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Airfoil-missing losses | 15 comments (15 topical)
Re: Airfoil-missing losses (3.00 / 0) (#11)
by finnsawyer on Mon Nov 28, 2005 at 09:39:46 AM MST

You seem to be contradicting yourself.  On the one hand you seem to be saying the leading edge of the blade must be rounded to avoid separation of the air flow, and on the other that all that is needed is for the blade to deflect the air like a ball bouncing off of a wall.  In fact, the air foil doesn't care what the air is doing.  It responds to forces on it, in this case a difference in pressure between the top and bottom surfaces.  This is true for a wing or for a sail.  The air foil shape is chosen because it gives the greatest lift for the least drag.  A board can exhibit lift, but has sizable drag.  It doesn't matter whether one views the process as being kinetic based or lift based.  They are the same, and I recently gave an explanation why?  I won't repeat that here.  You can look it up.  If you have some issues with my explanation, let me know.  From the stand point of anyone trying to manufacture efficient blades, I believe using the lift and drag coefficients for a given airfoil will yield the best results.  I wish we could put the myth that a kinetic approach and a lift approach are fundamentally different to rest.
GeoM
[ Parent ]


Re: Airfoil-missing losses (3.00 / 0) (#12)
by IntegEner on Tue Nov 29, 2005 at 09:07:07 AM MST

It is as plain as day. The blades must "shovel" the wind from one vector direction to another in producing a driving force from the wind, nothing more, nothing less. Blade thickness is not directly necessary to do this. Profiles such as the one suggested here by this individual in Europe are excellent for this purpose but are limited to a narrow range of TSR, outside of which the sharp leading edge will produce separated flow unless rounded.

Checking your posts and your files and diaries provides the insight that you have leaned more toward the electrical side rather than the windflow side of the discussions. I have leaned the other way toward the wind as it impacts the blades and, in this context, I can report that the approach that talks "turkey" like this, somewhat newer in terminology than all the aviation style of talking, is called the "Newtonian" approach (someone else said this first). It "puts the cards on the table", "lines the ducks up", "makes the case in black and white", and etc. instead of the haze surrounding all this "coefficient" talk, valid as it may be to some.

I hope others are reading this as well for it is a point that needs to be made better throughout this discussion list. Please direct me to your post that explains the "kinetic" vs. "lift" ideas and I will gladly look into it.

Knucks
www.integener.com

[ Parent ]



Re: Airfoil-missing losses (3.00 / 0) (#13)
by finnsawyer on Tue Nov 29, 2005 at 10:08:03 AM MST

The blade isn't "shoveling" anything.  It is a passive participant.  The wind is doing all the work.  This is seen most clearly when the blades are standing still and the wind starts blowing.  Insofar as a change in vector direction is concerned, if you mount the blade assembly in a long tube, the direction of the air entering and leaving the tube is the same.  No change in vector direction.  Nothing, nada, nyet, nicht.  Yet the blade assembly can still do work, which results from a change in the magnitude of the wind velocity vector but not its direction.
GeoM
[ Parent ]


Re: Airfoil-missing losses (3.00 / 0) (#14)
by IntegEner on Tue Nov 29, 2005 at 04:48:20 PM MST

Speaking as the intellectual I am supposed to be but have rarely found support for myself on this basis, here is a favorite quote:

Richard P. Feynman, the noted science researcher and Nobel Prize winner (1965 - quantum electrodynamics): "The behavior of fluids is in many respects very unexpected and interesting".

It serves no purpose to debate about some of these details when the wind plays tricks on even the best of us in its "unexpected" ways. Those who have been contributing to the discussions found on this website about the electrical details, the Faraday's Laws and all the rest of it, would be surprised to find that a very great level of detail of a similarly logical and compelling nature exists also within the framework of the interactions between the wind and each inch of the lengths along the blades in observing momentum and mass conservation.

The short answer to the objection you raise is that the walls of the tube just simply redirect all the air deflected by the blades back into a straight path again. This also is why wind tunnels have seen little application in wind energy work - unless the facility is very large the walls interfere with the wind turbine's true operation out in the open.

AVC, www.integener.com

[ Parent ]



Re: Airfoil-missing losses (3.00 / 0) (#15)
by finnsawyer on Wed Nov 30, 2005 at 09:19:04 AM MST

Given your simplistic answer, I will take a stab at it.  It is well accepted that off of the trailing edge of an airplane wing the air has a downward flowing component (your vector direction change?).  As such, it can be resolved into a component perpendicular to the wing and one parallel.  Now an airplane wing is not a good model for the windmill blade, as it is being dragged through the air.  The primary force is drag, the secondary force is lift.  In the case of a windmill blade the primary force is lift, the secondary force is drag.  A better model for the windmill blade would be an airfoil stationary in a wind tunnel.  If there is a change in air velocity parallel to the blade it would show up as a reduction, not an increase, due to drag.  In the case of a blade assembly in a tube, then this backwash can be resolved into a component parallel to the incident wind and a component in the direction of the blade rotation, which would cause a vortex in the tube.  There can be no component perpendicular to the tube walls (the blade assembly completely fills the tube).  The vortex, if it exists, would quickly disappear in the tube as it encounters the tube walls.  In free air the vortex would expand as it moves downstream until it disappears.  The energy in the vortex would be energy lost.  There may, in fact, be no vortex.  As Dr. Feynman implies fluids do not always do as we expect.  If that were the case, then there would be no change in the velocity vector's direction.  Perhaps the "Newtonian Principle" can shed some light on this.  But, as it happens, the drag coefficient accounts well for losses, while the lift coefficient accounts for the usable power from the blade assembly.  This is why I recommend anyone involved in windmill blade design use the lift and drag coefficients.  
GeoM
[ Parent ]


Airfoil-missing losses | 15 comments (15 topical)

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