Nov 17, 2012, 06:45 AM
greg
somerset, nj
Joined Feb 2005
377 Posts
Quote:
 Originally Posted by Crossplot I cannot find again a free download copy of "Understanding wing lift" by J.Silva and A.A.Soares. If you can find it it is a must read. Also look for Dr klous Weltner and Gail M. Craig

Quote:
 Originally Posted by Crossplot My quest is to establish for certain as to whether the downwash aft of the wing is an actual reaction to lift or is it a byproduct of the system created to generate lift.
i agree ... isn't this the key question

Quote:
 Originally Posted by ShoeDLG After going through the exercise, I was very surprised to find that my intuition (based on incorrect application of Newton's 2nd Law) was completely incorrect. It turns out that the rate at which the air's vertical momentum changes depends on how the air is bounded (even when the boundaries are very far from the wing).
is this implying that the momentum of the air can only increase if it moves, has some place to move to (boundaries), or that the air is unaffected?

If it has no where to move to it simply pushes back opposing the pressure exerted on it, like the chair i sit on has no where to move to (is the chair force = 0 because a = 0). And even if it can move, isn't there a resistance to its movement that partially contributes to the force exerted on it by the pressure at the wings surface?

greg
Nov 17, 2012, 09:42 AM
United States, GA, Atlanta
Joined Oct 2010
551 Posts
Quote:
The Coanda effect refers to a jet of air passing over a body. It can be used to explain the increase in lift seen on blown flaps, but not the general case of an airfoil in a uniform flow.
Nov 17, 2012, 12:16 PM
Registered User
Joined Dec 2011
62 Posts
Quote:
 Originally Posted by DPATE The Coanda effect refers to a jet of air passing over a body. It can be used to explain the increase in lift seen on blown flaps, but not the general case of an airfoil in a uniform flow.
Right, but Coanda has incorrectly become the sum of effects that keep flow attached . They also refer to "pulling down on air".

However do NOT miss the explaination of the centripetal acceleration that actually creats lift.
Nov 17, 2012, 12:38 PM
Registered User
Joined Dec 2011
62 Posts
Quote:
 Originally Posted by ShoeDLG
This is just a question. The wing is wonderful expresion of fluid dynamics. It dots the i's and crosses the t's. The problem is that I can find no reaction to lift in lifting line downwash. It is defined by the same circulation that defined lift but that is rather remote.
 Nov 17, 2012, 12:49 PM Grad student in aeronautics United States, GA, Atlanta Joined Oct 2010 551 Posts Crossplot, Thank you for giving something tangible to talk about in this discussion. I read the whole article and found many problems with it. They gave the well known equation for centripetal force but fail to offer any way of calculating the lift from it. There is no way to show that they are right or wrong because they do not provide any useful equations. To lead into this discussion they say "The air flow near the aerofoil follows the geometrical shape of the upper surface generating a pressure gradient and acceleration, both perpendicular to the streamlines and directed to the centre of the flow trajectory." The acceration is most certainly not entirely perpendicular to the streamlines. The flow near the surface accelerates rapidly past the leading edge and then. Yes some acceleration will be perpindicular to the surface, but consider the face that the change in the length of the vectors they show in the Fig. 4 indicates the velocity change parallel to the surface. They also discuss viscosity: "This variation of viscosity induces a decrease of air velocity inside the boundary layer from its outer frontier to the aerofoil surface." This is completely wrong. For a Newtonian fluid (like air and water), viscosity only changes with temperature. This only becomes important for very high speed flows. The velocity is lower inside the boundary layer because of the no slip condition at the surface. Their conclusion: "In summary, lift occurs when flow is shifted downwards." This is the first time they use the word "shift", thereby making this statement very ambiguous. In actuality, lift is created by sending more streamlines above the body than below. -David
 Nov 17, 2012, 02:00 PM Registered User Germany, BW, Stuttgart Joined Mar 2012 1,193 Posts While there is no connection between centripetal acceleration and the change in pressure along the air's streamlines, there is a definite connection between centripetal acceleration and the change in pressure normal to the streamlines: pressure gradient perpendicular to flow direction = density * velocity^2 / radius of curvature Suppose you started at a point many spans above the wing. You would expect the pressure there to be roughly equal to atmospheric pressure. Imagine you followed a path to a point on the top of the wing by always moving perpendicular to the local flow direction. You could determine the pressure at that point on the wing by integrating the equation for the pressure gradient given above. As long as the streamlines predominantly curved downward, the pressure would decrease as you moved to the wing, meaning the pressure on the top of the wing would be less than atmospheric pressure. You could follow a similar path from well below the wing to the lower surface. As long as the streamlines again predominantly curved downward, the pressure would increase as you moved to the wing, meaning the pressure there would be greater than atmospheric pressure. By adjusting your starting point fore and aft, you could end up at any point on the surface of the wing. This would give you a (horribly inefficient) way to calculate the lift on the wing from the centripetal acceleration. This illustrates that the wing essentially creates an area of lower pressure on the top surface and a region of higher pressure on the lower surface by deflecting the air downward as it passes above and below the wing. Is there momentum exchange associated with this deflection? Absolutely. However, you have to remember that a 3D wing is elsewhere deflecting air upward, so you can't draw any conclusion about net momentum exchange.
Nov 17, 2012, 05:15 PM
Registered User
Joined Dec 2011
62 Posts
Quote:
 Originally Posted by DPATE The acceration is most certainly not entirely perpendicular to the streamlines.

I was afraid that this question was going to come up. All accelerations are relative to the true inertial path of the fluid. that is its path relative to the remote still air. The path past a cylinder is a symetrcal cursive "e".
The primary acceleration is normal to this path. The only place that it is actually normal to the surface is at the "high"point. The acceleration that produces lift is the component of the primary acceleration that is normal to the surface.
As Shoe shows, the dynamic pressure change comes from the summing of the pressure gradient, dp/dr = density*v^2/2/r. Since decreases outwards at approx 1/r, from Abbot and von Doenoff "Theory of Wing Sections", r(surface curve) /2 will integrate the gradients across the flow.
Reducing, v^2/r*r/2 gets us back to Bernoullis equation which can be used everywhere.

Shoe, that is not a bad description.
Nov 17, 2012, 06:36 PM
United States, GA, Atlanta
Joined Oct 2010
551 Posts
Quote:
 Originally Posted by Crossplot IThe path past a cylinder is a symetrcal cursive "e". The primary acceleration is normal to this path.
I am not sure what you mean here, maybe you could draw it.

There is no disagreement that centripetal acceleration takes place and is proportional to the gradient of the pressure in the direction normal to the path. However, please re-read this quote:
"The
air flow near the aerofoil follows the geometrical
shape of the upper surface generating a pressure
gradient and acceleration, both perpendicular to
the streamlines and directed to the centre of the
flow trajectory."

This states that the acceleration is normal to the streamline, which is mostly not true. Consider the a fluid particle very close to the stagnation point. Its velocity is almost zero, so there is very little centripetal acceleration, but there is a large tangential acceleration. As you point out, flow over a cylinder illustrates this nicely:
http://www.desktop.aero/appliedaero/...cylinders.html
Also as you point out, the acceleration of the flow at the surface is only normal to the streamline at the top of the cylinder (theta=pi/2).
 Nov 17, 2012, 10:26 PM Registered User Joined Dec 2011 62 Posts [QUOTE=DPATE;23295127]I am not sure what you mean here, maybe you could draw it. I have been reluctant to refer to my web page because many find it a poor read. However it spends a lot of time with the identification of the real inertial flowpath. http://svbutchart.com .> However, please re-read this quote: "The air flow near the aerofoil follows the geometrical shape of the upper surface generating a pressure gradient and acceleration, both perpendicular to the streamlines and directed to the centre of the flow trajectory." This is not really wrong.remembering that the acceleration normal to the streamline is a componment of the primary turn acceleration. As in Bernoulli, the lift here is created by the flow speed relative to the surface but to the curve of the surface. The particle that arrives at the stagnation point has been accelerated from still to the velocity of the wing or cylinder. The tagential velocities are relative to the surface and are not real. Note the relative velocity at the top of the cylinder is 2V : one V is air moving aft and one V is the cylinder moving fwd. Last edited by Crossplot; Nov 17, 2012 at 10:33 PM. Reason: added rhought
 Nov 18, 2012, 06:40 AM greg somerset, nj Joined Feb 2005 377 Posts besides Coanda and centripedal force, the Silva and Soares article also says collision with the bottom surface all contribute to lift. (Wouldn't Coanda only need to be considered on the back side of the upper surface)? For me, suggesting that lift is due to multiple effects seems reasonable. Perhaps not all explanations are wrong, nor is any one completely correct if multiple mechanisms apply. And there are different boundary conditions such as ground effect. Apparently not all feel Gail Craig's Stop Abusing Bernoulli is accurate. Not sure that Airfoil Lifting Force Misconception Widespread in K-6 Textbooks provides any better explanation. But rather than dispelling bad explanations, i'd like to read a concise comprehensive description (less esoteric) that accounts for various effects (if there are various effects). I hope no one hasn't any complaints about John Denker's, See How it Flies. I read it 15 years ago. Maybe now i might be able to understand it better. greg forget the forest and trees, maybe i'm lost in the weeds
Nov 18, 2012, 09:24 AM
United States, GA, Atlanta
Joined Oct 2010
551 Posts
Quote:
 Originally Posted by Crossplot Note the relative velocity at the top of the cylinder is 2V : one V is air moving aft and one V is the cylinder moving fwd.
This is exactly my point. This shows that there is significant acceleration along the streamline (meaning not normal to the streamline) because the flow is accelerated up to 2V.
Anyways, I wasn't trying to lead the discussion in this direction; I think there are some good points being made.
 Nov 18, 2012, 02:38 PM Registered User Germany, BW, Stuttgart Joined Mar 2012 1,193 Posts Cross plot, I'm a little confused by your approach. It seems that you are trying to relate the pressure on the surface of a cylinder or wing to the accelerations of the air at the surface. As you point out in the discussion on your website, knowing the accelerations on the surface can only give you the pressure gradients there, not the pressure. In order to find the pressure on the surface, you need integrate the pressure gradients.
 Nov 18, 2012, 02:48 PM Registered User Joined Dec 2011 62 Posts Perhaps I was over enthusiastic about Silva and Soares. They make a lot of bad statements but were the one site that showed centripetal acceleration applied to the wing. ciurpita - I agree that multiple effects can be at play. Perhaps for "Coanda" attachment and the boundary layer. Gail Craig gave me my first heads up on normal acceleration John Denker, at least was, based upon the Bernoulli Principle which we can show is not at play. DPATE - If we look again at that 2V we find that the one v going aft is rhe real velocity of the flow and it is the same one V going fwd that it aquired at the stagnation point If we look at a wing we see the flow across the top is 1.2 - 1.3 relative to the wing. The true flow over the wing has SLOWED DOWN from the one V it aquired at the stag point
Nov 18, 2012, 02:59 PM
Registered User
Joined Dec 2011
62 Posts
Quote:
 Originally Posted by ShoeDLG Cross plot, I'm a little confused by your approach. It seems that you are trying to relate the pressure on the surface of a cylinder or wing to the accelerations of the air at the surface. As you point out in the discussion on your website, knowing the accelerations on the surface can only give you the pressure gradients there, not the pressure. In order to find the pressure on the surface, you need integrate the pressure gradients.
The surface pressure can be calculated from the centripetal acceleration, v^2/r, using the tangential velocity relative to the surface and the surface radius. Again, multiplying by Rho gives the pressure gradient at the surface. Multiplying the surface pressure gradient by the surface radius/2 integrates the pressure change built up across the flow field. Subtracting this sum from the total pressure is then the local static pressure. This calculation simplifies to the form of the Bernoulli energy equation. It now LOOKS like Bernoullis’ flow.

Since you missed it it means I need to do something to highlight it.
The r/2 came from Abbot and von Doenoff.
Nov 18, 2012, 03:01 PM
Registered User
Germany, BW, Stuttgart
Joined Mar 2012
1,193 Posts
Quote:
 Originally Posted by Crossplot If we look at a wing we see the flow across the top is 1.2 - 1.3 relative to the wing. The true flow over the wing has SLOWED DOWN from the one V it aquired at the stag point
Why is it surprising that (in the "still air" reference frame) air has slowed in moving from the stagnation point to the top of the wing?