Coanda effect

[Realms09]

This afternoon I took the time to sift through the indices of about 20 fluid mechanics and aerodynamics books in the engineering library searching for the keyword "Coanda effect." Of those, only one had "Coanda effect" listed. It was a brief mention in a paragraph regarding using blowers for boundary layer control.

I'm not sure where one can actually learn about this effect. It seems increasingly clear it has little to do with fluid mechanics problems of engineering interest, and I am baffled how it entered the pilot world at all.

 

[tgrayson]

brief mention in a paragraph regarding using blowers for boundary layer control.

That's about all I could find as well.

I am baffled how it entered the pilot world at all.

The "Understanding Flight" people are the most renown proponents of it, but they have their apostles.

Thanks for investigating.

 

[TonyC]

I'm not sure where one can actually learn about this effect. It seems increasingly clear it has little to do with fluid mechanics problems of engineering interest, and I am baffled how it entered the pilot world at all.

Why not try Google? I just got aout 106,000 hits. Even Wikipedia has an article about it, and you can quickly discern why it's important to aviation.

The Coanda effect has important applications in various high-lift devices on aircraft, where air moving over the wing can be "bent down" towards the ground using flaps and a jet blowing over a curved surface. The flow from high speed jet produces enhanced lift through turbulent mixing that does not occur above a normal wing. It was first implemented in a practical sense during the U.S. Air Force's AMST project. Several aircraft, notably the Boeing YC-14 (the first modern type to exploit the effect), have been built to take advantage of this effect, by mounting turbofans on the top of wing to provide high-speed air even at low flying speeds, but to date only one aircraft has gone into production using this system to a major degree, the Antonov An-72 'Coaler'. The McDonnell Douglas YC-15 and its successor, the Boeing C-17 Globemaster III, also employ the effect, though to a less substantial degree.

The internet is rich with information on the subject.




OH, and as far as engineering interest ... I would imagine it's important to those who design pumps, hydroelectric generators and dams, and waterways.
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[tgrayson]

The internet is rich with information on the subject.

And a lot of misinformation, too. How do you tell the difference?

For reliable information, I much prefer a textbook, where there is some degree of peer review. Not just any nut can produce a textbook.

Many of the hits for "Coanda Effect" have to do with wing lift, and is probably due to the influence of the "Understanding Flight" authors.

 

[TonyC]

And a lot of misinformation, too. How do you tell the difference?

For reliable information, I much prefer a textbook, where there is some degree of peer review. Not just any nut can produce a textbook.

Many of the hits for "Coanda Effect" have to do with wing lift, and is probably due to the influence of the "Understanding Flight" authors.

Excellent point, and the same can be said for any subject on the internet. One has to be critical and discerning.


I don't object to using textbooks, but they aren't perfect, either. At best, they represent the best of man's knowledge a year ago. I have several reputable texts with outdated, and inaccurate information. The same critical, discerning filter must be used on what's found there.




.

 

[tgrayson]

At best, they represent the best of man's knowledge a year ago. I have several reputable texts with outdated, and inaccurate information. The same critical, discerning filter must be used on what's found there.

Yes, they trail technology, but, honestly, in aerodynamics at this level, not much has changed in 50 years. Some of my best texts date from the 60's. The very basic principles were identified almost 100 years ago. Reading the old NACA papers is still very educational.

In fact, I bet in most fields, the basic principles haven't changed much in many years. If you go skimming the internet for the most current literature without ever mastering the basics, I'm skeptical that you will learn very much, except for a lot of unconnected trivia.

 

[B767Driver]

TG,

I really think the Coanda effect is described in "Flightwise" when the author talks about the development of the trailing edge and aft taper of the airfoil. While he doesn't use the term, Coanda, he does describe exactly what happens to the streamlines...and it's basically the Coanda effect.

The author states that "circulation" is induced over the airfoil....and a force mV^2/r is produced that accelerates the air rearward. And this force is what causes the airflow to "hug" the curved surface and it flows. The force required to make the turn at the trailing edge is too great....and the flow separates.

This is what I inferred in another post....when stating that airfoil shape will "prevent" separation from an adverse pressure gradient.

I think I said it would prevent an adverse pressure gradient...which you called me on. A better wording would be mitigating the effects of the APG.

 

[B767Driver]

Of those, only one had "Coanda effect" listed. It was a brief mention in a paragraph regarding using blowers for boundary layer control.

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Realms,

The mention of using blowers for boundary layer control affirms by post above. The Coanda effect, really, seems jargon for the circulatory effects designed into a curved surface.

 

[tgrayson]

I really think the Coanda effect is described in "Flightwise" when the author talks about the development of the trailing edge and aft taper of the airfoil. While he doesn't use the term, Coanda, he does describe exactly what happens to the streamlines...and it's basically the Coanda effect.

I do not agree at all. I do believe there are similarities in the behavior of the fluids, but that's all. This is the reason the concept is an attractive nuisance.

The air turns due to pressure gradients; the Coanda turns due to the viscosity of the fluid (as I understand it.) The only critical role viscosity plays in the production of lift is to ensure that the air separates at the trailing edge. The resulting vortex rotates in one direction, causing the air to rotate around the airfoil in the equal and opposite direction. This rotation increases to the point where the airflow leaves the trailing edge cleanly, generating no vortex. (The Kutta Condition.)

The Coanda effect doesn't require an airfoil shape; one of the most used demonstrations is the way water hugs the back of the spoon. The pictures of boundary control blowers that I've seen are placed after the point of minimum pressure and merely blown along the curved surface.

 

[B767Driver]

If you have Carpenter's "Flightwise", read Chapter 7 from pg 161 to the end of the chapter.

He basically states that circulation is necessary for the production of lift...and that circulation would not be possible without viscosity.

It seems to me, he's stating the Magnus Effect provides the force to move the airflow towards the trailing edge....and since it has viscosity it will adhere and move toward the trailing edge where it separates.

Could this be what you mean...when you say that viscosity causes the airflow to separate at the trailing edge?

So the force, Magnus Effect, provided by the circulatory flow...is moving the air towards the trailing edge. It seems similar to the Coanda Effect.

 

[B767Driver]

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The air turns due to pressure gradients; the Coanda turns due to the viscosity of the fluid (as I understand it.)
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That's my point. The circulator effects of the aft section of the airfoil (made possible by the viscosity of the airflow) is providing a force normal to the direction of rotation. This Magnus Effect force ...is accelerating the air rearward (not providing an upward force)...that causes the airflow to fight the adverse pressure gradient and make it all the way to the trailing edge.

This is what seems similar to the flow found with the Coanda Eff.

 

[B767Driver]

The Coanda effect doesn't require an airfoil shape; one of the most used demonstrations is the way water hugs the back of the spoon. The pictures of boundary control blowers that I've seen are placed after the point of minimum pressure and merely blown along the curved surface.

This still seems consistent with my understanding of the Magnus Effect. The camber introduced into the spoon...with a sharp trailing surface...seems like it would introduce circulatory flow....much like the aft section of an airfoil would. And this is where the circulatory effects would be most required...after the point of minimum pressure.

 

[tgrayson]

a force normal to the direction of rotation. This Magnus Effect force This is what seems similar to the flow found with the Coanda Eff.

Except that the aspect of viscosity that's important to the separation at the trailing edge is a force parallel to the direction of flow, not normal.

The Coanda effect is a force normal to the flow, as you say.

<<The camber introduced into the spoon...with a sharp trailing surface...seems like it would introduce circulatory flow...>>

The flow deflection happens regardless of the type of trailing edge. A sphere would produce the same effect, and the water would drip off the bottom, not circulate. Try it with an apple under your kitchen faucet.

 

[Realms09]

The internet is rich with information on the subject.

I prefer a college library. Most of the stuff on the internet seems cursory.

If I am right in saying that this effect is mostly irrelevant, I would love to know, on a physical level, why that is. Similarly, if I am completely wrong in saying this, I would also love to know why, on a physical level. What I am after, and what has remained elusive, is a physically well-motivated analysis of what the Coanda effect is. From that I can reason its importance and formulate a good idea of when it fits into the picture and for what reason. As far as I've looked, that explanation isn't out there.

 

[tgrayson]

As far as I've looked, that explanation isn't out there.

The only thing I found that seems plausable is that when you have a fluid flow, the bottom layer aheres to the surface and swings the fluid element above it as if it were on a string towards the adhering surface. This produces a force perpendicular to the direction of flow.

 

[av8rmsu]

i read this post and had to dig my old CFI manual out and find the link to this article...

It is a good article that is well explained. I'm sure you'll enjoy the read.

http://www.allstar.fiu.edu/AERO/airflylvl3.htm

Here is the coanda part of it...

Air has viscosity

The natural question is "how does the wing divert the air down?" When a moving fluid, such as air or water, comes into contact with a curved surface it will try to follow that surface. To demonstrate this effect, hold a water glass horizontally under a faucet such that a small stream of water just touches the side of the glass. Instead of flowing straight down, the presence of the glass causes the water to wrap around the glass as is shown in figure 8. This tendency of fluids to follow a curved surface is known as the Coanda effect. From Newton’s first law we know that for the fluid to bend there must be a force acting on it. From Newton’s third law we know that the fluid must put an equal and opposite force on the object which caused the fluid to bend.

Why should a fluid follow a curved surface? The answer is viscosity; the resistance to flow which also gives the air a kind of "stickiness". Viscosity in air is very small but it is enough for the air molecules to want to stick to the surface. At the surface the relative velocity between the surface and the nearest air molecules is exactly zero. (That is why one cannot hose the dust off of a car and why there is dust on the backside of the fans in a wind tunnel.) Just above the surface the fluid has some small velocity. The farther one goes from the surface the faster the fluid is moving until the external velocity is reached (note that this occurs in less than an inch). Because the fluid near the surface has a change in velocity, the fluid flow is bent towards the surface. Unless the bend is too tight, the fluid will follow the surface. This volume of air around the wing that appears to be partially stuck to the wing is called the "boundary layer".

 

[Realms09]

I think I'll try to find a big bad book of boundary layers sometime and report back.

 

[B767Driver]

Except that the aspect of viscosity that's important to the separation at the trailing edge is a force parallel to the direction of flow, not normal.

The Coanda effect is a force normal to the flow, as you say.

<<The camber introduced into the spoon...with a sharp trailing surface...seems like it would introduce circulatory flow...>>

The flow deflection happens regardless of the type of trailing edge. A sphere would produce the same effect, and the water would drip off the bottom, not circulate. Try it with an apple under your kitchen faucet.

If the air is spinning...or has the characteristic of spinning...in circulatory flow...the normal force would be parallel to the direction of flow, would it not? Let's say the air is spinning from top to bottom. The force induced would be from left to right. It's my understanding this is the basis of introducing circulation into the flow over the airfoil...is to take advantage of the viscosity of the air to advance it rearward over the airfoil to fight against the adverse pressure gradient.

If the airfoil did not have a sharp trailing edge...the flow would make the turn and follow the bottom surface just like an apple. That's the reason for the tight trailing edge...to ensure the flow separates.

 

[tgrayson]

If the air is spinning...or has the characteristic of spinning...in circulatory flow...the normal force would be parallel to the direction of flow

I started to say that having a normal force be parallel is like saying left = right , but I think what you're saying is that the normal force to the air flow is perpendicular to the airfoil, yes?

<<the flow would make the turn and follow the bottom surface just like an apple. >>

Let's make sure we're on the same page as to what Carpenter is describing....the flow does not separate on its progress from top to bottom, but instead separates in its journey from bottom to top.

I don't have the book with me, but he shows where the rear stagnation point would be in the absence of viscosity....it's on the top of the airfol. For that to be a stagnation point, the bottom flow must negotiate the sharp trailing edge, which it can't do. So it's the bottom airflow that separates, inducing the circulatory flow around the airfoil. Once circulation is established, there is no more separation at the trailing edge, and you have the steady circulation around the airfoil.

 

[B767Driver]

TG,

I don't think we're on the same page. I'll try to succinctly describe what I've gleaned from Carpenter's book.

1. Start with a sphere in inviscid flow. The streamlines over the top...equal the streamlines under the bottom. The result is a "no lift" condition because the pressure on the top and bottom are equal.

2. So now, how to you get a pressure differential between top/bottom? By Bernoulli. Increase speed over the top...slow it over the bottom.

3. This is accomplished by introducing circulation around the sphere in a clockwise manner (it could be counter clockwise...but let's use left to right circulation). We want to superimpose an additional clockwise rotation into existing flow over the top of the sphere. This will add to the velocity over the top. As the clockwise rotation rounds the bottom of the sphere...it butts against the bottom flow...and retards its velocity. This now provides a pressure differential between top/bottom and a lifting force. This is termed a "free vortex"...the introduction of circular streamlines going around a central point without the introduction of new air into the flow. (Like taking a spatula and stirring the flow clockwise.)

4. This circular flow around the sphere is only possible because the flow has viscosity. When circulation is induced...the air adheres to the surface and "pulls" the streamlines in a clockwise manner. This "pull" or force is the Mangnus Effect...which provides a force normal to the direction of rotation. So if the rotation is from 0 degrees to 180 degrees....the force is provided in a direction of 90 degrees.

5. How do you provide circulation? 1) You could spin the sphere clockwise (like a golf ball). (The more you spin a golf ball the higher it will fly). However, engineering spinning spherical wings is not a very reliable design for an airplane.

6. It was found that if you kept the sphere stationary...and placed a sharp edge at the rear....the air flowed in a circular manner. This caused "circulation".

7. Hence the importance of the sharp trailing edge. The sharp trailing edge introduces the left to right circulation into the airflow. Since the trailing edge induces circulation....there's no need for the airfoil to be spherical...so to minimize form drag the front is still rounded...and the tail section is tapered past the thickest point (point of minimum pressure) towards the sharp trailing edge. This keeps the adverse pressure gradient to a minimum and allows the air to flow to the sharp trailing edge which will induce the circular motion around the airfoil.

8. At the trailing edge...the air would like to flow like it does with a sphere...around the bottom. However, the force required for the air to make the turn is too great....and it separates cleanly at the trailing edge....resulting in the Kutta Condition.

Carpenter emphasizes that while the leading edge has many important characteristics....it is actually the Trailing Edge that is responsible for Lift! This is because that it introduces the circulatory flow...which is most responsible for accelerating the flow thru the adverse pressure gradient.

And viscosity, while seemingly detrimental because of drag effects, is vital to lift production...because circulatory flow could not be made possible without it.


This is bascially my "book report" on Carpenter's chapter 7. To me...it sure seems like a physical explanation of the Coanda effect...and maybe why it's not shown more in aero publications. It's there....but just described by the physical nature of the air in circulatory flow...and not by it's "slang" name.

I'm not saying this is correct, however. Just my observation.