Lift Generation

[Charge]

Not to mention the effects of laminar flow...

-C+

[SgtPappy]

Wow Bozon. I think I'm starting to get it. I read something on NASA recently ... about 'turning' the airflow and more 'turning' meant more lift. They actually gave the teaspoon example as well.

Don't worry about the physics; if you ever want to post the physics, that's fine since I really like the subject and I'm trying to get into it. Keep it simple though haha. At any rate, I'm having a little trouble visualizing what you said. Is the curved edge of the spoon supposed to be the leading edge and the back side the trailing edge? Sorry I'm a tad slow.

Perhaps you could explain to me in terms of Die Hard's animation of the airfoil.

[bozon]

Don't take the teaspoon example too seriously. Any flat surface that you put inside the liquid will do. What this comes to demonstrate is that the flow, when it meets a "corner", will tend to form a circular vortex flow behind it, and the direction of rotation will be "into" the corner.

Dip the "teaspoon" (or you other flat surface) only a little, so the widest area is in the water. When you move the teaspoon in the liquid, so that the direction of the flow is against the wide surface (concaved or convexed, doesn't matter), you will notice the circulation patterns on each side of the spoon, around the "corner". This is NOT like a lift-generating wing. What the wing tries to do is create this circulation only on one edge (trailing edge) but prevent a local vortex from forming at the leading edge. This breaks the symmetry and forces the counter rotation (that counters the vorticity in the trailing edge) to be formed elsewhere - around the entire wing.

It is a fragile (and unstable) pattern and this is why the wing is limited to a very small tilt (angle of attack) range of ~15 degrees. Beyond that, the large circulation will break into smaller circulations - typically forming the counter rotation only above the wing (behind it, from the flow point of view), instead of around it.

If you want to use Die Hard's image. Look at the yellow background one that says "accelerated" flow. Ignore the "accelerated" part and look only at the bottom left figure. You can see the trailing edge vortex, but no leading edge vortex. The "global" circulation is the air flow velocity difference between above and below (which is not drawn unfortunately). Any sheer in velocity is a kind of circulation. The mathematical definition is that the  path-integral of the velocity vector field, around the wing, is non-zero.

When you understand this type of vortex flow, you will realize that there has to be more, smaller, vortices in the flow (places where the velocity sheer is strong). This is where the real difficult physics is hiding and what makes this problem so difficult analytically. A professor of mine, an expert in hydrodynamics, used to say that he still thinks it is a miracle when he sees a plane actually lifts off the ground.

[SgtPappy]

Hah it is difficult but I suppose this is why I'm so interested in physics class. Thanks for the explanation Bozon. I'm understanding it much better now.

[Charge]

"Don't take the teaspoon example too seriously. Any flat surface that you put inside the liquid will do. What this comes to demonstrate is that the flow, when it meets a "corner", will tend to form a circular vortex flow behind it, and the direction of rotation will be "into" the corner."

Or expand the low pressure theory from there. Not that you could actually do it or notice any lifting forces but if you would move the spoon sideways through the fluid in straight motion, turned the way you normally use it with leading edge slightly up and according to low pressure theory there would form a low pressure in the "cup" which would create "lift" causing the spoon to slightly go upwards. But thats not how it happens but the spoon would want to go down according to vortex generation theory.

Or think about frisbee. Try throwing it upside down and see what happens. Frisbee is interesting because the shape effect is very evident there but how would frisbee fly if it did not rotate? Is rotation mandatory to its lift generation or is form effect enough. Or does rotation simply stabilize it in flight. If it does, why? One would think that due to laminar effect the frisbee would drift to opposite direction from its rotation unless thrown slightly tilted to compensate but I don't think it does. Why does not laminar effect work here if if works for a golf ball?

This is interesting approach to aerodynamics too: http://www-scf.usc.edu/~tchklovs/Proposal.htm   

Sorry if I'm degrading the conversation but I'm a simple man and I try to reach complex things through understanding simple examples first.

-C+

[Stoney]

if it works for a golf ball?

Golf ball has dimples.

[bozon]

Or think about frisbee. Try throwing it upside down and see what happens. Frisbee is interesting because the shape effect is very evident there but how would frisbee fly if it did not rotate? Is rotation mandatory to its lift generation or is form effect enough. Or does rotation simply stabilize it in flight. If it does, why? One would think that due to laminar effect the frisbee would drift to opposite direction from its rotation unless thrown slightly tilted to compensate but I don't think it does. Why does not laminar effect work here if if works for a golf ball?

A frisbee can fly upside down. Not so well though. When you flip the frisbee over, you change the "corners" that the airflow meets. The flow going over the leading edge now meets a sharper edge instead of the curved edge (now curved downwards). This encourages a vortex to be created over the leading edge - not good for lift generation. On the other side, the trailing edge is now curved upwards and produces a weaker vortex behind the frisbee.

The rotation of the frisbee serves the purpose of stabilizing it in flight, not generating lift. A sudden tip of the disc while in flight will lead to a precession that (if not to large) will decay quickly (it dissipates the energy to create small fluctuation/vortices in the airflow). The rotation does create some sideways "lift". Notice that the flight path of the frisbee (if not tilted) tends to curve to the side that rotating "into" the airflow.

As Stoney mentioned, the dimples in the golf ball serve to increase the lift generated by the rotation (better stirring of the air due to ball's spin). The stripe on the baseball serve a similar purpose and allow effective curve-ball pitches. The knuckle-ball pitch tries NOT to rotate the ball as it leaves the hand. This creates a very unstable airflow due to the interaction of the air with the stripes on the ball, that shifts the ball in random directions as it travels.

[Stoney]

And, primarily, the dimples keep the boundary layer flow attached, reducing drag and increasing the distance the ball will fly.  I'm not so sure that "lift" has as much to do with a golf ball's trajectory/distance.

[Charge]

"I'm not so sure that "lift" has as much to do with a golf ball's trajectory/distance."

The point with the golf ball and laminar attachment to those "dimples" is that while the laminar airflow is attached to ball's surface it will have effect on surrounding airflow at certain speed range IF the ball rotates around its horizontal axis which will alter its trajectory which can be described as lift, especially if it pulls it upwards.
"How a Golf Ball Produces Lift

Lift is another aerodynamic force which affects the flight of a golf ball. This idea might sound a little odd, but given the proper spin a golf ball can produce lift. Originally, golfers thought that all spin was detrimental. However, in 1877, British scientist P.G. Tait learned that a ball, driven with a spin about a horizontal axis with the top of the ball coming toward the golfer produces a lifting force. This type of spin is know as a backspin.

The backspin increases the speed on the upper surface of the ball while decreasing the speed on the lower surface. From the Bernoulli principle, when the velocity increases the pressure decreases. Therefore, the pressure on the upper surface is less than the pressure on the lower surface of the ball. This pressure differential results in a finite lift being applied to the ball."

"So, why dimples? Why not use another method to achieve the same affect? The critical Reynolds number, Recr, holds the answer to this question. As you recall, Recr is the Reynolds number at which the flow transitions from a laminar to a turbulent state. For a smooth sphere, Recr is much larger than the average Reynolds number experienced by a golf ball. For a sand roughened golf ball, the reduction in drag at Recr is greater than that of the dimpled golf ball. However, as the Reyn olds number continues to increase, the drag increases. The dimpled ball, on the other hand, has a lower Recr, and the drag is fairly constant for Reynolds numbers greater than Recr.

Therefore, the dimples cause Recr to decrease which implies that the flow becomes turbulent at a lower velocity than on a smooth sphere. This in turn causes the flow to remain attached longer on a dimpled golf ball which implies a reduction in drag. As the speed of the dimpled golf ball is increased, the drag doesn't change much."

What I'm trying to bring into this discussion is the consideration of how air itself can work on an object (other than a wing) to produce lift. Ball, pole (spear) etc.

-C+