Airfoil
ciechanow.ski
ciechanow.ski
https://aviation.stackexchange.com/questions/20798/are-naca-...
>You'd think that with modern computer software it would be possible to design better airfoils, but apparently, those shapes have already been mathematically perfected by hand and by experiment.
No, modern computer software indeed does better, but there's not a whole lot of room to do better, small changes to bump performance a percentage point or two. These are optimizations which can be (and are sometimes) skipped for many commercial projects.
[1] http://airfoiltools.com [2] https://www.youtube.com/watch?v=FHYTBguMfWc
BTW, all lift based flying objects have an L/D ratio (which depends mainly on the airspeed), this includes birds, fighter jets, commercial airliners; and the discrepancies can be pretty interesting. For example if one looks at the L/D of the Concorde vs a subsonic jet it becomes clear why it was so damn expensive to operate. Or why the U-2 looks like a glider :). I cannot find any aerodynamic performance data on any famous long endurance (>24h) unmanned drone, but I bet it's rather high as well.
Another good example is the space shuttle. It does actually glide back down. But it glides like a brick at first (1:1 during its initial braking into the atmosphere), and then like a less dense brick (2:1 while it's still supersonic), and then like a brick with shitty wings (a whopping 4:1 or whatever on final approach). Which is about what the Concorde is during landing, 4:1, yea.
Pretty crazy stuff
(Obviously the space shuttle was a tradeoff for, you know, getting it into orbit via rocket)
A brick’s L/D is much worse than 1:1. I’m seeing people say 1:10 online, but I can’t find a source and I think that’s incredibly high. A real brick is going to tumble and essentially not make any lift.
A less dense brick will have the same L/D. L/D is about the shape, not the mass.
Neither lift nor drag has anything to do with mass. They are entirely determined by the surface of the object, and are not affected at all by the interior properties, including density.
Something a bit misleading done generally is aircraft don’t have one L/D, they have many, depending on angle of attack. When you see one number, it’s usually the best one.
I thought the point was that aerodynamics change from one domain to the next as shockwaves cause flow separation or eddies on or behind surfaces.
This is not the explanation you are looking for, but "aha the heavier object takes the same path but drops faster" was what made me okay with L/D not depending on weight.
I mean it looks like a brick and it flies
“Steerable brick in an atmosphere”… or the slightly more accurate “orientable brick in an atmospheric reentry regime”…
Kinda like a single control plane missile that spins (rolling airframe?), except... without the control plane lol.
Tumbling itself can produce lift. The difference in drag between one side and the other can result in net pressure differences for a moving object. This is the basis of many baseball pitches. Spin a brick fast enough and it might just be able to climb if thrown horizontally.
If static airfoils are complicated, try looking into airfoils that rotate or otherwise move in relation to airflows. A Russian engineer once said that all problems in aerospace are placed on the tip of every helicopter blade.
Curiously, the rotation can also lend the ball's path greater stability against changing air currents/densities and crosswinds. Knuckleballs are famously hard to throw because they have very little spin, but they are also notoriously hard to hit because the trajectory is so subject to the vagaries of airflow between pitcher and batter.
This is to say nothing about when the axis of rotation is predominantly parallel to the direction of travel (e.g. rifle bullets and American footballs), where the Magnus effect effects the rotating objects ability to continue to rotate parallel to the direction of travel. Get it right and the spin makes the path more stable, but get it wrong, it becomes less stable. The hows and whys of that are beyond my understanding of fluid dynamics, but its fun to think about how complicated it can get.
edit: got my spin directions confused
NACA published empirically determined wind tunnel performance numbers for selected parameters, which was useful research but not a declaration of ‘these are the good values, you should only use these’.
It’s a bit like saying all satellites follow TLE orbits derived by NASA/NORAD in the 1950s - they do, but only because that’s just a standard way of writing down the orbital elements that describe a particular ellipse, not a catalog of ‘known good’ orbits.
Not to mention the content itself is great.
I'm taking notes.
:head-exploding-emoji:
Whenever I encounter a tricky subject I'm having a hard time learning, I start writing an article explaining it to someone else. It really forces me to confront the gaps of my knowledge because I can see so clearly that "Wait, I can't explain what happens between these two steps here. What am I missing?"
I guess I struggle with articles like this because it's already so intuitive as a mix of air and fluid dynamics. In fact, fixed airfoils are so boring when you see what a duck can do.
https://www.youtube.com/watch?v=-3CVZYY8xS4
So for all the fancy physics talk, this duck is literally just paddling air with his wings. The same physiology I use to stay afloat when treading water while swimming.
Where else could it possibly go?
GNU Terry Pratchett
https://en.wikipedia.org/wiki/Bombardier_beetle
(on a much smaller scale)
Once you have technology that enables flapping type motion, it's opening up the applicable physics to like 6 degrees of freedom versus zero in current wing technology (fixed = 0 degree of freedom); much more complex and interesting to study.
How else will we move toward ornithopter style wings, or vehicles that can hover via wing movement.
I guess “supersonic wing flapping” would have similar problems, but maybe there are more clever solutions than can be modeled, with so much degree of freedom?
And is “supersonic flapping” even possible?
Grossly speaking, sure. But I feel like this simplifies away a lot of the interesting bits. It's not as simple as, say, someone on a canoe paddling. Why is the duck's wing shaped just so, and not another way? Why does it move its wings just so instead of another way?
I'm reminded of an analysis of fruit fly wings, showing how they re-capture energy from the air when flapping[1]. Maybe the duck is doing similar; I don't know.
Of course, these animals make it look easy, thanks to millions of years of evolution (:
Also, in my experience there’s a huge difference between having an intuition for something and having an understanding of something to the point where you could model it.
https://en.wikipedia.org/wiki/Kline%E2%80%93Fogleman_airfoil
Very useful when making model airplanes out of foamboard.
Is this an airfoil that works for tailed aircraft but not tailless ones, perhaps?
Edit: I just skimmed the book on paper planes by KF and indeed they are using the variation with the step on the bottom for their paper planes.
I'm actually even more surprised now. How on Earth did they manage to patent the idea of reflex on a delta wing to give a tailless plane stability? This seems like the thing that (a) was known since early human-carrying gliders, and (b) implicitly discovered by anyone that folds a lot of paper airplanes. I will definitely read their book in more detail.
Every person who ever stuck a flat object outside the window of a moving car knows that you do not need a fancy shape to have lift.
And so many people are stuck thinking that the shape of the airfoil is responsible for the plane to be able to fly, supposedly because the air needs to run a longer way around the foil above the wing than below the wing. And this somehow causes pressure difference due to Bernoulli law and this is what keeps the plane up. Which is almost total BS because planes can obviously fly inverted.
Now I admit I only skimmed the article, and although the animations are beautiful, I am missing what really is key to understanding of what is happening.
I am looking for a bigger, far away view of the wing and showing what happens to the air BEHIND the wing.
Because how the plane really works is as it flies forward, it diverts large masses of air downwards. It pushes off of air.
Part of the air is diverted by the lower portion of the wing, but the much larger portion of lift is generated by larger masses of air above and behind the wing. Those can be thought as being sucked down behind the wing (if you look at it from the point of view of a stationary air mass, not from the point of view of the wing).
And the main role of the airfoil is to keep that mass of air behind the wing stuck to the airfoil at wide range of angles and speeds as possible, because a flat sheet is very poor at doing this.
But the same I found myself unable to pass by someone pushing "flat plane at an angle".
Fortunately my exams were open ended not "fill in the circle in answer sheet" so worst case I'd have written a more complete answer and fought it out.
Worrying about having to fight against "answer key" is part of why only one person (and only on a lark) took computer science on Matura exam in my class - which was CS-math-physics focused one
FWIW, aerospace engineering degree, used xFoil, did tons of fluid sims, etc.
For reference, actual "proper" discussion of lift in textbooks on aerodynamics have tendency to start with a sphere/cylinder.
EDIT: This is a good starting point for the frankly awesome material from NASA Glenn Research Centre: https://www.grc.nasa.gov/WWW/K-12/VirtualAero/BottleRocket/a...
Unfortunately it partly bitrotted due to using java applets for interactive demos, but I think most of it is still reachable - I'll try to find it later when I'm at the desk.
Personally I learnt from a 1980 book that was still part of mandatory reading for glider pilot course in Poland in 2005.
If the diagram shows lift but doesn't show the air being directed downward after leaving the tailing edge of the wing, I basically stop reading. That's the whole thing.
If you take a step back there is a simple way to think about this. In order for the object to stay up there, there needs to be equal and opposite force from some other body. What is that other body? It is the mass of air that is being directed in the opposite direction of the lift force acting on the plane.
This view also implies that most of the lift is happening on the very front edge of the wing which I doubt is accurate otherwise we would have very skinny wings.
It's super frustrating when wrong is very pretty.
But there's quite different flow and drag around it, which was used as opening for for adding rotation (which would add viscosity effects including lift from rotation) and other changed shapes in better way than starting with flat plane.
As the article says, you can have lift by just changing the inclination of a symmetrical airfoil, but an asymmetrical one can generate lift even without inclination (and with lower drag). The article also explains that acrobatic airplanes have symmetrical wing sections exactly because they need to be able to fly just as easily inverted.
Both of these sub-clauses are true, but the "which means" connecting them aren't. There's no law of physics saying a fluid that has a longer path ahead of it speeds up in anticipation.
As a child, I quickly outgrew the airfoil explanation when I realized this.
Both give exactly the same results and are convertible mathematically.
For wind tunnel work it was easier to measure pressures.
I'm with you I don't think the standard hand wavy explanation gives you the ability to attack the problem mathematically. So it's basically wrong.
The reason wings produce significant lift anyway is that they deflect air far beyond their surface. Air several metres away from the wing is also deflected downward, even though it doesn't actually hit the wing itself.
So yes, Newton's third law is involved, but in a "spooky action at a distance" form, where the wing somehow manages to deflect a bunch of air it doesn't even touch!
In a normally flying airplane, the wing compresses and pushes an amount of air under its wing. But there is actually even greater amount of air sucked down by the region of underpressure created above the wing and by the laminar flow directing it downward. Here, the drawing at the top of the page makes it clear: http://www.amasci.com/wing/airfoil.html
When you have a stall condition, what happens is that the air below the wing is still being compressed and directed downwards, but the air above the wing becomes turbulent and "unsticks" from the surface of the wing. Rather than being nicely directed downward, it just dissipates a lot of energy in turbulent motion that is not directed in any particular direction.
This turbulent air not only ceases to provide lift, it also prevents the air from below the wing to be directed downwards efficiently.
The main job of an airfoil isn't to create a pressure difference, it is to create conditions for the air to be laminar at as wide range of speeds and angles of attack as possible to make the plane nicely behaving and possible to takeoff and land. It is super critical for landing as you need to have higher the angle of attack the slower you fly and all planes essentially are driven as close to stall as possible during landing. Similar happens at high altitudes and high speeds, but for a bit different reason (read up on "coffin corner" if you are interested in that sort of thing).
Many (most?) planes cannot sustain inverted flight.
For instance, the engine no longer receiving oil at negative 1 g, or fuel, as the system is designed for gravity flow.
Stunt planes and airplanes capable of long inverted flight need special oiling and fuel systems to keep the engine from starving from either.
This article did provide a barn door model also, but it was quite far down.
The shape is mainly about efficiency and increasing the range of reasonable angles of attack, and then further nuances.
https://news.ycombinator.com/newsguidelines.html
> When disagreeing, please reply to the argument instead of calling names
I also publish articles (though nowhere near as good or ambitious as this one) online and the comments I look forward to most are the constructively critical ones. They are the reason I publish in the first place.
My only goal of giving and receiving constructive criticism is to improve our collective understanding of the world. There's nothing sinister or ill-natured about it as another commenter suggested.
(This extends to comments as well. I really appreciate you prompting me to check my tone.)
I have noticed this "it works or it doesn't work. Everything else is nuances." binary thinking among SWEs. It's odd.
But for explaining how lift appears, it is an irrelevant detail.
The purpose of modeling is not to mimick reality at high fidelity but to focus attention on the o parameters that matter for a specific situation. When you change the situation (going from explaining how lift happens to trying to fly) it is not surprising to have to switch to a different model.
<div class="slider_viscosity" id="fdm_hero_sl0">(...)He elaborates later on, but you're changing the Reynolds Number - a calculated value from the velocity, fluid density, viscosity and length. The cool thing about a Reynolds Number is that you get identical (in theory) airflow characteristics for two setups with the same Reynold's Number, even if e.g. the airspeed is different.
function draw_car(ctx, rot) {
ctx.save();
let sc = 0.04;
ctx.scale(sc, -sc);
ctx.lineWidth \*= 1 / sc;
ctx.translate(-286, -51);
ctx.beginPath();
ctx.moveTo(463.93652, 9.89137);
ctx.bezierCurveTo(462.12793, 6.72347, 461.22363, 3.5344, 461.22363, 0.32417);
ctx.bezierCurveTo(447.58911, -1.16177, 434.20691, 2.81333, 434.85754, 5.10777);
...
The 2d part though is probably generated.
It looks like maybe an SVG file converted into JS? Do you know if there is some standard tooling that generates this?
https://twitter.com/BCiechanowski/status/1522067904522428417
> The wings act like a scoop forcing air downward behind the wing
Only bottom side of the wing acts as a scoop, creating positive pressure. Upper side, in opposite, creates negative pressure which "sucks" the plane into it, creating additional lift.
It surprised me how much lift is coming from the negative pressure - about a half: https://aviation.stackexchange.com/a/16202
Notice that this isn't a separate effect from the effect of pressure - it's just a different way of seeing the same effect. The wing is accelerating the air both upwards and downwards, but because the pressure is higher below the wing than it is above it, more air is accelerated down than it is accelerated up - which lifts the airplane, but makes the air go down.
If you take the difference between the pressures above the wing and below the wing, you get a negative number.
A thing not existing absolutely can still exist relatively.
They (or their stackexchange source at least) are - like the referenced article and as is commonly done in aero engineering - subtracting out ambient pressure as a reference pressure, and then viewing pressure above the wing as ‘negative’ and pressure below as ‘positive’. It’s a convenient choice to make, for various reasons, but it is essentially an arbitrary one.
The problem comes when you then go on, like OP did, to come across statements like “how much lift is coming from the negative pressure - about a half”
Now, since in analyzing the pressure we have subtracted the reference pressure and made a zero point in between the low pressure value above the wing and the high pressure value below it, it actually shouldn’t surprise us at all that ‘about half’ of the lift seems to be attributed to the positive pressure below the wing, and half to the negative pressure above the wing.
This is just saying that half the lift on the wing is attributable to the first half of the pressure differential across the wing, and about half the lift attributable to the other half.
One of the problems of using a relative pressure and thinking about negative air pressure is that it gives the impression that negative air pressure, like positive air pressure, can grow arbitrarily large. It can’t. You can’t have a negative air pressure lower than negative ambient air pressure, because the absolute air pressure cannot go below zero.
But what you’re talking about is a relative pressure differential. We can have an arbitrarily large negative pressure differential because we can have an arbitrarily high pressure on one side of it.
> made a zero point between ... shouldn't surprise us
Whether or not you are surprised is immaterial, but it is not guaranteed a priori -- you could get a net upward force with ambient pressure above the wing and positive pressure below or with ambient pressure below the wing and negative pressure above (meaning gauge pressure, relative to the ambient pressure distant from the wing, to be clear). The person who started this thread seemed to be implying that the former was a good mental model, and the person you replied to was just saying that in fact for practical wing designs it is somewhere in between.
FWIW it is very common to talk about positive and negative gauge pressure. Some people may say that without understanding what is going on, but it is a mistake to assume that they don't understand just because they use that language.
Yet a 747 can produce 850000 pounds of lift with only 729000 square inches of wing? Feels like a very incomplete description at best
In order to use "scoop" approach for lift, you need to have either very low wing loading (think paper airplanes) or very high speeds (above transsonic range).
No, it wouldn't.
I think the article does a pretty good job building a more complete understanding than the simplistic "deflection" mental model.
If we could somehow "draw a box around" the entire plane+air system, then the plane's upward lift will create a corresponding downward force on the box, one way or another.
So, in the broad sense that you push the earth away from you when you jump, the plane also pushes the earth away from it when it flies (mediated by a bunch of fluid dynamics).
Or, classic example: if a (sealed) truck full of birds is jostled so that they start flying, does the truck weigh less? [1]
Certainly if we flew the plane very low over the ground, the air pretty directly pushes down on it, and the hypothetical scale would register something. Just look at the grass when a helicopter hovers over it.
As the aircraft flies further up, we'd need a bigger scale to capture the full area affected, and if it's moving there would be increasing lag between the location of the plane and the (large) area where the downward force hits the ground.
Or do you disagree with that? At what point does the scale stop working?
Obviously there would be practical limitations — that force is so spread out that it would be hard to measure. But let's not have practice get in the way of theory (:
Planes fly through gas, not solid particulate. Gas has intrinsic kinetic energy when energized. Diffusion plays a huge role in all this of course.
The airflow is split at the leading edge. The area of positive pressure is not entirely below or focused under the wing. The top and bottom of the airfoil are both involved in turning the air flow.
The pressure under the airfoil increases a bit, but the pressure above decreases by as much to much more depending(2-3x or more). This hypothetical scale is under the aircraft but much of the lift occurs by decreasing forces on the top surface.
Scales measure weight/mass. Barometers measure changes in atmospheric pressure. So it's not even the tool for the job even if the stone skip theory of lift was accurate.
Perhaps my mention of Newton's third law gave you the impression that I was advocating for that "stone skip" theory — I assure you I wasn't! Especially as presented on that page, it is obviously junk (:
But surely you agree, broadly, that if birds are flying inside a sealed box, the box still weighs the same amount as if they were standing, right? (modulo some fluctuations)
All of the pressure differentials and whatnot have the net effect that an upward force on the wing results in a downward force elsewhere. The purpose of the scale is to measure that force — like measuring the weight of the box with birds in it.
In the hovering helicopter example, wouldn't you agree that a (large) scale directly under the helicopter will measure a weight corresponding to the helicopter's lift force? Like if I blow directly onto a kitchen scale — it will measure some grams.
Edit: I feel we are kinda re-hashing the Bernoulli/Newton discussion also addressed at the nasa site: https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a... — 'So both "Bernoulli" and "Newton" are correct ...'
For reference, the correct Newtonian explanation (flow turning) is also covered: https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a...
Actually that is a bit of a lie, the airfoil shape only falls out due to a third implied force that needs to be accounted for. the wing needs to be strong enough to hold itself up. if you had infinitely strong materials the deflector shape that would fall out would be like a slightly bent piece of paper.
A clarification note on fluids: you are deflecting fluids, and everything this implies. just because I say newtonian deflection don't think I mean billiards balls, or if it has to be billiard balls think trillions of them simultaneously
Theory of Wing Sections by Abbott and von Doenhoff (1959)
https://www.amazon.com/Theory-Wing-Sections-Aeronautical-Eng...
https://www.youtube.com/watch?v=NBsvzMi9-f8
So yeah, fans are puzzling too.
I'm guessing the initial puff creates a high pressure area on top of the paper, rolling it downward and back. Them after the puff has pushed the air away, there is now a low pressure zone on top of the paper which lifts it up as the air below is rushing upwards around the sides of the paper.
This is only partially true, though; a totally flat wing can also support flight. The shaped nature of the wing contributes to its efficiency (and other factors) but do not make other wing shapes incapable of supporting flight.
The reality is that the Wright brothers' innovation was not the airfoil shape or even the lightweight motor. It was the control surfaces, to allow the operator to adjust the plane's attitude on the three axes of rotation, allowing actively stabilized flight.
Paper airplanes and kites demonstrate all the same principles of heavier-than-air flight (the Wright brothers even had a kite version of their airframe they used for testing), despite the fact that they generally do not exhibit shaped airfoils.
Without those controls, flight is basically impossible, and with them, you could use nearly any airfoil shape (modulo engine power, drag, and stall speed considerations) and achieve heavier-than-air flight.
For example, yes, the air above the wing moves faster than the air below the wing, and it's related to shape of the airfoil.
However, it has nothing to do with magical "air has longer to travel".
It starts with how combining flows at the trailing edge of the airflow create a vortex which induces an opposite vortex around the wing, which is a bit counter-intuitive (but it has nothing on why swept wings work, which can be summarised for practical aircraft design purposes of "because if we calculate at an angle we get better values and reality is crying in the corner")
Wait, I was under the impression this Cutta circulation was a computational simplification and the "real" reason were the pressure differences as explained in this submission. What am I missing?
One comment already mentioned how position of flaps could have visible effect on pressure sensors in front of the plane, and this is slightly mentioned in how the pressure created by front of the air foil has an impact on air "at a distance" from the airfoil.
The vortices created around the airfoil result in significant change of flows, which especially at low speeds provides big chunk of the pressure changes necessary for the creation of lift, with the effect IIRC getting lower as you go faster, with transsonic regime breaking it - because that's when the resulting speeds go beyond speed of sound at given pressure in the air, which in very simplified way means that air can't move towards front of aircraft anymore in those areas, breaking all sorts of flows you depend on at lower speeds.
We still don't have a very good understanding of turbulence.
Navier Stokes equations still make Aerospace engineers drink.
Yes its stupid simple if you care about the simplest of analogies, but if you try to understand it, there are reasons why 80% of people drop out.
It's the size of the aerodynamic forces and the complexity of the physical mechanisms that create them that many people have trouble with. In particular: intuitions can be pretty wrong, most simplified explanations are wrong under simple experiments, and the problems exhibit scale variance that is unfamiliar (e.g. Reynolds number).
One time I was working on air data computer for a transonic aircraft that could fly up to about M0.95 - during flight test, an air data probe mounted on a nose boom was used to supply impact and static pressures, angle of attack and sideslip etc. for various air data calculations like airspeed and altitude.
I was fascinated that there was a term in the calculation that related to the aircraft flap position - what's happening way out on the trailing edge of the wing actually has a meaningful effect of pressures measured on a boom out the front of the nose during certain regimes of flight.
I mean, actually, it isn't - that's the whole point about scale variance and Reynolds number and why wings that work for insects are not the wings that work for jumbo jets.
A tail wind is just saying that the air is moving in a certain direction with respect to the ground (the same direction the plane is flying). The plane doesn't give a shit about that.
Fortunately that kind of instantaneous change doesn't happen in real life.
Jet stream boundary is usually not this sharp, and the airplane would fly much faster than the difference anyway.
You can feel a much stronger pressure in the sail when moving towards the wind on a fast windsurfer/windfoil as you can do 15-20kts 45deg towards the wind, giving you an apparent wind that is 10-14kts stronger than the true wind.
On the same craft, going away/downwind, you will feel the apparent wind at a similar angle 10-14kts less. In fact, because of the change in drag and forces, you'll probably be going faster and feel even less wind on the downwind leg.
When you turn, this can be a big benefit for going downwind (jibing) as at some point the sail feels zero apparent wind (your motion cancelling out the true wind), feels very light in your hand, and easy to rotate to face the other way. Even knowing the physics of it, the timing and execution is still something that takes a lot of practice...especially on big race gear with a 9.0m2 sail.
This stops being true for quick changes - because there's still the inertia of the aircraft. So if the wind speed changes quickly, the aircraft can't immediately move along with it.
This is why gusts are so dangerous to landing aircraft. A strong gust from behind can cost you all your lift, and a strong gust from the front can temporarily stall your wing.
The only connection a plane has to the universe is the air around it. It simply does not know or care what the ground is doing until the ground is quite close.
A small plane in a very high wind is perfectly happy having a "backward" ground track.
Same thing as if you were trying to swim upstream in a fast river. How fast you move through the water doesn't have anything to do with how fast the water is moving across the land.
Lift only comes from the interaction of the air and the wing, so if there's zero relative motion then you will fall out of the sky, regardless of if you have a 200 knot groundspeed.
This also means that, if the wind at altitude is above your plane's stall speed, you can hover in place by flying straight into the wind! (example here: https://www.youtube.com/watch?v=n_e6ijREScE)
Similarly, if you are in a packet of air that is moving at 200 knots, the fact that you are moving at 500 knots indicated airspeed does not mean you are flying supersonic from an aerodynamic perspective, despite having a groundspeed of 700 knots.
How does viscosity work?
I had generally previously thought of viscosity as "how slow" a fluid is. High viscosity means high "thickness," which means it flows slowly (like molasses vs. water).
But as presented on this page, viscosity is actually a measure of "how fast" — how fast the effects on one molecule can spread out from there to neighboring molecules. Perhaps you could think of sounds waves moving through a substance — a "thick" substance like solid metal propagates those waves quickly (on a molecular level), while with a "thin" substance like air it's much slower. In the more precise language from the article: "viscosity controls the diffusion of momentum..."
So, because this diffusion happens quickly in a high-viscosity situation, little whorls of turbulence are inhibited, because the forces governing those whorls get spread out/diffused quickly.
Perhaps you missed the part of the article talking about diffusion, or did not see the connection? The link between that and viscosity was not immediately apparent to me, either.
Though I don't think I missed a part of the article, I feel more like the author did ;)
What I still don't get is what the difference between high and low viscosity looks like on a particle level. I don't understand why he introduced the collision between two molecules and then never explained that.. :)
You are saying (force / area) / (1 / time). I add two distances that cancel out: (distance * force / area) / (distance * 1 / time) and get (energy / area) / speed, which is energy used per area and speed. I can feel that, and it seems to be what you are saying, right?
https://webglreport.com/ , or check about:support if you're on firefox.
The fan still roars.
Is it just me who sees diagonal lines on the first interactive graphic? Are they a bug? They appear after a little while, fade out, then reappear
https://gist.github.com/RobinL/aa4a5e14d35e61e46c1e99c8198d0...
Most automobiles are pretty heavy, so the engine has to do significant work just to get it to move. At a certain point, the vehicle can change gears to get the engine to do less work and use less fuel. But around the same time, the force of the air is increasing. By the time an automobile goes over about 50mph, the air forces are getting increasingly strong, and the engine has to work harder to keep the vehicle moving. At this point, beginning to lower the air's coefficient of drag on the vehicle will lessen the work the engine needs to do to keep the car moving at speed. So you can optimize the design of the vehicle's exterior to reduce the drag coefficient, which will reduce things like flow separation and turbulence, creating fewer rear pressure zones and causing less drag.
So you might wonder, why aren't more cars teardrop-shaped like the airfoil? The answer is, it depends. Most people want something that looks good more than they want efficient operation at speed. But sometimes having more drag actually helps. For example, the Lotus Elise: while it is smaller and looks more sleek than a Tesla Model 3, it actually has a much worse drag coefficient than a Tesla Model 3. The Lotus has way more force acting against it at speed than the Tesla. But the Lotus is a sports car, and sports cars benefit greatly from increased traction, and you can get more of that traction by increasing the downforce on the car. So the Lotus's design sacrifices top-speed drag coefficiency in order to gain some downforce which helps traction when cornering at speed.
What about pickup trucks? Even though modern pickups actually have lots of subtle design changes to improve drag coefficient, they all tend to have open beds, which is terrible for drag. It creates this giant messy turbulent pressure area in the bed which drags on the tailgate and the rest of the car. By adding a truck topper, the drag is significantly reduced, but you don't see most trucks driving around with a topper on. But trucks naturally have worse gas mileage, so nobody really thinks twice about the aerodynamics.
(To be fair, the air's impact on gas mileage is minimal unless you're going quite fast. But for trucks with extremely bad gas mileage, like 18-wheelers, it makes much more difference. That's why they often have airfoils on the front of the truck, gaps between cab and container closed, and skirts to reduce drag from the undercarriage. Strangely though, the biggest improvement to reduce drag coefficient actually comes from modern European big-rigs whose containers are actually tapered like a teardrop. The rear of the vehicle's shape makes the most difference to how severe flow separation is, and thus how big of a pressure area develops, pulling on the rear of the vehicle. If we wanted to make trucking more fuel efficient globally we'd change the shape of the containers to be more like teardrops, but that would make handling and shipping them much more awkward)
You'll usually only see these effects on automobiles at higher speeds, due to the vehicle needing to overcome gravity before the air forces build up. Lighter vehicles (say, bicycles) with less impact on them from gravity will be impacted earlier (at lower speeds) by the force of the air, so optimizing drag coefficient is much more important, which is why bicycle racers have to put so much into aerodynamics at significantly lower speeds than an automobile. Interestingly, the drag coefficient on a bicycle and rider is actually equivalent to that of a small car.
I'd love to spend my time working on such articles when I'm retired :)
* still the classroom is what it looked like in 1878
* still a lot of learning is about remembering useless facts
* still there are many places where the curriculum is a static thing which does not reflect the needs of youngsters but their grannies
* still the exams are solo even though later in life one works in a group and it was shown that in a group one manifesta better
* many places still don’t teach topics such as personal finance, digital hygiene, business management, soft skills, etc as universal mandatory classes even though the world we live in requires these skills on a daily basis
Neil Postman makes in depth analysis in his books, and particularly The End of Education.
I can also go for ages with examples from my practice and what I see, and you can downvote me as much as U like. It doesn’t chang the fact that organised schooling is a byproduct of Industrial Revolution and it was designed to cater to its needs. Not the needs of the ages we live in… like so many things tbh which absolutely do not make sense to a teenager at this day.
The downside of WebGL is its complexity, but there are many libraries to help with that.
Edit: interestingly, it seems to only be the first animation. If I scroll it out of view the others all seem to render fine.
I know what I'll be spending a stupid amount of time reading today :)
The mechanical watch post is 6th on the list
Imagine how much more you'd know, be able to do and understand.
I really wish good education was valued more highly in society.
Anything that fits the above as was stated :)
I wonder will there be articles in the future with more math and code snippets?
Bartosz's dedication and craftsmanship is really inspiring.
Bernoulli and Coanda are important but without thrust/velocity there is no lift
Where the pilot said what is probably the most British thing ever said in history:
> Ladies and gentlemen, this is your captain speaking. We have a small problem.
> All four engines have stopped. We are doing our damnedest to get them going
> again. I trust you are not in too much distress.
The key difference is that a rock (like any ballistic projectile) accelerates until terminal velocity. In contrast, a 747 (like any airplane) descends with a constant vertical velocity when they lose power.