Talk:Coandă effect

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Come help with Wikipedia:WikiProject Fluid dynamics moink 23:19, 27 Dec 2003 (UTC)


Just a thought ... wouldn't a diagram for this entry be really useful?

( I just added one. I created it in the GIMP and boy do I hate the bezier tool in that program. Anyone is welcome to clean up my efforts. --Elijah 19:05, 18 May 2006 (UTC) )

The Coanda effect has nothing to do with the lift over an airfoil! This is a fallacy; lift is entirely produced by circulation (i.e. a bound vortex) and a proper fluid dynamic explanation doesn't need viscosity at all except as an initial condition at the trailing edge (while the Coanda effect is a purely viscous). --Knotnic 00:25, 8 August 2005 (UTC)

removed "The Coanda effect is important in the understanding of an airfoil's lift."
see [1] for a well-written discussion by a physicist and flight-instructor
So you assert that flow-attachment is unimportant in understanding lift? Such a strange position requires a detailed defense, not just an assertion.
So, why do you think that air is deflected from a straight path by the upper airfoil surface? Denker's website on lifting force is otherwise excellent, but the bit about Coanda effect is distorted: it contains both derogatory language as well as a major straw-man fallacy. The trick with the spoon and the jet of water is a very bad illustration of Coanda effect, yet he labels this demonstration as "the Real Coanda effect." No, the real Coanda effect involves flow attachment of gases as well as liquids, so it would require that the water jet take place in an underwater environment... or that the demonstration be performed using a spoon and an air jet in air. The term "Coanda effect" has approximately the same meaning as "flow attachment," so anyone who debunks Coanada effect and applies derogatory labels such as "fairy tale" is essentially trying to debunk (and to attach derogatory lables to) a genuine phenomenon: "flow attachment" in gases. Without flow-attachment, the air flowing above an airfoil would take a relatively straight path (i.e. the airfoil would remain permanently in Stall.)--Wjbeaty 00:47, 30 May 2006 (UTC)


I disagree. You may understand lift in terms of circulation, bound vortex, etc, but the average layman (i.e. the reader of an encyclopedia) probably doesn't, at least at first. It's far more intuitive to talk about lift as a turning of the airflow, creating lift by reaction - that is after all what is happening. The Coanda effect is important in that respect because it explains why the air should stick to the wing surface as it turns. If you read the page on lift, all explanations are given equal weight, and that is right - we need to cater for all audiences, not just aerodynamisicts (what are they doing reading this anyway?). So the statement is right, but might need to be qualified in some way. Graham 00:51, 8 August 2005 (UTC)
Aerodynamisicts are reading this pulling their hair out. Water sticking to the side of an object is in no way related to the lift of an aircraft. This article should be written by people who know aspects of fluid flow at a high level. They should be the ones who simplify the subject for the layman, not laymen themselves. —Preceding unsigned comment added by 74.197.176.46 (talk) 03:37, 19 November 2009 (UTC)
I'm all for alternative explanations when they're equally valid. But the fact remains - you can have lift *without* the Coanda effect. So I don't think it should be given equal weight as an explanation... but am fine for mentioning it as a related fluid phenomenon. I see now that there is extensive discussion on the lift page, so I'll probably not add more here. Knotnic 19:49, 21 August 2005 (UTC)
You mean that we can have lift *without* flow attachment at the upper airfoil surface? But the airfoil would then be well into stall, and only the lower surface would provide significant lift. An explanation of lifting force without Coanda effect (without upper-surface flow attachment) is an explanation of Stall-regime flight, not of normal flight.
Unfortunately the Circulation-based explanation of lifting force makes the unspoken, un-discussed assumption that the flow remains attached to the airfoil surfaces. The turning of an air flow because of flow-attachment is an interesting topic in its own right. I find it odd that anyone would try to debunk it, or try to attach derogatory emotional labels to the concept!--Wjbeaty 19:30, 15 December 2005 (UTC)

The inviscid lift theory works without so-called "flow-attachment", because, if the flow did not remain attached, there would either i) be a vacuum behind the object, or ii) the fluid behind the body would be entrained to "infinity". Neither of these is plausible physically. Even in an inviscid universe, pressure gradients cause fluids to accelerate. The vacuum behind the object would quickly be filled by fluid from the surrounding atmosphere, and lo and behold, there is your boundary layer attachment. Nothing to do with van der Waals forces, electric charges, viscosity, surface tension, or any other type of energy. There is therefore no need for the Coanda effect to explain the lift of an airfoil. Inviscid airfoil theories give surprisingly accurate predictions of lift without taking viscosity into account, provided the airfoil is not stalled. Now drag on the other hand.... Skr777 19:49, 27 June 2007 (UTC)

Coanda effect has significant impact on the lift over an airfoil. It can be applied to control the flow separation which caused by the trailing vortices. Actually the lift is generated by the geometry of the airfoil itself. The type of the airfoil will reflect on how much lift could be generated by the airfoil and this is depending on what is the Reynolds number of the flow. This is applied to either laminar or turbulent flow (turbulent will happen when Re > 500,000). Other consideration is the angle of attack of the airfoil because this will influence the airfoil to become stall or not. At certain angle of attack, there will be turbulent boundary layer at the back of the airfoil which caused by the trailing vortices. At here, the coanda effect could be applied to re-attach back the flow on the airfoil as a method of flow separation control by injection of fluid to the flow. The device that can be used in this case is synthetic jet actuator, which produced oscillatory flow that been injected back to the flow over the airfoil so that the boundary layer will remain laminar. This is what coanda effect deals with the flow over the airfoil. The method said will reduce the drag caused by the trailing vortices and hence, improve the lift!

By definition, the concept of a boundary layer no longer applies to the stalled portion of an airfoil. The turbulence in the wake of a bluff body has nothing to do with boundary layer turbulence. On just about any aircraft larger than a radio-controlled toy, the boundary layer over the wing is always turbulent, except very close to the leading edge. In fact, you may notice longitudinal "ribs" on the suction (upper) surface of commercial aircraft. These are used to intentionally "trip" the boundary layer to a turbulent state during takeoff, when, because of the lower speed (i.e. lower Reynolds number), the transition of the boundary layer to a turbulent state occurs further downstream along the wing. The turbulent boundary layer, because of mixing, has more momentum near the wall, making it less prone to separation due to an adverse pressure gradient (i.e. more stall-resistant).Skr777 19:49, 27 June 2007 (UTC)


From PNA/Physics[edit]

  • Coanda effect - attracts crackpots (due to "flying saucer" relationship), but the current version may be O.K. --Pjacobi 18:14, 24 September 2005 (UTC)

Questionable statement[edit]

"Professional aerodynamicists regard this theory as a fallacy." I'm skeptical. Please point out which aerodynamicists. --Wjbeaty 01:21, 30 May 2006 (UTC)

This statement is wrong, so I'm fixing it: "The flow from high speed jet produces enhanced lift through turbulent mixing that does not occur above a normal wing." On the contrary, conventional wings remain out of stall because of turbulent mixing in the boundary layer. As I understand this, the only wings which don't employ turbulent mixing are the so-called "Laminar flow" wings. So in other words, "Coanda effect" does apply to wings: it's part of the normal boundary layer physics, and part of the phenomena of Stall. On the other hand, the flow from a high speed jet above the wing will actively enhance the lift (rather than just delaying stall,) because, as the jet remains attached to the curved surface, its curved streamlines lead both to a lowered pressure above the wing, and also to a downward deflection of massive air parcels. --Wjbeaty 01:09, 30 May 2006 (UTC)

Article content RfC[edit]

This article contradicts, or at least seems to contradict Lift_(force)#Common_misconceptions. This should be worked out. —Preceding unsigned comment added by 82.181.17.213 (talkcontribs) Pjacobi 00:17, 25 May 2006 (UTC)

Supporting this anon comment [2], I've put this article on RfC. If we can't verify it the one way or another, we should just drop the content. --Pjacobi 00:15, 25 May 2006 (UTC)
Exactly who is of the opinion that boundary-layer attachment (coanda effect) plays no role in airfoil explanations? Those who put the Coanda effect in the misconceptions section need to justify their actions. Denker's page on airfoils is no justification, since Denker argues that "Coanda Effect" only involves a liquid stream flowing through gas. This is a very odd claim to make. After all, Coanda originally was led to research the boundary layer effects because the output of a jet engine was attaching to the fuselage of Coanda's early jet plane and causing heat damage. Also, the Coanda-effect hovercraft employs a thin radial sheet-jet of gas in a gas environment, as do the Coanda-effect gas burners, etc. "Coanda effect" encompasses fluid flow-attachment in an environment of the same fluid, and so is perfectly legitimate physics: it is part of the legit phenomena known as Stall or as boundary-layer attachment/detachment. --Wjbeaty 00:56, 30 May 2006 (UTC)

Demonstration image[edit]

I took the liberty to modify the image made by Eli_the_Bearded used in this article. I interpreted the "Demonstration" part of the article in this way: there are mainly four stages represented by the four images in the animation loop: 1) Only water running. 2) The convex object brought close to the waterstream. 3) The "Venturi effect" creating a pressure decrease in the air between and thus bringing the stream towards the object. 4) The "Coanda effect" taking over. I hope this modification can be useful. --Profero 02:06, 30 May 2006 (UTC)

I hate to say it, but even though this image does a good job of demonstrating the effect, it is obnoxious and distracting. Perhaps the GIF can be turned into a sequence of images instead? Either way, it can't stay how it is. Axda0002 03:08, 21 June 2006 (UTC)
I agree with you that animations, especially on the article pages, are most often very distracting. Although I don't know if this one is more distracting than the one in the article, there should be an easy way to start and stop them, or link to them. Yes, you are right, they are often obnoxious too. Perhaps a good idea is to generally create sub-articles containing movies and animations? So I made this still image with link as a suggestion. --Profero 11:18, 21 June 2006 (UTC)
I think that's a good suggestion. Either that, or have the image currently on the article page loop only once or twice. That way, the animation will terminate on the frame that is most descriptive of the effect. Axda0002 13:49, 21 June 2006 (UTC)
I think that if an animation stops, one should be able to start it again without reloading the complete page. I don't know if this is possible. Perhaps you know? In any case I also think that your suggestion of also (or alternatively) showing the individual sequences one by one would be a very good idea in a lot of cases, as they would be easier to understand. After all we are working with computers and should use the potentials we can to best illustrate articles – even more underlining the good idea of having separate good illustration pages with optimal imagery and not clogging up the text. (If you like you can perhaps also share your opinions at The deletion talk page and/or on Talk:Coandă_effect_movies concerning this issue.) --Profero 15:33, 21 June 2006 (UTC)
now a still is referenced? why not LINK to the moving http://commons.wikimedia.org/wiki/Image:Venturi-and-Coanda-effect-2.gif (but i do not know how to techincally do it...) :21:26, 9 April 2007 (UTC)

Effect photo[edit]

I added an actual photo of the effect because...why not? Axda0002 03:21, 21 June 2006 (UTC)

I think that is a good illustrative image. --Profero 15:35, 21 June 2006 (UTC)

test patient-001[edit]

Is the reference to test patient-001 in the article of interest to anyone or noteworthy? There's no article in Wikipedia about them, so I'm assuming that they're not well known whatsoever. I went to the link in the article and their website doesn't have much in the way of...information of any kind (save for one song download). I say remove that section. Any objections? Axda0002 02:30, 22 June 2006 (UTC)

After I removed this section, it has reappeared as an edit by 65.78.214.112. It reads:

The Coanda Effect is a progessive metal band from Point Pleasant, WV. They have an album due out this fall and you can find out more at http://www.terraincognita.echoz.com. The band is made up of guitarist Matthew King, singer James Lilly, and bassist Noah Tyree. The percussion is created digitally using computer generated drums.

This is inappropriate for this article. First of all, it smacks as an advertisement. Second of all, this is an article pertaining to a physical phenomenon, not music. If the user feels that this band is notable, which they aren't, then they should be bold in creating an disambiguation page. Axda0002 23:44, 29 June 2006 (UTC)

Does the Coanda effect occur in a vacuum?[edit]

Anyone know? (Stream of water against the back of a spoon, would the path of water be be pulled across the curve of the spoon, or would they bounce off?)

It would certainly help in the aerodynamic lift confusion. Please supply references.--JayJayPlant 16:15, 8 August 2006 (UTC)

If it were possible to test in a vacuum it wouldn't occur. In a vacuum, the water would instantly vaporize. The gas would instantly disperse. However, if you could somehow keep these two things from happening, there would be no pressure differential caused by entrainment of surrounding fluid to force the steam against the airfoil/spoon.


Your proposed experiment can be prefoermed if you use a stream of liquid mercury kept at a low temperature to minimize the presence of mercury vapor. My suggestion is at a temperature, T in Kelvin, such that 235K < T < 315K which should place the vapor pressure below 1Pa according to the information provided @ http://en.wikipedia.org/wiki/Mercury_(element) . Jatosado (talk) 21:31, 31 July 2009 (UTC)

Cause[edit]

At present, the article completely lacks any explanations of the causes of the effect, merely giving its applications. --82.181.61.48 22:13, 18 August 2006 (UTC)

Mathematics vs. Physics.[edit]

The vortex model to explain lift of an airfoil is a mathematical approach. It provides a means to abstract arbitrary airfoils' physical properties to a generalised mathematical model so that performance or other physical properties of any airfoil can directly be compared with any other one. The physical - or real - cause or explanation of lift is indeed the intuitive bernouilly approach. The spoon does not at all show the so-called coanda effect, but merely cohesion and adhesion effects between water molecules reciprocally and the surface of the spoon, together with surface tension effects. The coanda-effect's pseudo-scientific aura has even been amplified and confirmed by this WIKI entry, since it seems impossible for coanda-believers to unambigously demonstrate what it is about.

An attempt to clarify "Coanda effect" and "boundary-layer attachment"[edit]

In this article and the article on the lift force, and especially in the discussion pages that go with them, there seems to be considerable confusion regarding how the Coanda effect relates to boundary-layer attachment in ordinary aerodynamic flows. Here is an attempt to clarify the issues.

What the term "Coanda effect" properly encompasses

The work that Coanda himself did, which resulted later in the coining of the term "Coanda effect", was limited to powered jet flows in which the jet is of the same phase (gas or liquid) as the surrounding fluid and has higher total-pressure than the surrounding fluid. The phenomenon that Coanda observed was the tendency of a relatively thin jet to attach itself to an adjacent solid surface, and it is due to the strong tendency of jets, especially turbulent jets, to entrain surrounding fluid. The boundary layer in an ordinary aerodynamic flow is not a jet, and its tendency to remain attached is not a direct result of entrainment, so it seems that to apply the term "Coanda effect" to flow-attachment phenomena in general would be to broaden it far beyond what it originally meant. I am a practicing aerodynamicist, and I have discussed this issue with a number of my colleagues. I have yet to find one who thinks that the term "Coanda effect" properly applies to flows other than powered jets.

The phenomenon in which a small vertical stream of water from a faucet deviates from its original vertical path and follows a curved surface is obviously not a result of the same turbulent-jet entrainment that is responsible for the regular Coanda effect. Instead it seems to be due to an actual molecular attraction between the liquid and the solid surface, and to the fact that the stream resists being torn apart, because of the surface tension. So it seems improper to cite this as an example of the Coanda effect.

It also seems improper to apply "Coanda effect" to the flow around an ordinary airfoil, as is advocated by Wjbeaty and has been done in books by Anderson and Eberhardt, and by Craig. But the problem is not just one of semantics. Those who have introduced "Coanda effect" into the discussion of airfoils and lift seem to be under some misapprehensions regarding how flow attachment happens in general, which brings us to the next topic.

What "flow attachment" or "boundary-layer attachment" really involves

Anderson and Eberhardt erroneously see the Coanda effect as implying that viscosity (or turbulence) plays a direct role in the ability of an ordinary aerodynamic flow to follow a curved surface. They assert that viscous forces in the boundary layer tend to make the flow turn toward the surface, specifically, as they put it, that the "differences in speed in adjacent layers cause shear forces, which cause the flow of the fluid to want to bend in the direction of the slower layer". Actually, there is no basis in the physics for any direct relationship between shear forces and the tendency of the flow to follow a curved path. To arrive at a correct understanding of the role of viscosity, it helps to look first at what would happen without it, and then to look at what happens when it is present.

Much of the early theoretical work in fluid mechanics dealt with inviscid ("ideal") flow, and solutions were generated for 2D inviscid flows around many simple body shapes, including airfoils. In the theoretical inviscid world, flows follow curved surfaces just fine without aid from the effects of viscosity, contrary to what Anderson and Eberhardt claim. To support their position, Anderson and Eberhardt argue that the need for a Kutta condition in inviscid airfoil theory somehow demonstrates that inviscid flows don't naturally follow curved surfaces. This is incorrect. The Kutta condition just determines how far along the airfoil chord the flow follows the surface, not whether it follows the surface. The Kutta condition simply rules out flow patterns in which the flow has to whip around the sharp trailing edge, and a correct physical interpretation of it is that it is viscosity that prevents such flow patterns. But this by itself implies a direct role for viscosity only in the immediate vicinity of the trailing edge, not all along the surface, as Anderson and Eberhardt assert. In any case, theory tells us that fluids without viscosity would have no trouble following curved surfaces.

Real-life fluids not only have viscosity, but they interact with solid surfaces in such a way that the tangential velocities of the fluid and the solid are matched at the interface (the no-slip condition). At all but very low Reynolds numbers, a thin boundary layer in which viscous effects are important forms along the surface, and outside of the boundary layer the flow behaves as if it were inviscid. Near the front of a body, the boundary-layer flow is usually laminar, but in most cases of practical interest it transitions to a turbulent state before it reaches the back of the body. Over a wide range of conditions, the natural state of a laminar or turbulent boundary layer is to remain thin and to remain attached to the surface. But under some conditions, the boundary layer will separate from the surface. What determines whether the boundary layer separates or stays attached?

Within the boundary layer upstream of separation, the flow velocity is nearly parallel to the local solid surface. The distribution of this surface-parallel velocity with distance off the surface, from zero at a stationary surface, to a high velocity at the edge of the boundary layer, is referred to as the velocity profile. In addition to viscosity, the velocity profile is strongly influenced by the pressure distribution in the flow direction (parallel to the surface). The curvature of the surface has almost no direct effect, but only an indirect one to the extent that surface curvature affects the pressure distribution. Whether the boundary layer separates or stays attached depends on what happens to the low-velocity fluid at the very bottom of the boundary layer, which is strongly influenced by the pressure gradient in the local streamwise direction. If the pressure is constant or decreasing (a favorable pressure gradient), the low-velocity fluid will continue to move in the direction of the local surface, and the boundary layer will remain attached. Boundary-layer separation from a smooth surface in a 2D flow generally requires rising pressure (an adverse pressure gradient) to stagnate the low-velocity fluid. Counteracting the effect of an adverse pressure gradient are the viscous forces by which the higher-velocity fluid farther from the surface drags the low-velocity fluid along. So boundary-layer separation is determined by a tug-of-war between an adverse pressure gradient and favorable viscous forces. If the adverse pressure gradient is not too strong, the viscous forces will win, and the boundary layer will remain attached, just like the corresponding inviscid flow would under the same conditions. The amount of adverse pressure gradient that can be withstood is greater if the boundary layer is turbulent than if it is laminar, which is why even so-called laminar-flow airfoils are generally designed to have laminar flow only over part of the airfoil chord, with the boundary layer transitioning to turbulent before it encounters the strong adverse pressure gradient that usually prevails over the aft part of the airfoil.

A turbulent boundary layer's resistance to separation improves as the Reynolds number increases, no matter how high the Reynolds number becomes. Thus Wjbeaty's supposition that an airfoil flow at an extremely high Reynolds number would always be stalled is incorrect.

We've seen that the role of viscous forces in maintaining boundary-layer attachment is to help the low-velocity fluid at the bottom of the boundary layer keep moving, and that the viscous forces are needed only in situations where the pressure gradient is adverse. Viscous forces have nothing direct to do with causing the flow to turn and follow a curved surface. However there is an indirect association between surface curvature and the need for viscous effects that may have contributed to Anderson and Eberhardt's confusion in this regard. Convex surface curvature is often, though not always, associated with an adverse pressure gradient, in which case favorable viscous forces are needed to prevent separation. But the viscous forces prevent separation by dragging fluid along in the direction of the local flow, not by directly contributing to the turning of the flow.

If viscous forces make no direct contribution to the turning of the flow when the surface is curved, what actually causes the flow to turn? The answer to this question lies in the interplay between the velocity field and the pressure field, which works in the same way whether the fluid is viscous or not. When a flow turns to follow a curved surface, it is able to do so because the pressure field adjusts so as to provide the force needed to accelerate the fluid toward the center of curvature. Thus the centrifugal force generated when the flow follows a curved path is countered by a pressure gradient perpendicular, or normal, to the local flow direction. The normal pressure gradient and the flow curvature have a reciprocal relationship in which they cause and support each other simultaneously. Note that the normal pressure gradient is perpendicular to the streamwise pressure gradient that we considered earlier, and that it is the streamwise pressure gradient that plays the important role in determining whether the viscous boundary layer separates or remains attached.

A good counterexample to the Anderson and Eberhardt argument is the flow around a rotating circular cylinder. Tangential motion of a surface, due to rotation, can affect the location of separation. In this case, the flow follows the curved surface farther around the side of the cylinder where the surface is moving with the flow, and separates earlier from the side on which the surface is moving against the flow. But if we apply the Anderson and Eberhardt argument to this flow, it predicts the opposite of what is observed. On the side where the surface is moving with the flow, the viscous stresses are reduced or even reversed, and the ability of the flow to follow the curved surface should be reduced, according to their argument, but in fact it is enhanced. And vice-versa for the other side of the cylinder. The observed effects on both sides are consistent with ordinary boundary-layer theory, which correctly accounts for the effects of pressure gradient and surface motion.

What this indicates for the encyclopedia articles

The article on lift should point out that attachment of the flow to the airfoil upper surface is important, but that attached boundary-layer flow is the normally expected condition up until stall, and that the "Coanda effect" is not applicable. The article on the Coanda effect should be revised so as not to conflate the effect with ordinary "boundary-layer attachment". It should also explain what the effect really is and how it works, not just assign a name to it and give examples of where it occurs. Of course the water-faucet example should either be removed, or it should be revised to explain how this effect differs from the real Coanda effect. J Doug McLean 20:39, 4 December 2006 (UTC)


Excellent posting! Let me try to drill down to three critical issues. First one. You say 'The boundary layer in an ordinary aerodynamic flow is not a jet, and its tendency to remain attached is not a direct result of entrainment.' Interesting. Here's a thought experiment. Suppose we have an enormously wide jet which flows over a cylinder. This would be an example of boundary layer attachment, agreed? (Let the width of the jet be >> than the cylinder diameter. Assume laminar flow for clarity.) If we then let this jet decrease in width, eventually we'll arrive at a "Coanda effect" situation. I'd like to understand why there is a fundamental difference between the mechanism which attaches a wide jet, versus the mechanism which attaches a narrow jet. Why is a narrow jet attached by entrainment, while a wide jet is not? (Or equivalently, why is a narrow jet *not* attached by boundary layer pressure distribution, while a wide jet is?) Also, as we go from a wide jet to a narrow one, if the attachment mechanisms change completely, then what does the transition from "entrainment" to "boundary layer attachment" look like? I ask because I'm under the impression that there is no difference between attachment mechanisms, and that they are just two alternate approaches for explaining a single phenomenon. --Wjbeaty 05:49, 5 December 2006 (UTC)
You're right that the distinction between ordinary boundary-layer attachment and flow attachment in flows involving jets isn't always as clear-cut as I made it out to be in my posting. You make a very good point that you can define a continuum of situations between a thin jet and a thick one, and that there is no apparent switchover from one physical phenomenon to another. But I don't think that means that we're dealing with just one physical mechanism across the whole possible spectrum of such flows, and I'm going to argue that we should still maintain a clear distinction in our terminology.
To define what I consider to be the purest prototype of the Coanda effect I'd ask you to go a step beyond your thin-jet-to-thick-jet thought experiment. Consider a thin jet issuing from a nozzle that is separate from any downstream surface to which the jet might attach. The nozzle exit has sharp edges from which the internal flow separates cleanly as it leaves the exit, and a free jet is formed downstream of the exit Even if the nozzle flow isn't turbulent, the jet downstream quickly becomes turbulent For simplicity, think of a 2D flow from a slot nozzle, issuing into otherwise still air. In the absence of an attachment surface downstream, we get a classic 2D turbulent free jet that flows straight out along the nozzle axis. Now bring a 2D, curved attachment surface in from above or below the jet, a 2D version of the spoon in the faucet-and-spoon illustration in the article, but in this case it is in a single-phase flow. Even before the 2D spoon reaches the edge of the turbulent jet, the jet will be drawn to the surface and attach to it. The jet goes "out of its way" to attach to a surface that is entirely outside the former boundary of the turbulent free jet. This kind of attachment is clearly due to turbulent entrainment and would not happen in the absence of viscosity or turbulence. The corresponding steady inviscid-flow "solution" in this situation is a straight, constant width stream of fluid with higher total pressure than the surroundings, separated from the surroundings by zero-thickness slip surfaces. Until the spoon actually touches this stream, no steady, inviscid attached-flow solution is possible. So there is a range of spoon positions for which the viscous (turbulent) flow would be attached to the surface, but for which no inviscid attached-flow solution exists.
There is thus a sharp distinction between this prototypical version of the Coanda effect and ordinary airfoil flows, because in the case of an airfoil in an external stream, attached flow solutions exist for both the viscous and inviscid cases. In other words, in the prototypical Coanda case we need viscosity (turbulence, actually) to establish any attachment at all, while in the airfoil case we don't.
But for jet flows in which the nozzle is not distinct from the attachment surface, as for example when the downstream surface is an extension of one wall of the nozzle, we no longer have this sharp distinction. This situation, in which the jet starts out attached to a surface, is often referred to as a "wall jet". Whether such a jet is a thick one with a non-turbulent "core", or a thin one with an external flow outside of it, the situation is more like that of an ordinary boundary layer in an external flow, for which attached flow is the expected condition up until an adverse pressure gradient is strong enough to cause separation. In these wall-jet cases in which the nozzle fairs smoothly into the attachment surface, attached-flow solutions exist in the inviscid case, and viscosity is not needed to establish or maintain attachment any more than it is in ordinary boundary-layer attachment. This also means that the prototypical Coanda case, in which the jet starts out free and then attaches itself to the surface is the only one that needs entrainment to establish attachment, and that the cases in which the jet starts out already attached don't need entrainment. This particular distinction hadn't occurred to me when I wrote the above posting, so I'm glad you brought the issue up. Perhaps we should recognize a distinction between a "free-jet Coanda effect" (my "prototypical" case) and a "wall-jet Coanda effect", but in cases with a moving external flow, the wall-jet version would just be another name for the well-known technique of delaying boundary-layer separation by tangential blowing, which might be giving Coanda more credit than he deserves.
As I pointed out in the posting, in ordinary boundary-layer attachment, viscosity is needed to maintain attachment only when the pressure gradient is adverse, and even then the role of viscosity isn't the same as the direct role implied by Anderson and Eberhardt. So I still think we should avoid using the term "Coanda effect" in connection with ordinary airfoil flows because it implies that viscosity plays a much more direct role in maintaining attachment than it does in reality. J Doug McLean 01:48, 10 December 2006 (UTC)
I respect your expertise in this area, but I think your expertise regarding the differences in the types of flows has blinded you to the obvious commonalities, in particular, the aerodynamic mechanisms at play. You state that "this kind of attachment . . . would not happen in the absence of viscosity or turbulence." Fair enough. Now give me an example of a fluid that has no viscosity. Air has viscosity, and as air travels over an air foil, it is pulled down in conformance with the air foil shape because the pressure differentials above and below the stream. The pressure below, is reduced as a result of entrainment, shear, and viscosity (all inter-related concepts). Therefore the mechanism at play in your "prototypical version" of the Coanda effect is the same as in the instance of airfoils. If the mechanism at play is the same, why not use the same term, as it is descriptive of the mechanism? --Lenehey 21:50, 17 June 2007 (UTC)

I think J Doug McLean's posting is about as accurate treatise of this topic as I've come across. Can I add that I emailed Scott Eberhardt of Understanding Flight, by David Anderson and Scott Eberhardt, some years ago and he told me that they did not intend to imply that the Coanda Effect has anything to do with lift on a aerofoil. He said they used it to explain Newton's Laws to the layman and were considering removing it in future editions as there was so much resulting controversy. I got the impression that his co-author saw more value in mentioning the coanda effect than he did. Tt261 (talk) 18:16, 13 May 2008 (UTC)

If Scott Eberhardt said he and Anderson "did not intend to imply that the Coanda Effect has anything to do with lift on a aerofoil" I don't believe him. On page 21 of Understanding Flight it says "This downward-traveling air is the downwash and as we will see is the source of lift on a wing. Why does the air bend around the wing? The answer is in an interesting phenomenon called the Coanda effect. The Coanda effect has to do with the bending of fluids around an object."
See my critique of the book at Talk:Bernoulli's principle/Archive 2#Understanding flight. I'm not surprised there was great controversy about the book's contents - the book is scientifically unsound on many points. Dolphin51 (talk) 02:49, 27 October 2008 (UTC)

light and the coanda effect[edit]

it would seem that light also bends round and 'sticks' to objects, expecially convex ones. might not gravity also - and in which case - could this be made use of? —Preceding unsigned comment added by 203.27.90.208 (talk) 01:58, 12 October 2007 (UTC)

Aerospike Engine[edit]

Is the Coanda Effect employed in the (linear) Aerospike Engine, causing the exhaust flame to adhere to the vane? LorenzoB 04:32, 21 October 2007 (UTC)

WikiProject class rating[edit]

This article was automatically assessed because at least one WikiProject had rated the article as start, and the rating on other projects was brought up to start class. BetacommandBot 09:46, 10 November 2007 (UTC)

Causes, definition, limitations, citations[edit]

Although the article is pretty good considering the amount of misinformation surrounding the Coanda effect, one body citation (zero before I put one in) isn't enough in the body. There is a wealth of scientific literature on the effect; Henri Coanda is neither the beginning nor the end. He didn't discover it (it was known as far back as 1800, probably observed earlier) or explain the causes of it (which is still a topic of active research). He just happened to patent a significant application. These are some of my specific concerns:

1) The causes section is lacking and contradictory. It should at least not offer two explanations for the thin jet attached to a spoon, but should also offer a more complete and referenced explanation, covering the current state of the research.

2) The Coanda effect is not a synonym for "boundary layer attachment" and convexity is not a necessary condition. I addressed these problems with the definition by inserting is the most most general definition, which is common enough in the literature that it doesn't need to be cited.

3) There should be some discussion about the limited applicability of the effect or discussion of popular misconceptions, perhaps integrating this article with the mention in the Lift article. It's appropriately limited to a jet of fluid impinging on a curved surface, but there are still other references online that incorrectly lift as consequence of the effect. The article, thankfully, does not make the lift claim. But it comes up frequently enough still that it needs a mention.

I'll return to it when I have time, but for now I put the unsourced tag on to to alert the reader to check the facts and be careful about taking it as the definitive reference. Mbelisle (talk) 01:46, 18 April 2008 (UTC)

Working models[edit]

Photos and videos of two radio controlled flying models that use the Coanda effect. http://jlnlabs.online.fr/gfsuav/index.htm —Preceding unsigned comment added by Bizzybody (talkcontribs) 00:46, 30 May 2008 (UTC)

News from France[edit]

The so-called Coanda effect strongly depends upon a largely ignored factor, viz: the width of the defllected jet, as compared with its radius of curvature. Appropriate experiences show that Coanda effect does not occur at all if the ratio width/radius is greater than 0.3: obviously the Coanda version of stall.

Extensive information about this factor can be found in the wikipedia french edition “effet coanda”: see the article, then the page 9A in the pdf in reference 6, and the discussion which provides photos with additional information in wikipedia commons.

The same experimental information is available in english in the following reference:

Kadosch M., “The curved wall effect” in 2nd Cranfield Fluidics Conference, CAMBRIDGE 3 Jan 1967 --Marcel kadosch (talk) 14:03, 10 February 2009 (UTC)

New Material Added[edit]

Sept. 8, 2009

Kim Aaron, PhD Aeronautics, Caltech 1985.

I revised the discussion quite a bit today. I rewrote the "Causes" section to match the explanation as I learned it at Caltech. I supplied an honest-to-goodness text book as a reference which shows very convincing photographs of the phenomenon and explains it the same way I learned it. I have not scanned those images and pasted them in due to copyright reasons. I'm talking about "Physical Fluid Dynamics" by D.J. Tritton. It is an excellent book explaining all kinds of fluid phenomena in easy to understand language. When I purchased the paperback version in the early 1980's, it cost me $11.55. Of course, with inflation, that will be higher now, but it's a very reasonable price and I highly recommend it.

I particularly revised the demonstration of the spoon in a stream of tap water to explain that it is NOT due to the Coanda effect, but is rather due to surface tension. I also added a few real examples.

_____

Let me add my two cents to the discussion above about flow past a wing. When a wing moves through the air, there is no concentrated jet. Just because there is shear in the boundary layer does not mean it is an example of the Coanda effect. It is true that the flow resembles what would happen if you aimed a jet over the top of the wing in that the flow remains attached to the upper surface and the flow follows a curved path over the top (and the associated change of momentum contributes to the lift on the wing). But in the main uniform flow past a normal wing in flight, there is no entrainment. At best, there is the potential to entrain flow where the wing is because the boundary layer would try to mix with fluid that would be where the wing is if the wing were suddenly not there. But that is really a stretch in calling it Coanda effect. Really, the fluid tries to stay attached to the wing due to continuity of material. The pressure distributes itself to try to make that so. When the fluid does break away from the surface (during stall), then there will be entrainment along the highly sheared flow, and in some circumstances, that entrainment will pull the flow back down to the surface leading to reattachment. That might reasonably be cited as as example of the Coanda effect, although I personally don't think of that as being Coanda effect. Lots of things can be going on in a fluid flow and they bear resemblance to similar but different flow situations. Because one thing looks like another does not mean it *is* that other thing.

Kimaaron (talk) 21:54, 8 September 2009 (UTC)

Deleted this image because it is misleading:

Demonstration of Coandă effect

It looks like the spoon in the stream of water, which I have discussed as being NOT an example of the Coanda effect.

Kimaaron (talk) 22:44, 8 September 2009 (UTC)


  • If it's not right, it's not right. I'm no expert. I created that image because that's what I'd been led to believe the Coandă effect was, and the article didn't make it clear. Can you create an illustration that is more accurate? --Elijah (talk) 22:58, 10 September 2009 (UTC)
It is a great illustration of the flow past the back of the spoon, which previous editors had described as illustrating the Coanda Effect (a VERY common misunderstanding). There are some photographs in the book I cited (Tritton) that illustrate the Coanda Effect very well. I wish I could simply scan them and paste them in, but that would violate copyrights. Maybe I will write the publisher and ask for permission to display the images (with appropriate credit). in the meantime, I'm not sure exactly what a replacement image should look like. I'll think about it. Maybe I can come up with something. Maybe I'll sketch something by hand, scan it, and put it in the discussion so someone with the right skills can reproduce it in a similar illustration to what you produced so well.
Kimaaron (talk) 05:58, 11 September 2009 (UTC)
Well your PhD was three years before mine so I guess you take precedence although I don't know about Caltech versus Cambridge. However I think the wording of the explanation should be more along the lines of "one useful way to understand it is". There are a whole load of ways to understand Coanda including from Bernoulli, Newton's Laws and entrainment. All of these can be done right or wrong, and each have their share of people who think they do not work because they have only heard a wrong explanation based on them. The trouble with entrainment as an explanation is that it is quite a difficult concept to start with. Of course explaining to graduate students that way is one thing but Wikipedia might manage explanations with fewer intermediate steps. The Oxford English dictionary used to be criticised for defining pepper as a "pungent aromatic condiment" which gave a child three words to look up instead of one. Coanda is a similar level of concept to entrainment and not really reliably defined in terms of it. :)--BozMo talk 13:26, 11 September 2009 (UTC)

Y'all might want to add this too. The Ching disc golf company claims that it's putter takes advantage of the coanda effect from the indentations "contours" in the flight plate of their putter. here is the link. they have a patent and everything, haha. http://www.ching.us.com/sports/index.php/d-r-a-f-t-systems Grrbrown (talk) 02:49, 31 March 2013 (UTC)

Experimental material[edit]

Sept 24, 2009

Defining Coanda effect just as the tendency of a fluid jet to be attracted to any nearby surface is questionable, and Coanda effect is also limited as follows.

-Assume first the nearby surface to be convex towards the jet : in most of quoted applications which involve a rather high Reynolds number: Re = vb/kinematic viscosity , where v is the mean fluid velocity and the length b is the lateral extension (or width) of the jet, the fluid jet is attracted only if the ratio: R/b = radius of curvature/width of the jet ≥3, as measured by Kadosch M., see: “Déviation d’un jet par adhérence à une paroi convexe” in:

Journal de Physique et Le Radium, avril 1958, p 9, or : ajp-jphysap_1958_19_S4_A1_0.pdf p 9,

The fluid jet is separated when R/b <3

The critical ratio R/b at which the jet is separated increases at small Reynolds numbers: up to R/b =7 if Re = 500, R/b=10 if Re=200, and to R/b = 20 if Re=100, as measured by T. Vit , F. Marsik , see: “Experimental and theoretical study of heated coanda jet” in:

fluid.ippt.gov.pl/ictam04/text/sessions/.../FM2_12062.pdf, 2004.

-If the nearby surface is a flat plate at an angle a , the jet is first separated then re-attached further at a significant length, when a increases from 30° to 60° or decreases with an hysteresis of 15°, as measured by Bourque C. and Newman B.G. , see: “Reattachment of a two-dimensional, incompressible jet to an adjacent Flat Plate “,in:

The Aeronautical Quarterly, vol XI, aug 1960; pp 201 and seq.

If a< 30°, the bubble of separation is too small and its length is not significant.

- What if the nearby surface is concave? Why not test the popular spoon suspended under a jet getting into the spoon instead of around? Surface tension or not, the spoon is thrown out and the formerly deflected jet is straight again , then the spoon comes back and is thrown again etc, a relaxation oscillation occurs, with a period depending upon the mass flow.

Now an impulse turbine (a Pelton wheel) may be thought of as a number of spoons mounted on the same axis for continuous rotation. Most people believe that the jet impulse spins the wheel, but Coanda-lovers may argue that the wheel is spinned by the attraction of the jet to the back surface of the spoons. This is no longer physics, just metaphysics --Marcel kadosch (talk) 22:55, 23 September 2009 (UTC)

Fundamental flaws in this article[edit]

This article constantly mixes up the attraction of liquid to a surface (which is caused by a molecular attraction between the two) and flow over aircraft wings (which involves no such molecular attractions). There is no way this is the same thing. Precisely what the Coanda effect is needs to be agreed upon and used consistently. —Preceding unsigned comment added by 74.197.181.171 (talk) 06:40, 4 November 2009 (UTC)

Answer to 74.197.181.171, and additional experiments[edit]

Maybe I am mistaken (I speak french) but I understood that “ in this article” means “in the above article”, titled: "Experimental material". All the quoted experiences concern Coanda effect with air jets attracted by a nearby convex or flat surface. I suggested to add an experience concerning either Coanda effect (gaz jet in gaz) or Teapot effect (liquid jet in air), surface tension or not, when the nearby surface is concave: an extension which is involved by the new definition of Coanda effect introduced by Kim Aaron in the article “Coanda effect”, 8 september 2009, as given by Tritton: he does not quote him for copyright reasons, and unfortunately I could not read him. But many people say:”when I hear the word Coanda, I reach for my spoon”: that’s what I did.

Meanwhile I take the permission to quote the experiences made by C. Duez et al, who finally discovered the real cause of the teapot effect: “a novel hydrocapillary adhesion phenomenon coupling inertial flows to a capillary adhesion mechanism”, and took a laser photograph of it! See it in:

"Wetting controls separation of inertial flows from solid surfaces", C. Duez, C. Ybert, C. Clanet, L. Bocquet, Physical Review Letters 104 084503 (2010). --Marcel kadosch (talk) 21:18, 15 April 2011 (UTC)

Image at top of article[edit]

Is anyone interested in making a higher-quality one? No offense to the OP, but kinda unclear and low-budget looking atm. a13ean (talk) 17:51, 8 October 2012 (UTC)

I've gone ahead and replaced it with a photo from Commons -- 82.32.198.178 (talk) 20:55, 8 October 2012 (UTC)

Prior to Frost[edit]

I recently, some time about a month ago, came across a diagram of a vehicle that used the disks that Frost was experimenting with in the image seen here: [3]. This was part of a UK research group's output from the WWII era, and the imaged showed four of these disks arranged around the corner of a bus-like vehicle. It suggests that Frost's work was inspired by this group, which is certain worth mentioning. Has anyone else seen this image? Maury Markowitz (talk) 11:23, 26 August 2013 (UTC)

Ping-Pong Demonstration Section[edit]

Most focus is on the effect, so the ping-pong section was overlooked. My correction addresses the following:

A) The air does not necessarily curve all the way to a downward direction. It only has to have enough downward acceleration for M*A to equal the very light ball's weight. This may only be a small change (reduction) in the vertical component with an additional small horizontal component set by the jet's off-vertical angle. Testing with a handball sized ball and a cotton tuft probe showed a 70 degree total change in airflow direction from 20 degrees off vertical to horizontal.

B) Using the word "suction" mirrors the false explanation of lift using Bernoulli's pressure difference and ignores the air momentum change and the required reactive force.

In addition, a hefty vacuum cleaner can blow a ping-pong ball across the room, so something heavier is recommended (ball or light bulb), or a lighter air stream. A heavier ball should also show more total air-stream deflection.

Also, I see a common objection to calling sheet adhesion the Coanda effect. I won't inject my opinion into the discussion of whether it is a jet vs. sheet being the Coanda effect but say that Coanda's initial 1936 US patent, 2,052,869, explicitly recognizes an adhesion effect in both jet and sheet form factor. In addition, his aircraft had a sheet rather than a small jet. So perhaps it is what von Kármán originally defined it as. I also recognize that writing a good patent requires using the most general description possible, so perhaps Coanda was generalizing, with no basis, to the sheet form factor. -- Steve -- (talk) 01:07, 8 July 2014 (UTC)