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Last Site Update:
July 4, 2017
The Aerodynamics Studies main section in the center of this home page on the right received a good, brief definition of the term, "Blade Second Elements" along with their advantages. Studies at the high academic level have been initiated on what awaits more thorough investigations of such a feature.
Don't forget the earlier changes. A more complete treatment of the NoDrag Flap blade mod, earlier introduced briefly, is now presented. It brings into better understanding the difference between the ordinary wind, which is in the earth frame of reference, and the wind in the blade frame of reference, more appropriately known as the "relative wind", which is much faster and at a narrow angle of attack.
Also, the "Hands On(!)" page (link above) has now four videos with sound showing the early development of the double element blade concept done by IntegEner-W in small scale. These entertaining productions provide a good idea of how the blades look and how well the blades perform. This has been a basis for further work.
Titles of Earlier Subsections Removed
Wind Turbine Blade Airflow Deflection Under Analysis
The Issue of How Wind Energy and Aviation Differ
Converting Aviation's Wing Lift and Drag to
Wind Energy's Blade Driving and Thrust Forces
Solving For The Blade Forces That Matter
Air In Motion Does Not Want To Be Deflected!
The Kutta-Joukowsky Condition To Some Degree Invalidated
The Surprising Airfoil Aspect Ratios - Both In Aviation And Wind
Stream Function CFD Software in QuickBASIC
Aerodynamics Engineering Textbook Studies
Wind Energy Airflow Deflection Theory
(See pages one, two, and postscript.)
The Surprising Benefits Suggested by Theory of Pitching Blades
Not Positive, Not to Zero Degrees, But Actually Negative
Evidence From Practice
FRED A Wind Turbine Blade With a First Name
Designing blades with additional blade elements that reach an axial distance upwind of only 12% of the rotor diameter length is what would be necessary to achieve a "full rotor energy delivery" or "FRED" possibility.
The MultiElement aka "Wind Harvester" Blade As Providing Structural Reliability Against Excessively Strong Winds
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The earlier material continues to be valid and is still available by means of email or telephone request. This is a free service and consists of receiving a copy of the last version of the website home page as it stood before this latest change. As always, the material is of academic interest, based on published work within the field as well as original work contributed by IntegEner-W. The subsection titles are all listed above. The copyright still holds but is subject to "fair use" as is acceptable under copyright law. There is to be no abridgement of privacy so long as the copyright is honored. The "Customer Data Base" and "Home Projects" website pages have not been given such abbreviation and so remain intact.
Blade Second Elements
Airfoils Attached Alongside Blades On The Upwind Side With a Gap Between
In as few words as possible, the magic of "blade second elements" is that the wind can be captured while still upwind of and before it reaches the cross section of the two dimensional rotor plane. This is wind that, due to the high speed tangential motion of the blades, may not even find the blade directly ahead of it but would pass between and otherwise escape the blades. It is not necessary to reach very far upwind at all. It is so much easier to access more wind by blade second elements added to the existing three blades than to add entirely new blades beyond three to the rotor. As the blade lengths increase for higher rated turbines, such an advantage has no ready alternative. The three bladed turbine with blades of no more than present day existing lengths can go on by this means to greater power ratings found to be yet unachievable.
The "NoDrag Flap" described hereunder is within the category of a blade second element feature.
The NoDrag Flap Wind Turbine Blade Mod
Blade Cross Section with the Approaching Relative Wind at a TSR = 5
For example, a blade tangential speed of 150 mph and a wind speed of 30 mph
The blade, having approximately a zero degree pitch angle, sees relative airflow moving from the left at an attack angle of 11.3 degrees. It is a little like a sail on a sailboat close hauled in a course headed in an upwind direction. It can be very efficient. The sailboat can go much faster than the wind. The driving force comes from simple airflow deflection. The airflow leaves the trailing edge of the blade moving at about a zero degree pitch angle. So it has deflected 11.3 degrees. This deflection causes the force on the leading edge and above the blade, a combination of driving force and bendback force.
Now look at what is happening. The attack angle is so small that even with a wide blade the approaching airflow sees ahead of it or "faces" only a blade dimension that is equal to "1" in the diagram. To widen the blade for more airflow to face it would take a much larger blade chord dimension with little effective gain in blade faced by the airflow. The idea herein is to keep blades narrow and yet provide plenty of airflow deflection as if the blades were wider.
But if another blade element were to be added beneath the trailing edge as is shown above, much more effective blade width would be gained with little cost or increase in actual blade width. The blade dimension that is faced now would be equal to "2" in the diagram, which is 57.5% larger than "1".
This also makes the blade effectively 57.5% wider in encountering the wind without making the blade actually wider in this case. More wind is deflected and greater power is produced. Included in this website are evidences from testing and practice of successful implementation with improved performance of second element mods such as this to blades.
A Fault in Airfoil Theory
That May Bear on Wind Energy
Drawing From Airfoil Theory vs.
Photo Of Actual Streamlines
Drawing From Wind Blade Patent Application vs. Photo Of Actual Streamlines
Drawings tend often to show streamlines over upper airfoil surfaces that assume a greater Coanda Effect than may actually occur. The flow appears to be receiving a large deflection downward all the way to the trailing edge and beyond. The air has density, which gives it momentum, and does not want to deflect as readily as is assumed therein. This misconception may have an impact on wind turbine blade performance not generally realized.
Added to this is the missing fact that the airflow velocity is not only increased above the airfoil near the leading edge as indicated but is increased also beneath the airfoil near the trailling edge, which causes a lowered pressure there as well.
Corrective measures are sometimes being considered. The vortex generators introduced on the downwind sides of blades have the purpose of increasing the Coanda Effect. However, experience has shown a relatively small effect gained. A possible solution is offered by a second blade element. The element may be a thin flap added under the trailing edge as superimposed in the photo - as is - on the left.
In the image on the right, a better view of the potential effect of this second element is gained. An approximate conversion of the image to the Earth Frame of Reference is obtained by means of skewing. The flow attack angle ahead of the stagnation point then is fixed at minus 90 degrees beneath the airfoil to eliminate the blade tangential velocity vector. It becomes quite clear that the additional element beneath the airfoil moves to the rear providing more effective blade width as shown. If the flow streamlines were adjusted to suit, a better deflection of the entire trailing flow downward and closer to the blade chord line would be seen. Greater airflow deflection, of course, means more power production.
Answers such as this are needed. Small one kilowatt turbines using double element blade concepts have been a remarkable success. Shows to go (!).
The theory which sets the flow lines to follow the airfoil surfaces closely and leave the trailing edge smoothly following its direction is found in early aviation theory publications¹ (page segment copied in "fair use" on the left for review purposes).
What is happening here is that a "clockwise flow circulation" has been postulated around the airfoil profile. The strength of this circulation, termed "gamma" and notated by the Greek letter Γ, is being determined by how well the flow thereby adjusted deflects sufficiently to leave the trailing edge smoothly. This has been given a name: "The Kutta-Joukowsky Condition".
The value of gamma found thereby is seen as the last line in the analysis. But the underlying assumptions have made this analysis superficial and unwarranted. The flow has momentum that overrides any circulation present. The actual value of this parameter can not be found in this manner. In fact, the actual value has never been adequately studied and found as is presently known. There is some deflection occurring but how much is a more complex problem than this. Suffice it to say that this logic has been given more credence than is justified. The evidence from practice now demonstrates that the "Kutta-Joukowsky Condition" is not a valid deflection theory hypothesis. Wind turbine blade design is better approached without it.
And yes, small wind turbines can sometimes readily and quickly cover gaps in the theories. Small is beautiful (!).
¹ From page 52 of Theory of Wing Sections by Abbott and von Doenhoff, 1959, ISBN 486-60586-8, Dover Publications Inc., Mineola, NY.
This is for your benefit, polar bears. Wind has been called an "infinite" source of energy (!)