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nacaairfoil September 28, 2009 14:52

Coanda effect
 
hi
Could somebody give me some explanation about coanda effects. I searched internet, it seems that due to the entrainment between wall and ambient fluids, the flow will move fast close to the wall. According to Bernoulli principles, it will produce a lower pressure and then bent the streamlines.

However I have several questions?
1) what is the effects of viscousity?
2) what is the effects of surface friction? if there is no surface friction, does still coanda effect occur?
3) what is the effects of flow separation on the wall?
4) what is the relationship between coanda effects and surface tension.

Thanks a lot

nacaairfoil September 29, 2009 10:19

Could somebody give me some hints. Thanks a lot....:)

nacaairfoil September 29, 2009 21:01

could somebody please help me with these quesitons? Thanks a lot

radiatn September 29, 2009 21:55

1) what is the effects of viscousity?
A: The higher the viscosity, the higher coanda effect
2) what is the effects of surface friction? if there is no surface friction, does still coanda effect occur?
A: Yes, still occur even if not surface friction, but in reality there is always surface friction i.e. law of the wall, y+
3) what is the effects of flow separation on the wall?
A: If there is separation, the flow of the coanda is disturbed
4) what is the relationship between coanda effects and surface tension.
A: The higher surface tension, the bigger coanda effect

Regards! But the answers should be your area of study, there is no quick simple answer in reality.

nacaairfoil September 30, 2009 09:53

Thanks radiatn.
However, I still do not quite understand. It seems that the coanda effect is due to the Bernoulli effects which basically is inviscid effect. How does the viscoucity connect to this effect? What is the meaning of stronger coanda effects(what kind of quantities can be used to quantify the coanda effects, since the flow deflexion angle is always determined by the inclination angle of the walls(always parallel to the wall).

If I put a needle into to tape water, the water will follow the needle. Is this coanda effects? Since there is no much entrainment occurring for this case, it seems a purly surface tension effects. Can I also call this type phenomena coanda effects?

Thanks a lot...

CharlyAndy October 11, 2009 09:34

Coanda effect at the particle level
 
In http://arxiv.org/abs/nlin/0507032 I have tried to explain the cause of the Coanda effect. CFD is the study of the computation of the behavior of fluids. It is not necessarily the study of the causes of the behavior. The fluid approximation is primary in CFD. I think, however, that the explanation of the Coanda effect requires that one thinks of a fluid, correctly, as being made up of particles. To use a macroscopic term, shear in the fluid near a convex surface is the cause of the Coanda effect. The Bernoulli effect and the Coanda effect are quite different. Please let me know what you think.

Ahmed October 11, 2009 12:59

Quote:

Originally Posted by CharlyAndy (Post 232151)
In http://arxiv.org/abs/nlin/0507032 I have tried to explain the cause of the Coanda effect. CFD is the study of the computation of the behavior of fluids. It is not necessarily the study of the causes of the behavior. The fluid approximation is primary in CFD. I think, however, that the explanation of the Coanda effect requires that one thinks of a fluid, correctly, as being made up of particles. To use a macroscopic term, shear in the fluid near a convex surface is the cause of the Coanda effect. The Bernoulli effect and the Coanda effect are quite different. Please let me know what you think.

Time ago, I was involved in the analysis of the internal air ducts used to cool the blades of a gas turbine, at this high rotational rotor speed, one would expect the centrifugal forces to be strong enough to control the air flow, but the deflected air jets were leaning towards the rotor axis, as you say, the Coanda effect and the Bernoulli are not relatives.
Thanks for the reference, worth reading, truly.


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CharlyAndy October 12, 2009 07:17

I'm interested in knowing more about your project. How was the Coanda effect used?

CharlyAndy October 13, 2009 06:08

Coanda and Bernoulli
 
To nacaairfoil,

In http://arxiv.org/abs/nlin/0507032 I try to explain
the difference between the Bernoulli and the Coanda effects.
The Bernoulli equation uses the principle of the conservation
of energy to relate changes in altitude, pressure and velocity
in the flow. It does not address the way in which the fluid
interacts with itself and with the walls of the Bernoulli tube to
produce these effects. The Coanda effect, on the other hand,
can only be explained by the interactions between the fluid
and itself and the surface. Viscosity is a macroscopic notion
that is meant to reflect in a very general way these interactions.
Of late, CFD researchers have introduced a complex
viscoelastisity to explain some of these phonemena and at the
same time maintain the fluid approximation. I think this
approach amounts to putting mathematics and computation before
the physics. We need a whole new computational approach.

CharlyAndy October 22, 2009 15:42

To nacaairfoil.:

1) what is the effects of viscousity?
Viscosity is a measure of the interaction of the fluid with a surface. This interaction could be very complicated and turbulent.

2) what is the effects of surface friction? if there is no surface friction, does still coanda effect occur?
I assume you mean surface roughness, e.g. the roughness of a wing. Since all materials have a granular structure, they are made of molecules and atoms, there is always surface friction. The Coanda effect is caused by the interaction of molecules in the main flow with more or less stagnant molecules in the boundary layer.

3) what is the effects of flow separation on the wall?
Flow separation occurs when the pressure in the boundary layer pressure builds up. This rise in pressure is reflected in a complex and turbulent boundary layer. Can you send a drawing?

4) what is the relationship between coanda effects and surface tension.
In a liquid, surface tension is caused by the attractive intermolecular van der Waals forces. In air or any other gas these forces are negligible since the molecules are in each others' near vicinity for such a short time. The phenomenon of a stream of water clinging to a spoon is due to the van der Waals forces. The Coanda effect in air has a very different cause. The shearing air flowing along a convex surface is attracted to the surface. When the wing is working properly, i.e. not stalled, the low pressure on its front top surface is maintained.

nacaairfoil October 22, 2009 18:33

Hi Charlyandy
I am very insterested in your paper and work, just did not have time to fish the reading. My project is involved in airflow during phonation. During phonation, the flow is a very asymmetric inside the glottis. A lot of people think that is due to the coanda effects. I am trying to find the real cause for this type flow asymmetry. That's why I am looking at what is exactly cause behind this coanda effects. Again thanks a lot for your posting. I might have some question to ask you after reading your paper.

CharlyAndy October 23, 2009 07:11

Coanda effect in phonation
 
To: nacaairfoil,

Since the throat forms a sort of Bernoulli tube, I suspect that the overall pressure and velocity there are predicted by Bernoulli's equation. However it would seem that localized pressures very near the surfaces, e.g. the glottis, will fluctuate according to the curvatures that are created by the muscles in the throat. I am really intrigued by this. Maybe in phonation,
and other activities like Tuva singing, we make use of the Coanda effect. I was listening to the call of the Lyre bird yesterday. Incredible!


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