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)


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.

Causes section[edit]

An un-authenticated user inserted the following comment into the article:

In the below paragraph the example of spoon seems to be wrong. I am no master in physics, so if any reader is, please check whether the example is correct or not. The example has being contradicted some 10-15 paragraphs below.;-)

This belongs on the talk page, not in the article itself. I'm going to remove the comment. And since I agree that the paragraph is problematic, I'm going to comment out the paragraph until we reach some consensus about what it should say. Mr. Swordfish (talk) 14:44, 25 August 2014 (UTC)


Delving in a bit farther, while it is true that Anderson & Eberhardt (the cited source) ascribe the cause to viscosity, this is not widely supported and another article states that this is a misconception (Lift_(force)#Misconception_regarding_the_role_of_viscosity. I'm removing the section pending consensus. Mr. Swordfish (talk) 15:15, 26 August 2014 (UTC)

Causes[edit]

The Coandă effect is a result of the viscosity of fluids.[1] Viscosity leads to a velocity profile for the fluid, creating a boundary layer in which the fluid velocity increases from zero at the surface of an object to the free-stream velocity when the distance from the surface is sufficiently great that its presence is negligible. The resulting adjacent streamlines within this boundary layer travel at different speeds, thus creating shear forces that bend the fluid in the direction of the slower-moving streamlines closer to the surface. A corresponding reaction force is then generated on the object in the direction of the flowing fluid, thus "entraining" it.

The existence of this force can be demonstrated by bringing the base of a spoon held vertically into contact with a downward-flowing stream of water; the spoon experiences a force pulling it towards the fluid flow.

References

The removed section is above. Glrx (talk) 23:35, 11 October 2014 (UTC)

"The Coandă effect is a result of the viscosity" - Isn't part of Coandă also related to the fact as mentioned above, that if flow didn't remain attached (or a vortice formed in the case of a stall) to a convex surface, a vacuum would be created? Some have called this aspect of a flow following a convex surface as "void abhorence" effect. I though this was considered to be part of Coandă effect Rcgldr (talk) 21:12, 28 February 2015 (UTC)

Explanation for ping pong ball in diagonal stream[edit]

There's a side article showing a ping ball ball suspended in a diagonal stream, mentioning that Coanda effect is why the ball remains in the stream. It also mentions combined with Magnus effect, but the angle of the stream can be reversed (moving the blow dryer while changing the angle so the ball doesn't move much), and even though the ball is initially spinning the "wrong way", it remains stable, so the primary stabilizing factor is the Coanda effect.

Not mentioned is why Coanda effect stabilizes the ball. The stream diverges outwards as it slows (mass flow conservation, ignoring viscous interaction with surrounding air), so the outer portions of the stream are angled more outwards relative to the direction of the stream than the inner portions of the stream. If the ball gets offset within the stream, then the Coanda effect diversion of the wake due to the difference in outer and inner angles of the stream result in a wake that is diverted outwards, which coexists (Newton third law pair) with an inwards corrective force on the ping pong ball.

Rcgldr (talk) 21:03, 28 February 2015 (UTC)