The definition and causes of the Coanda effect

[Fluids]

I am confused about the following points regarding the definition and causes of the Coanda effect:

• whether the Coanda effect is the same as normal boundary layer attachment to a surface
• whether the Coanda effect is only defined for convex surfaces over which a jet (of the same state as the surrounding fluid) is tangentially blown
• what causes the Coanda effect: the jet entraining the ambient fluid or a balance of pressure and centrifugal forces, or something else?

Any answers to these, or recommendations of useful textbooks/papers, would be much appreciated.

Thanks.

 

[boneh3ad]

The Coandă effect is the tendency of a moving flow to follow the contour of a surface it is in contacting. One way to think of it is the fact that nature abhors a vacuum. Assume that a flow does not follow the contour of a surface for a second: the air would pass over the surface and as the surface begins to fall away, the flow would tend to entrain the fluid that is near the surface, pulling it along with the flow. That would tend to create a vacuum against the wall, so the flow will seek to find equilibrium by coming back to the wall. This would theoretically apply to a concave surface as well, but I can't think of any situations where you would actually see it since there is usually a strong favorable pressure gradient that would tend to hold the flow against the surface anyway.

This is not the same as the boundary layer, which has to do with fluid particles near the wall not slipping against the wall. This gives rise to the boundary layer. Interestingly, the Coandă effect can be overcome if the boundary layer has a strong enough pressure gradient retarding its motion. Then the boundary layer can "separate" and pull away from the surface. It overcomes the Coandă effect by pulling fluid in backward from upstream, creating a sort of bubble of circulating fluid against the surface.