When surfing the net yesterday I came across a long-since closed thread in this forum (dated 15 to 16 Nov 2006)about the Coanda Effeect, containing several references to Flightwise Vol 1, of which I am the author. It occurred to me to offer my two-penny-worth on the topic, in case it helps.
I suggest that the use of the term "Coanda Effect" in aviation aerodymanics is really a bit of a red herring, and as I suspected it doesn't appear in the Index of any of the serious aerodynamics books that I have at my elbow. The most often quoted example is that of water from a tap flowing downwards onto a horizontal cylindrical object such as a glass tumbler or a bottle on its side. The water certainly "clings" to the cylinder, but there is a major difference between this situation and that of an aerofoil in an airflow. The aerofoil is totally immersed in its fluid (air), whereas in the flowing water case the fluid involved (water) has a free surface between it and the surrounding fluid (air). Also water is a liquid (which at rest finds its own level) whereas air is a gas (which fills the entire volume available to it).
The "Coanda Effect" is explained by the fact that the water is flowing, whereas the surrounding air is at rest, being scarcely disturbed by the flow of water at all. Since the water has a velocity, the pressure within the water is reduced below atmospheric (by Bernoulli's principle), but the pressure in the surrounding air, being effectively at rest, remais at atmospheric pressure. The resulting pressure difference acts inwards on the free surface of the water, deflecting its path inwards towards the centre of the cylinder, as observed.
This would indeed produce an outwards reaction on the bottle, or a sideways "lift" force, which could easily be demonstrated by suspending the bottle on strings, and letting the flow pass over one side of the bottle only. So does the coanda effect explain aerofoil lift? There is certainly a similarity: reduction in pressure due to increase of speed causes lift in both cases. But in the case of the aerofoil, at any point on the upper surface there is a pressure gradient outwards from the surface, corresponding to a velocity gradient (the velocity is greatest close to the surface, ignoring the boundary layer), whereas in the water-on-bottle effect theree is a step in velocity and hence pressure at the water-air surface. The lift-producing effect is similar.
Therefore I suggest that one is not incorrect in using the term Coanda Effect to explain aerofoil lift, but that there are distinct differences that make it not the most useful analogy.
By the way, the suggestion that the water somehow "sticks" to the cylinder through viscosity is incorrect and totally misleading. The viscous force is purely tangential to the cylinder surface, and can have no normal (inward) component at all! On the other hand, at extremely slow flow speeds of water over a bottle, there is another force which will come into play around the edges of the water stream, and that is surface tension, perhaps being relevant for example when a trickle of water is passed over the back of a spoon.
Interesting discussion - wish I'd got involved at the time it was live! But, if you are interested, I have just launched a brand new web site to give readers of Flightwise the opportunity to discuss with me and each other points that arise in the books or from your experience. Its URL is flightwiseforum.webplus.net . Being a brand new site, it may have teething problems for which I take full responsibility (it being entirely my own effort), but I look forward to meeting some of you there.
Chris Carpenter
I suggest that the use of the term "Coanda Effect" in aviation aerodymanics is really a bit of a red herring, and as I suspected it doesn't appear in the Index of any of the serious aerodynamics books that I have at my elbow. The most often quoted example is that of water from a tap flowing downwards onto a horizontal cylindrical object such as a glass tumbler or a bottle on its side. The water certainly "clings" to the cylinder, but there is a major difference between this situation and that of an aerofoil in an airflow. The aerofoil is totally immersed in its fluid (air), whereas in the flowing water case the fluid involved (water) has a free surface between it and the surrounding fluid (air). Also water is a liquid (which at rest finds its own level) whereas air is a gas (which fills the entire volume available to it).
The "Coanda Effect" is explained by the fact that the water is flowing, whereas the surrounding air is at rest, being scarcely disturbed by the flow of water at all. Since the water has a velocity, the pressure within the water is reduced below atmospheric (by Bernoulli's principle), but the pressure in the surrounding air, being effectively at rest, remais at atmospheric pressure. The resulting pressure difference acts inwards on the free surface of the water, deflecting its path inwards towards the centre of the cylinder, as observed.
This would indeed produce an outwards reaction on the bottle, or a sideways "lift" force, which could easily be demonstrated by suspending the bottle on strings, and letting the flow pass over one side of the bottle only. So does the coanda effect explain aerofoil lift? There is certainly a similarity: reduction in pressure due to increase of speed causes lift in both cases. But in the case of the aerofoil, at any point on the upper surface there is a pressure gradient outwards from the surface, corresponding to a velocity gradient (the velocity is greatest close to the surface, ignoring the boundary layer), whereas in the water-on-bottle effect theree is a step in velocity and hence pressure at the water-air surface. The lift-producing effect is similar.
Therefore I suggest that one is not incorrect in using the term Coanda Effect to explain aerofoil lift, but that there are distinct differences that make it not the most useful analogy.
By the way, the suggestion that the water somehow "sticks" to the cylinder through viscosity is incorrect and totally misleading. The viscous force is purely tangential to the cylinder surface, and can have no normal (inward) component at all! On the other hand, at extremely slow flow speeds of water over a bottle, there is another force which will come into play around the edges of the water stream, and that is surface tension, perhaps being relevant for example when a trickle of water is passed over the back of a spoon.
Interesting discussion - wish I'd got involved at the time it was live! But, if you are interested, I have just launched a brand new web site to give readers of Flightwise the opportunity to discuss with me and each other points that arise in the books or from your experience. Its URL is flightwiseforum.webplus.net . Being a brand new site, it may have teething problems for which I take full responsibility (it being entirely my own effort), but I look forward to meeting some of you there.
Chris Carpenter
