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The Secret of Flight 11: Story of Vortex

(pleasant music) (plane engine roaring) – [Narrator] The Secret Of Flight. A series of programs on aerodynamics. Program 11. The Story Of The Vortex. Your host is Doctor Alexander Lippisch. – When I explained to you in one of our last programs, the generation of lift as a simple position of rotation and motion around the wing and the straight flow, we call that the translation. Then you might have thought that these fabulous lift forces which carry these large airplanes with several hundred thousands of pounds through the sky. That these lift forces are, in some way, generated by a vortex motion. And that, this vortex motion is necessary to make the lift of an airplane. Well, there are many other cases of such rotation and motion in the fluid. And since the vortex is so important in many cases of fluid motion. Let’s talk a little about
this strange fellow. Well the last time, I think I showed you, some ring vortices out of this box. And well, since it is so nice let’s make some more of these. I think you see it
better when I stand here, and shoot it across our tunnel. Let’s see. Now you see ’em? They go straight out, and it looks to you as if you shoot just air through air. And when you look at them, you observe at once one very important law of vortex motion. See, the smoke comes out like a bullet. That means that all the particles of air which contain the smoke, or which are smoky air, stay together in this ring which actually is nothing
than a vortex ring. That means, we know at once that if fluid is bound in a vortex, it stays together and goes
through the other fluid as if it would be a solid body. On the other hand, when
you look at this close, then you see that this rotation around the vortex is different than the rotation of a wheel. On a wheel, the outer peripheral paths move faster than the center. On the vortex it’s just opposite. The center moves the fast, and the outer moves slower. Now how do we produce this? Well, let’s think about it. Let’s have a cross section of this thing. This is a box and there we have the bottom with a hole in it which has very sharp corners. Now we punching at this membrane which we have back there, and then we produce a pressure wave which goes through here, hits these corners, and
blows out a bubble of air, and then the bubble of air comes out here. This takes little particles which just are sitting here with it, and then new air has to flow towards the point where you have taken away these particles. So this motion, and this motion produces a spiral motion which makes a kind of mushroom and out of the mushroom
finally comes your rain vortex. We have made some high speed pictures, some slow motion movies of this generation of the vortices, and let’s now look at these pictures. First, we see a front view of the box. And it’s very interesting to observe first the radial motion of the air, and then the bubbles formed and come out as ring vortex. In the next picture, we see
the same process from the side, and since we have a membrane back there which is elastic, the bubble
jumps in and out first before it finally travels
away as a ring vortex. Here we have another tube that we have another kind of membrane on it, so this doesn’t happen, and it looks like an
atomic explosion actually. But the most interesting view is to see it from in front, which is this picture again, and observe the vortex formation before it comes out. This is most interesting, and I think it’s only shown
in this high speed picture. You never can observe
it with the bare eye. Now what we are seeing here, well that the formation depends on a flow across a sharp edge, and behind this sharp edge, the vortex is forming. Then we remember that we saw this once on an inclined plane. Actually, the formation
of the wing tip vortex is nothing else than it hits
the sharp edge of a wing tip, and then the flow rolls over it and forms this wing tip vortex. Let’s look at this thing
again in our wind tunnel. And we have in this case, we have just a part, the ending of a flat plate in it, and when we turn the blower on and we have some smoke, then we can very well see, again, the formation of this spiral vortex from this corner where the flow actually soars away and new air from the side moves in and forms then this spiral motion. You see it here very well. When you look at it from the side, then you can actually see the three dimensional development
of this spiral motion. Now you know, we have
made these vortex rings and the fluid vortices in the air. And you might think, can you do that also in other fluids? For instance, in water? Well, I will show you you can actually do the smoke rings in water, and we have here a water tank. We don’t have fishes in it, but we have a hole at the center and underneath of this tank, we have a little container with colored water. We have some ink in it, and there’s a membrane underneath. And when I push this membrane, then we make the same smoke rings here, the water rings, as we did it in air. Now watch this. See, I push it up. Then it goes lightly, and there’s the smoke ring. Do you see ’em? Another one. Another one. When I make it a little bit more violent, then you see that it even
jumps out of the water. Watch this! Or this. Now we don’t want to color this all up, because then we don’t
see our rings anymore, but I will just make one more. You see? Therefore, we can make these vortices also in such a dense fluid as the water is. Now, when you look at the rings, and you see that they
jump out of the water at least at the center, you think about what sense
of rotation is in there? Let’s talk about this for a moment. See, we have actually here our bottom, and there’s the hole in it, and now when we push, then the water comes out of it and as I told you, forms just a bubble,
like this, a mushroom, and out of the mushroom
comes finally this ring, which covers up there as a true vortex ring. The center of the vortex
is somewhere here. And the highest motion is in the center. Now the center of rotation
will be then this. When we push this way, we have a rotation which goes around this way and this way. That means the outer turns back, and the inner goes up. And when we now hit the water surface with such a ring, where the rotation is less way then we have a water column which now jumps out of the water here, since we have this center we have the velocity
against the water level and the other ring disperses along the water surface. Now, you have probably yourself observed in water
another type of a vortex. You know, when you open the sink, or the bathtub, let the water out, you often observe that there is forming a kind of a funnel, and that the water begins to rotate and form such a funnel of air and surrounded by
rotating water particles, which actually forms a straight
vortex going into the sink. I think we can show that
also here in our water tank, when we let it out. First due to this continuous motion, due to some disturbances
which aren’t there, the water which all wants
to flow towards the center misses this center a little bit, and due to this, a rotation begins which
finally forms the funnel. Now it slowly begins. We have to wait a little bit, and this depends on the level of the water when this vortex actually develops and the size of the opening. I can see from here now that the rotation begins to form. And pretty soon you will see that due to the low pressure which is in the center of the vortex, there will develop a
little dip in the water which shows us the beginning of the vortex formation, as it is now. Now you see it beginning, and then it’s pulled down. You see it now coming down, and going towards the outlet. See now? And this gets stronger and stronger. You get a little wet with these experiments, but don’t bother. And you can see now this
vortex funnel very clear going down into the sink. The rotation at the
center is quite strong. Well, let me explain to
you a little bit more how this comes to such a rotation, and while it runs out we can see a little bit still. We will talk about the
cause of such a vortex. The main thing is that you have to have a sink. That means what is first with
the formation of the water? What is first? The chicken or the egg, and what is here the egg and what is here the chicken? Well the chicken is the vortex, and the egg is a low pressure area where everything’s close to it. Now, when it does this, when we have this hole in the center, we pull the plug out. Then all the water wants
to go towards the center, because the pressure here is lower than the pressure in
the surrounding water. Since we pulled the plug at this point, we have no water pressure from above. Now, theoretically these lines and these flow lines should
all come into the center, but due to little disturbances which are on the floor and which are in the water anyhow, some of the particles will
not have the straight line into the center, but
just miss it a tiny bit. Now, when one of these
lines misses this point and shoots over here, well then it is back into it and so this rotation stops. And then since this missed it, the other particles that wanted to go here comes also around and does the same thing, and then finally all of them do this and there we have the vortex. Due to the courtesy of the Hydraulics Research
Institute at the University, State University of Iowa, we have some nice films of such vortex formation, and we can see them here, especially a film which
is taken from above. Where you look through the
vortex formation from above, and you see very well in these films the velocity distribution in the vortex. You see where the center rotates the most, and towards the outer,
the rotational velocity gets smaller and smaller. Now we see here again the development of the
funnel from the side view. First, a very fine line which gets wider and wider when the rotation increases here due to the gravitational force which pushed the water
out to the bottom sink. And finally, we have a big funnel with a very strong
rotation around the center. The low pressure at the center is so low that we have instead of water, we have pure air in it because the water level itself
goes down up into the sink. Now we have put some dye into the funnel, and you might see the flow around the funnel itself made visible by putting
some dye into the fluid. As the big fill receives the
outer motion of the fluid. If you see these experiments with the water funnel, you might remember that we have seen this too in pictures, or even if you are from
the South, in nature. As a big tornado funnel. And you might think that there is some striking similarities between the funnel of the tornado and this outlet vortex in a water vessel. I think before we talk about this more closely, let’s look
at some of the pictures which we got about the tornadoes. So we can see them and
observe their formation. (wind roaring) Now the sight of this most
violent of all wind storms, the tornado, which presents
a vertical vortex column, we can observe in nature also horizontal formations of vortices, and I have here an experiment built up where we can see a
special vortex formation due to the fact that we have a hot plate underneath, on the bottom, and a very cold one on top. And when we let the flow
go through this channel, then due to the convective motion, the hot plate keeps air on the bottom, the thing goes up, the air goes up, hits the cold layer on top and returns again. And you see in this row of
counter rotating vortices. Let me explain just shortly this little apparatus. We have here a box here with a glass wall on
top, here on the side, where we can look through the channel. There’s the channel itself, on the top we have a metal plate with dry ice on it so that’s real cold. From underneath, we light it and have a glass wall so the light comes on the glass wall and heats the glass wall so that the glass wall is hot. And then we set this off by a blower. When we turn on the blower, then we can see this– (machine grinding) Makes a little noise, huh? Smoke develop. We have to look then
into the thing from here. And when we look this way in, then we can see a picture which looks almost like this. There we have the hot plate that’s hot. That’s cold. And there we have counter
rotating vortices. So the air goes here up, there down, here up, and there down. If we have this in nature, where the air goes up, clouds are forming and we see long, mile
long cloud streaks forming and we have cold air on top, very cold air on top and the ground is heated by the sunshine. The energy which makes all these vortices comes in this case from the heat, and in that once we are
also due to the layers there’s different pressure and density. You can observe also vertical vortex forming from the ground, which we call the dust devil. And we have here a small glass box where we can make little
models of such dust devils developing from the warmer
surface of this box. It is enclosed so that we actually can see better how
these vertical vortices, these dust devils develop. The main thing is that
we have a very warm layer on the surface itself, and when it rolls over
due to the slow motion, it begins to roll and
then a vertical vortex spools out of it as you see it very well in this box. The warm layer is what
we see here as smoke, and all these spooling up lines are actually vortices which are produced from
this warm air going up and rotating as it goes up, because the air can only go to other air if it assembles into a vortex. And you see here very well that every of these fine lines is actually a vortex. Now you see very near to the formation, and very near with the camera, and we can observe more distinct this rotational motion. But let’s get back to our aerodynamics. We get too deep into meteorology. A vortex which we saw was a wing tip vortex, and here’s a picture of a
cross section of such a vortex which we took in the 3D tunnel. Isn’t this a striking similarity to something that you might have seen sometimes in pictures
from the great telescopes which survey our stars in the sky? When you look at the picture of the wing tip vortex formation, and you see a picture of a galaxy, of a spiral nebula, you see that something is
similar in both pictures. I don’t know what it is, and I doubt that we
have a clear explanation of this similarity and whether there is any
spiral motion in the sense as we observed it here. But you sit here on such instruments, and apparatus, and see these strange things develop, and you think it over and you finally end up
that anyhow many things stay unexplained to you. As a great poet says, “Mysterious
even in the light of day, “may Mother Nature offer real reprieves, “unless she willingly reveals to you, “nothing you’ll force from her. “This level of a scope.” Goodbye, until next time. (pleasant music) – [Narrator] You have just
seen The Story of the Vortex, the 11th in a series of programs explaining the secret of flight. Your host has been Dr. Alexander Lippisch, director of the Aeronautical
Research Laboratories of the Collins Radio Company. This program was produced for the Educational
Television and Radio Center by the State University of Iowa. (jet engine roars)


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