Bernouli, Coandă & Lift
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2/25/2015 +: Bernouli, Coandă & Lift http://www.terrycolon.com/1features/ber.html 1/9 E What’s What and What's Doing What ver see someone demonstrate lift by blowing over a piece of paper? While this is fun and interesting, the usual explanation given is misleading by claiming it's an effect of the Bernoulli principle — the correlation between air speed and pressure, as speed increases pressure decreases. Many folks were taught this is how planes fly, and they pass this "wisdom" along. But they're fooling themselves, and you. Aerodynamic engineers have know the correct explanation of lift for decades, yet the misconception of Bernoulli principle lift lives on in the public imagination. However, it is not how an airfoil generates lift. That's a whole different kettle of fish. Read either How Planes Can Fly for a breif overview or See How it Flies to get the complete skinny on lift. By the way, the illustrations here are not true diagrams, but visual aids. Fluid behavior is a complicated business which can seem like organised chaos, what with tumbling and swirling, i.e., turbulence. We're not going to get into these details, but concentrate on simplified basics. The Bernoulli Principle What does this correlation between speed and pressure mean? Here's a way to imagine it. If you've ever watched a road race you know the cars slow down and bunch up in tight turns. Speeding up out of the turn they get stretched out. In the picture, a race official with a stopwatch at the turn times the cars as they pass, they are one second apart, though close together. A second official times them on the straight away, where they are also one second apart, though with several carlengths between them.
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Bernouli, Coandă & Lift
Transcript of Bernouli, Coandă & Lift
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EWhat’s What and What's Doing What
ver see someone demonstrate lift by blowing over a pieceof paper? While this is fun and interesting, the usualexplanation given is misleading by claiming it's an effect
of the Bernoulli principle — the correlation between air speedand pressure, as speed increases pressure decreases. Manyfolks were taught this is how planes fly, and they pass this"wisdom" along. But they're fooling them selves, and you.
Aerodynamic engineers have know the correct explanationof lift for decades, yet the misconception of Bernoulli principlelift lives on in the public imagination. However, it is not how anairfoil generates lift. That's a whole different kettle of fish.Read either How Planes Can Fly for a breif overview or SeeHow it Flies to get the complete skinny on lift.
By the way, the illustrations here are not true diagrams, butvisual aids. Fluid behavior is a complicated business whichcan seem like organised chaos, what with tumbling andswirling, i.e., turbulence. We're not going to get into thesedetails, but concentrate on simplified basics.
The Bernoulli PrincipleWhat does this correlation between speed and pressuremean? Here's a way to imagine it. If you've ever watched aroad race you know the cars slow down and bunch up in tightturns. Speeding up out of the turn they get stretched out. Inthe picture, a race official with a stopwatch at the turn timesthe cars as they pass, they are one second apart, thoughclose together. A second official times them on the straight away, where they are also one second apart, though withseveral carlengths between them.
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The same number of cars will pass each official in thesame amount of time, but the faster they go the more spacethere is between them. This is what the Bernoulli principle isabout, only with liquids and gasses, not cars. The faster airflows the more it gets stretched out so there's less air in thesame volume, it's less dense.
As speed increases pressure decreases, and viceversa.Higher speed means more force in the direction of flow. Imean, you'll take a bigger hit from a fast car on the straight away than a slow car in the turn. This higher speed has aneffect, too. After all, trees get blown over in a wind storm. Isthat because of a change of pressure or the force of theairflow? Let's return to wings and airflow and such.
Paper Lift: Let's have a look at a the "lift" demonstrationmentioned in the opening and see what it's about. Take apiece of stiff paper and curve it like a wing. Hold it up underyour mouth and blow on it. It lifts! (This works similarly with aflimsy sheet of paper because gravity pulls down the far endcreating a curve.)
The common explanation is the fast airstream over top haslower pressure (the Bernoulli principle) which sucks the paperup. Problem is, this paper lift really demonstrates the Coandăeffect rather than the Bernoulli principle.
The Coandă EffectYou can easily demonstrate the Coandă effect for yourselfwith a tablespoon and faucet. Conveniently, these are often
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found together in the kitchen, no need for highly technical labequipment. Follow the steps you see below. Gases behavepretty much like liquids, so when you see the water behaving
strangely with the spoon, that's what the air does with thecurved paper.
Dangle the spoon as shown next to the stream comingfrom the tap. I say dangle because you want to hold it looselyenough so it can swing back and forth a bit. (It helps to attacha piece of tape at the handle end to act as a hinge.) Move thespoon up to the edge of the stream so it barely touches.When you do the water will flow around the bowl of the spoonand off the bottom deflected to the side and the spoon willmove into the stream.
Just as water flowing around the spoon's curved surfacedraws it into the stream, air blown over the curved paper iswhat causes the lift in that common paper lift demonstration.Imagine the stream of water turned 90 degrees and you cansee we have the same situation as in the paper lift, only withwater you can see instead of air which you can't.
Notice how unlike a wing in flight it is, flow on one sideonly. The Coandă effect only works in specific conditionswhere an isolated jet of fluid (or air) flows across a surface, asituation which is usually manmade. You don't find it much innature and doesn't happen over a wing, either. Just so youknow, there is no Coandă lift on an airfoil.
The Coandă effect is a ballancing act between manyfactors, among them speed, pressure, molecular attraction,and a centrifugal effect if the surface is curved. How exactly itworks is unimportant to understand the demonstrations we'llexamine. But do the spoon test and you'll see that it works. Ican't explain gravity either, but we both know it's real enough.
This effect is named for Romanian air pioneer Henri
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This effect is named for Romanian air pioneer HenriCoandă, an interesting character in his own right. Read abouthis jet plane built in 1910. If you are still curious how theCoanda effect works you might read this explanation.
More misleading "lift" explanationsClashing Balloons: Hang two balloons from strings an inchor so apart. Blow a stream of air between them and theymove together. Now, I show a wind sprite in the picture, but ifone is not around do it yourself.
Railroad Bottles: Set a pair of empty soda bottles about aninch or so apart on a counter or table. Blow a stream of airbetween them and they move together.
The explanation usually given for both phenomena is byspeeding up the air between the balloons and the bottles youreduce the pressure in the airstream as per the Bernoulliprinciple. The lower pressure pulls the objects into the streamso they come together. Unfortunately, that's not quite the wayit works. Notice you're blowing a steam across a curvedsurface. If you suspect the Coandă effect is involved, yoususpect correctly.
The airflow across the surface of the round objects in bothcases give you a Coandă effect pulling them together. Onceagain, just as with the paper lift, the Bernoulli effect alone isnot doing the trick as advertised. Again, this does notdemonstrate how a wing generates lift either.
Cone and Ball Levitation: Put a light ball in an upside downfunnel or cone and blow through the small end, or otherwiseget some fast airflow through it. The ball will not be blown out,
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get some fast airflow through it. The ball will not be blown out,but will levitate in the cone as shown.
The explanation sometimes given for this phenomenon isthe Bernoulli principle creates a low pressure zone in thecone so the ball doesn't fall out. But it's not so simple as that.In this instance we've added a force to reckon with, the stiffcone barrier around the flow.
One thing you need to know before we proceed. When youblow a stream of air, like out of a hair dryer, you don't get adistinct "tube" of air punched through the surrounding air.Fluid behavior is rarely so simple. Instead, you get somethingmore along the lines of a fast river constantly eroding a sandyriverbed. It carries what it's rubbing against along with it.
Something similar happens with airflow, the ambient airbesides the flow gets swept along where they intermingle,which is entrainment of fluid. This creates a sortof cone of air,fast in the center, fanning out and gradually slower along side.Air not only flows in the hair dryer's inlet and out through theblower tube, but also is drawn in from surrounding areas andswept along.
Returning to the cone business, as the airstream exits thetube into the small end of the cone you get the same entrain ment of air as explained above. New air streams toward thespace where the air is being blown out forming a ring vortexshaped like a donut around the fast airstream in the center.The ball levitates in this vortex as if it were flying rather thanbeing held up by the Bernoulli principle.
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One important aspect of this effect is a ball is the onlyshape that works. You have to have a ball so the airflowaround it is smooth so not to disrupt the vortex which is what'sdoing the trick.
One Last Demonstration With a Bit of EverythingBall Levitation: Take a hair dryer blowing straight up andintroduce a pingpong ball into the airstream where it will float.Tilt the blow dryer up to about 45 degrees, the ball doesn't fallout but "levitates" in the airstream. (I've also seen it work witha vollyball and a strong leaf blower.)
The commonly given explanation for this phenomenon isthe Bernoulli principle creates a low pressure zone in the fastflowing airstream compared to the air along side so the balldoesn't fall out. However, it's not that simple. What we havehere is what's referred to as parachuting. How a parachuteworks seems obvious and simple, but it's not.
Air fills the inside of a parachute in effect turning it into ahalf ball of air with a fabric skin on top. The falling parachute"scoops" out an area of low pressure over top. (You might liketo know a parachute doesn't fill out from increased pressurebelow, but because of decreased pressure above.) As withanything moving through a gas, a vortex forms and the airfrom below races up along the curved fabric towards thelower pressure over top creating Coandă and Bernoulli effectlift.
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The air "rolls" along the fabric in a ring vortex not unlikethe one in the cone from above. These cannot last forever (forreasons too boring to get into) and are periodically shed inwhat's called periodic vortex shedding aptly enough. To keepit simple let's just say a parachute not only has some airresistance underneath, it has lift over the top, which isparachuting.
Not being only a half ball but a whole one, a pingpong ballin an upward blowing stream is similar, but different. (As YogiBerra's son once said, "Our similarities are different".)Because of a ball's aerodynamic shape, being round on thebottom in any position, there's less air resistance underneaththan with the half ball parachute. Compare the differencebetween a ball and a wad of paper. The ball hovers about 3inches from the blower end, the paper wad is blown about twofeet up and out. The wad displays no tendency to stay in thestream at all, a good hint pressure differences alone aren't thefull story.
The paper wad gets more upward push from the airstreambecause the irregular shape inhibits flow and traps more airunderneath. On a ball the air slips around the sides and so
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there's less upward push and equal flow and pressure allaround keeping it centered in the stream. In fact, a ball isabout the only shape that will float this way in a vertical
airstream.
When you tilt the stream and get the dramatic "levitation"effect, the ball doesn't stay in the center of the stream, ithangs down with the upper part in the faster center (A) andthe bottom in the slower section of entrained air (B). The ballfalls to a spot with different airspeeds and pressure above andbelow where it has enough parachuting lift to stay up.
After you tilt the stream past a certain angle you reach "thetipping point" and the ball falls out of the airstream. If you'veever done any house framing you'll understand the tippingpoint concept. Once you get past 45 degrees upright it getseasier and easier to tip up your wall as more of the weight istransferred down vertically through the wall rather than onyou. One way to think of it, a vertical slab is a wall and ahorizontal one is a roof. At 45 degrees it's half wall and halfroof, that's the tipping point.
Something like this happens to the levitating ball. Themore you tilt the more the flow pushes horizontally thanvertically, so you're relying on the lift above more and moreuntil you reach the tipping point. The ball falls out becausegravity overcomes the decreasing upward push. (I told you itwasn't all that simple.)
Only a ball shape works for this demonstration. A wad ofpaper or puff of cotton of the same size an weight simply don'twork. That's because it's not just pressure differences, a ballcreates parachuting lift and flies.
Summing UpWhat's sometimes omitted in the misleading explanations isthe confounding forces besides a pressure reduction as perthe Bernoulli principle. There is also an increase in dynamicpressure. Plus, every bit of aparatus like a funnel, a curvedshape, or something redirecting the flow will have an impact.You need to look at the whole picture.
You can always count on increased air speed to reducepressure as described by the Bernoulli principle. However,once it encounters a new force, like a wall or you sitting therebeing cooled off, additional principles also apply. How a fancools you is another can of worms we won't go into. But I willsay this, it has nothing to do with the Bernoulli principle or the
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say this, it has nothing to do with the Bernoulli principle or theCoandă effect. At least, I'm pretty sure it doesn't. I'll look into itlater.
© Terry Colon, 2007
Special thanks to Terry Day whose invaluable assistance, andpatience, helped me understand it. Mr. Day is a fluiddynamics engineer at New Fluid Technology
How Planes Can Fly Forget Bernoulli and Coandă, ThinkLanchesterPrandtl
A detailed, comprehensive explaination of lift:See How it Flies
