfaces. What they are experiencing could be called turbulence on a much larger scale, or could just be called "wind direction". Similarly, even if a bird is riding a thermal like a paraglider, it's still experiencing laminar flow over its wings, because if it wasn't it would be falling rather fast. Birds are of course also subject to the same downdrafts which occasionally slam unwary paragliders into the ground at high speed, but again they are much better at instinctively recognising such situations and reacting to them.

Impressive things, birds.

H.

Sorry to bore all the non-physicists, but that could be one of the more useful applications of a Reynolds number I've come across.
The damn things never worked out to what I wanted them to be when I was designing plastic pipeline.

B

Impressive things, birds - did you know they extract oxygen from the air when breathing out as well as when breathing in.

Alji (Member of The Guild of Wizards @ U197895)

All mammals do that to a lesser extent as well...

B

Their lungs are designed to extract the maximum amount of oxygen.


Alji (Member of The Guild of Wizards @ U197895)

Maybe even more impressive than birds: bumblebees. It seems you can prove they cannot fly with laminar flowing. They somehow use and increase the turbulence to keep them in the air.

There's a pretty definitive discussion on how the impossibility of bumblebee flight story came into being at http://www.math.niu.edu/~rusin/known-math/98/bees

I never believed in 'bumblebees' anyway.

The bumps and jumps you get in aircraft when the captain says 'we are experiencing some turbulance right now' affect smaller aircraft much more significantly than they effect large aircraft. If you think about it will take more force to bump a 747 up 3' into the air than it will take to bump up a 2 seater.

So - is what pilots call 'turbulence' actually turbulence, as you explained it, Hoo? Or is it the same word being applied much more loosely?

And are birds even more vulnerable than light aircraft, or is something different going on at 50' compared with what is going on at 35,000'?

B

There are basically two different things going in with "turbulence" affecting aircraft.

One is bulk movement of the air in which the plane is flying. Anyone who has piloted a paraglider (pretty much the lightest of light aircraft, given that you can without much effort carry one up the side of a hill in a bag) into a thermal will tell you that it can be a violent experience. The aircraft is passing from a region of air moving more or less horizontally into one moving quite quickly and vertically. The effect on an aircraft whose total weight including the pilot is less than 150kg is quite dramatic. However, flow over (and in this case through) the wing remains laminar. The turbulence, such as it is, is large scale, and the wing continues to work.

For an image of the difference between large scale and small scale turbulence, imagine stirring cream into your tomato soup with a slow moving spoon, then imagine doing the same thing with a hand blender.

Anyone who has seen "Top Gun" might remember the scene where Tom Cruise's
One is bulk movement of the air in which the plane is flying. Anyone who has piloted a paraglider (pretty much the lightest of light aircraft, given that you can without much effort carry one up the side of a hill in a bag) into a thermal will tell you that it can be a violent experience. The aircraft is passing from a region of air moving more or less horizontally into one moving quite quickly and vertically. The effect on an aircraft whose total weight including the pilot is less than 150kg is quite dramatic. However, flow over (and in this case through) the wing remains laminar. The turbulence, such as it is, is large scale, and the wing continues to work.

For an image of the difference between large scale and small scale turbulence, imagine stirring cream into your tomato soup with a slow moving spoon, then imagine doing the same thing with a hand blender.

Anyone who has seen "Top Gun" might remember the scene where Tom Cruise's F15?thread=There are basically two different things going in with "turbulence" affecting aircraft.

One is bulk movement of the air in which the plane is flying. Anyone who has piloted a paraglider (pretty much the lightest of light aircraft, given that you can without much effort carry one up the side of a hill in a bag) into a thermal will tell you that it can be a violent experience. The aircraft is passing from a region of air moving more or less horizontally into one moving quite quickly and vertically. The effect on an aircraft whose total weight including the pilot is less than 150kg is quite dramatic. However, flow over (and in this case through) the wing remains laminar. The turbulence, such as it is, is large scale, and the wing continues to work.

For an image of the difference between large scale and small scale turbulence, imagine stirring cream into your tomato soup with a slow moving spoon, then imagine doing the same thing with a hand blender.

Anyone who has seen "Top Gun" might remember the scene where Tom Cruise's F15 passes through the jetwash of another plane, and falls out of the sky like a rock. I *think* (someone correct me please if I'm wrong) that this is a case of the turbulence being so violent that the laminar flow over the wing breaks down and it stops working as a wing. It's a serious risk to planes like that, because if your wing isn't working, neither are your control surfaces - there's not a lot you can do but hope, or eject. It's worth noting that turbulence this violent is only to be found directly behind a jet travelling at high speed - no weather condition could produce it. The bumps and jumps you get in airliners are, I think, almost entirely the first kind - large scale turbulence where the plane is passing through masses of air which are moving in different directions. And yes, there is something different going on at 50 feet than is going on at 35000' - boundary layer friction. In the pipe example I gave above, I mentioned that in turbulent flow in a pipe, almost all the liquid is moving forward down the pipe at the same speed. The exception is the liquid right next to the pipe wall. It's slowed down by friction with the wall. Exactly the same thing happens in the air. Air speeds near the ground are quite slow. If you get the chance, look at the sail on a windsurfer. If it's rigged right, you'll see an aerofoil shape - but not a constant one. Down near the bottom of the mast, it's very full and deep to give as much driving force as possible. Nearer the top, it's almost flat, because at the top of the sail the wind will ALWAYS be moving faster than it is at the bottom. The difference is very noticeable, even over the less than 5 metres of the height of a sail. At ground level, where the earth acts as a friction brake, wind speeds of 60mph or more are rare. Seven miles up, where airliners fly, windspeeds of 100mph are an everyday occurrence. This explains, among other things, why it takes longer to fly to the US than it does to fly back (or is it the other way round?) and how it's possible to fly a balloon right round the world. H.

Wow - I think I'm starting to understand it all now. Birds are affected by 'turbulent flow and larimer flow' - is that right? Would they experience any tingles or jiggles? (I don't think that's a real word - never mind). A friend of mine is hoping to be a Physics teacher so if I ever have enough time I might ask her to teach me some stuff. Is it just me or does it all seem kind of poetic?

"Wow - I think I'm starting to understand it all now."

Excellent!

"Birds are affected by 'turbulent flow and larimer flow' - is that right?"

The word is LAMINAR - not coincidentally similar to the word "laminated". If flow in a pipe is laminar, you can imagine the fastest bit as a rod down the middle, the next fastest as a cylinder "laminated" onto that rod, the next fastest as a larger cylinder around that, and so on.

If flow in a channel, like a flat bottomed river, is laminar, it's just like a bunch of sheets on top of one another - the sheet at the bottom (near the bed) is moving slowest because of friction, and as you move up, each one gets faster and faster because there's less and less friction. (This ignores the effects of the banks)

"Would they experience any tingles or jiggles? (I don't think that's a real word - never mind)."

Tingles? Only if they've drunk an Aftershock! Jiggles? I think you're talking about bulk air movement here, so yeah.

" A friend of mine is hoping to be a Physics teacher so if I ever have enough time I might ask her to teach me some stuff."

Make time. You'll be glad you did, if your friend is a good teacher. You might also like to check out "Six Easy Pieces", by Richard Feynmann, one of the great physicists and teachers of physics of the twentieth century.

"Is it just me or does it all seem kind of poetic?"

Poetry, physics, what's the difference? Which scientific discipline actually wrote down the music of the spheres in notation we could understand. It's beautiful, isn't it?

H.