Can anyone explain to me what causes Coandă effect?
In essence, frictional forces keeps the boundary layer attached to the curved surface. Without frictional forces, adverse pressure gradients would make the streamlines flip off the surface. The Coanda effect prevents this from happening in the first place.
The Coanda Effect has been discovered in1930 by the Romanian aerodynamicist Henri-Marie Coanda (1885-1972). He has observed that a steam of air (or a other fluid) emerging from a nozzle tends to follow a nearby curved surface, if the curvature of the surface or angle the surface makes with the stream is not too sharp.
If a stream of water is flowing along a solid surface which is curved slightly from the stream, the water will tend to follow the surface.
You may find more info here http://jnaudin.free.fr/html/coanda.htm
But what is the cause of those frictional forces? Can it be somehow explained by taking viscosity into consideration?
Well, viscosity is of course a numerical measure of the force of friction; friction itself is a complicated macro-scale manifestation of electro-magnetic forces of attraction.
It's a combination of friction at the surface, and viscosity in the gas / fluid itself. Both are needed for coanda effect.
In addition, the flow of a fluid or gas can create a void, for example a bus traveling leaves a void just as it passes through a volume of air. This creates a low pressure area that accelerates the surrounding air into. This is not coanda effect, but just nature abhoring a vacum.
There are some descriptions of lift that imply coanda effect is a big factor when explaing lift, but lift is mostly a combination of "void" and deflection effect (a solid pasing through a gas leaves a moving void behind, in the case of a wing, the void and deflection are introduced in a mostly downward direction (lift) and some forward (drag)) , although viscousity also affects lift, along with laminar versus turbulent air flow, ...
is the Coanda effect also the cause of acid running along a stirring rod when poured into a beaker? I feel that is exactly what is being described here, but that is not mentioned in any of the articles I glanced through.
The way I understand it, the velocity of a fluid at the convex surface is zero or close to it and it increases with distance from the surface. Differences in the velocities of the adjacent streamlines create shear forces which cause the streamlines to bend toward the slower moving streamlines. However, I'm still looking for a physical explanation of what is the cause these forces.
As already stated: electromagnetic attraction. I'm not sure what else you are looking for...
Oh, now I get it.
Not so fast. In liquids the viscosity is due to electromagnetic attraction, but these play very little part in the viscosity of gases.
For gas, as far as I can tell, viscosity and entrainment are due to adjacent air streams mixing momentum (either due to particles in one air stream preferentially moving to another or simply due to collisions between the particles tending to even out the momentum.)
I've given a theory for how this viscosity causes the coanda effect (at least in the boundary layer) in another thread. Essentially, the viscosity in the boundary layer causes the particles there to have lower momentum tangent to the curve, which allows these particles to preferentially populate the region where a glass curves away.
The Coanda effect is seen even when the item is at rest with respect to the ambient air (that is, even when there is no void due to the item moving). However, in that case viscosity comes to rescue again as it allows entrainment and displacement of the stationary air below the horizon of the curve.
I would be careful claiming viscosity is due to electromagnetic attraction. Electromagnetism is a conservative force, viscosity is a dissipative process.
The Coanda Effect, by its speed over a surface, lowers the static pressure between itself and that surface. The following gas simply flows into the reduced pressure area. The Coanda Effect, when applied to lift on airfoils, can only be understood after lift is fully understood. Lift is not due to Bernoulli effect though the Bernoulli equations may be employed to calculate it.
The Coanda Effect does not naturally occur on airfoils. Lift is due to vorticity.
The Coanda Effect involves applying an artificial, high speed jet to a surface. This accidently occurs rarely in nature and does not occur on wings naturally. The applied wall jet simply lowers pressure, (no matter how you dissect whats happening layer by layer) between itself and the surface. The following gas simply flows into the low static pressure area even around up to 180 degrees. Applied at a special bluff trailing edge of a foil the resulting suction shifts the rear stagnation point around to the underside of the trailing edge. This embeds the foil back to within the "bound vortex" and significantly postpones stall at high attack angles, greatly enhancing lift. Lack of understanding of Coanda Effect assisted lift is due to the Bernoulli lift explanation error and the Coanda Effect over foils error. For that and also non lift situations such as "fluidics" the mechanism is quite simple. Increased kinetic energy (speed) replaces static pressure so the total energy locally remains the same (neglecting losses). The reduced static pressure along a curved surface causes the stream to follow the curve.
My PDF "Lift and the Coanda Effect" is from my website WWW.newfluidtechnology.com
This seems a rather odd statement given that the Coanda Effect is seen in situations having nothing to do with lift.
My apologies, I am so used to most people connecting Coanda Effect with foils. My PDF explains where some of those non lift examples are found. The value of "Lift and the Coanda Effect" is that it largely clarifies the mechanism of the Coanda Effect and defines how it is produced artificially and a few places it is found in nature. As you know it is the basis for "fluidics". There it is also artificially produced.
Static pressure is not reduced by the relative speed between a flat surface and a gas or fluid. There's a boundary layer with zero relative motion at the surface that extends out to where the gas or fluid is flowing at about 99% of the speed of the adjacent gas or fluid, the 99% by definition of 'boundary layer'. The boundary layer pressure is essentially the same as the gas or fluid just outside the boundary layer. Static ports on small aircraft are mounted in the sides of the fuselage, and sense the static pressure of the ambient air outside the aircraft, in spite of any relative speed between air and aircraft.
If the surface is cambered, then a combination of skin friction, viscosity, and 'void effect' (the gas or fluid fills in what would otherwise be a void if the gas or fluid flowed in a straight line away from a cambered surface), results in a centripetal acceleration of the affected gas or fluid, coexistant with a reduction in pressure because of inertia effects.
I'm not sure I would say it is "artificially" produced. This might be a matter of semantics, but to say the Coanda effect is artificially produced would suggest that something significantly unnatural were done to accomplish the effect itself (say, for example, adding glue to a surface).
I would say that your point is that most of the time the Coanda effect is seen, it is in a situation that is not found in nature...I'm not sure how relevant that is.
I noticed a few problems with the PDF:
i) Your discussion of how Coanda comes about does not utilize viscosity. Without viscosity the momentum of the air carries the packets beyond the curve (in many cases entrainment of stationary air is the way viscosity helps, but I don't see that in air foils).
ii) You claim Coanda does not occur under an airfoil that does not specifically have coanda jets. I don't see any proof of that. On page 2 you say "the above demonstrates there is no possible way it can occur naturally on wings," but there is nothing above that I see showing that. The Coanda effect was first reported when viewing (relatively low velocity) smoke bending around a curve.
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