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Click here for the latest thoughts from this Internet site's editor and author. Also now, for an expanded section on topics of current interest in wind energy engineering that have received our attention, click here. Such topics include suggestions for studies on wind turbine blades that need doing.
A Remarkable Improvement In Power Delivery
News of a Blade Mod of Real Significance
On only a twelve foot tall tower at a local test site in winds measured with a hand held Kestrel wind meter of some 20 - 22 mph, it was creating electrical arcs of such intensity as to blacken and pit the instrument leads. Routine, constant rotation speeds of 1000 rpms (tip speeds of 175 mph) and greater, while lighting full bright eight 50 watt car headlights strung in two parallel rows became the nominal performance. This was something we have seen before in lesser dimensions (look in the left hand margin for the "Power Gap Blades" image).
This concept has worked well for us perhaps due to its similarity to the use of leading edge slats for the high lift case of aircraft wing profiles. (See Abbott and von Doenhoff Section 8.6 on page 225 et seq. of the 1959 edition.) So, in pursuing theories and tests as described herein, we are finding ourselves taking on a task of adding to written theories regarding even aviation development. More to follow as updates are posted. A limited number of these blades, as modified, are available for purchase on the Home Projects page. Click on the link above.
Does the "Gamma Circulation", so highly prized for aviation wings, have relevance to wind turbine blades?
Batchelor says it right out in plain words point blank: "The fact that, in a two-dimensional field, fluid in irrotational motion exerts a side-force, but no drag, on a steadily moving body round which there is a circulation is turned to advantage in engineering. The side-force may be used for instance to support an aircraft against gravity, or it may be used to generate axial momentum of the fluid when the body is one blade of a rotating propeller or turbine..."
(Section 6.7, page 435). Written before wind energy (in 1967), this would also apply to the other situation, that is, the removal of air momentum while extracting its energy. Again, a swirl behind the rotor in both
situations is missing. He, of course, is referring to the ideal case of inviscid flow using pure theory. In practical terms,
some swirl and some flow deflection occurs as is seen in the test data. It is actually some fun to consider what he is
saying. No drag? A "side-force" always at exactly right angles to the oncoming flow? Even (and this is the hard part)
no flow velocity change in magnitude nor direction past the airfoil? Truly miraculous. But this is now what this website
has been coming to accept. It is no wonder that this takes some time to unravel. Wind energy demands nothing but
the very highest perfection in efficiencies in making a go of it. Past habits and conceptions must all be subjected to the
most intense of scrutinies for possible correction. Look at vertical axis. It has fallen before this sword for no other
reason than it still lacks the depth of study that it has always needed.
Further revisions and additions to this topic may be made.
Wind Energy's Blade Driving and Thrust Forces
Solving For The Blade Forces That Matter
Airfoil lift/drag "polar" graphs such as the one at the left (click to enlarge) are a convenient means of selecting airfoil profiles and adjusting their pitch angles for best performance. Examples of such graphs may be obtained by manipulating the "Aerodynamic Characteristics of Wing Sections" graphs in Appendix IV of Abbott and von Doenhoff. Lift and drag force vectors point in directions that have little meaning in wind energy unless some form of conversion is done using them. Also, wind blade aerodynamics includes a second angle in addition to the angle of attack seen in aviation airfoil aerodynamics. The additional angle is the "TSR" angle, the angle between the rotor plane and the incident wind ahead of the rotor plane, as seen by the rotating blade. Click here to continue.
Air In Motion Does Not Want To Be Deflected!
The Kutta-Joukowsky Condition To Some Degree Invalidated
The image on the left is a photograph of streamlines around an airfoil. The flow,
due to its momentum, is not being fully deflected and is breaking off before
reaching the trailing edge, demonstrating what is all too often not given full
enough consideration in blade design. Air in motion resists being deflected.
Airflow angular deflection is all-important in wind energy, where it directly
impacts energy conversion. Airflow angular deflection tends to be greater as
produced by the blades of wind turbines than the wings of aircraft. Conflicting requirements enter the mix due to drag
considerations but it can be said that obtaining good angular deflection requires added attention in wind turbine blades.
Note that the Coanda Effect is maintained here. Turbulent stall is not occurring. The flow beneath the airfoil is taking
a sharp turn around the trailing edge and reaches the upper surface before leaving. The Kutta-Joukowsky Condition,
a basic premise in Fluid Dynamics study and a requirement for the determination of the gamma circulation, G , and,
accordingly, the lift coefficient, is not being met, casting doubt on the benificence of earlier theories.
IntegEner-W has taken steps to address this issue. A wind turbine blade design that reinforces airflow deflection above that
of typical blade designs has been tested on a small scale with good results and submitted to the USPTO for filing and the
first stage of legal protection.
Here is the story. Roughly defined, the aspect ratio is the airfoil length divided by its average chord. Aircraft wings of
high aspect ratio, that is, that are extra long and of a narrow chord, deflecting the incident airflow downwards at a
small angle, can support the fixed weight of an aircraft with less power from the engines. Aircraft wings that are much
shorter but have a wider chord pitched at a larger angle for greater airflow deflection require, to support the fixed
weight of an aircraft, a larger amount of power from the engines...........Click here to continue
STREAM FUNCTION CFD SOFTWARE BASED ON ABBOTT & VON DOENHOFF - Software programs that draw computer screen images such as this airfoil (using the Joukowski model) and stream function y(x,y) line patterns such as these to the right, comprising TEN progressive CFD programs in BASIC, are now available. The example output screens demonstrate a flow pattern under study. Up to 7 input parameters can be adjusted. A BASIC compiler is also included (normally supplied in MS Windows). This help is invaluable for the teaching and learning of material in fluid dynamics textbooks as applied to wind energy problems. To download the .2 MB directory, click here.
Note that a flow pattern of "circulation-free deflection" is also covered in this study as provided by the FLOWDEF01.BAS program in the above list. This makes use of one each of the stream (and cross stream) functions available in Section 6.5 of G. K. Batchelor, "Use of the complex potential for irrotational flow.." A typical output screen image is on the right. Energy transfer is implied here, which makes this a more suitable representation of how wind turbine blades perform as opposed to aviation wherein a high lift-to-drag ratio is the main requirement. Nevertheless, again, all of these programs make excellent course work study for those entering the field. A background in calculus, normally necessary in reviewing the textbooks mentioned below, is not required.
Engineering textbooks Theory of Wing Sections by Abbott and von Doenhoff, An Introduction to Fluid Dynamics by G. K. Batchelor, and Advanced Mathematics for Engineers by Reddick and Miller are well known as being classics covering Fluid Dynamics Theory. As an aid in elucidating the material and converting the analyses more directly to wind energy while retaining their rigor, a sequence of written papers in .pdf format has been placed on-line. To download the 3 MB directory in compressed .zip format, click here.
Traditional theory, as discussed in the books above, makes use of superposed stream functions to return pressure distributions along the surfaces of the airfoil cross sections, requiring integrations to find the forces. The air is necessarily assumed to be inviscid, that is, to have no viscosity. In a somewhat different theory, lift and drag can also each be described as force vectors acting on the airfoil that follow directly from Newton's Law rewritten for averaged fluid flows in the Eulerian specification. Here the air viscosity and resulting profile drag can be given some attention. This less developed theory, although more readily appealing to intuition, awaits further work to give flight its full capability. The approach supported herein is to consider both theories together. A well known photo on the left shows a highly developed "downwash" occurring behind the aircraft, which is more in line with the force vector theory. For a good, brief, personal summary of the questions involved here, click here. To proceed further for a download of the detailed linear deflection force vector theorizing, click here.
TEST SERIES VIDEO ONE This is the same 51 inch diameter blade rotor under test as seen in video two just below mounted on the 14 foot wooden tower and powering a much larger generator now than the one previously used. It is a Baldor one horsepower 90 volt DC motor used as a generator. Each blade consists of one 18 inch metal cambered blade plus one 24 inch flatter sheet metal second blade with a gap between connected together with struts designed to give the rotor more airflow deflection capability. The load is now much greater, about two dozen 12 volt lights paired in series, including many 50 watt halogen auto headlights. Just click on the image for a fast download of the 530 kb video of the blade rotor powering the lights.
Several sites where can be found electrical energy generating equipment, particularly diesels and gas turbines, of ratings suitable for standby utility-grade application near wind energy projects are the following:
Two important periodicals that cover the latest on the renewables in the electrical energy field can be found at the following locations:
An overlooked idea on transportation that would interest EV and pedal-power supporters (recommended for its to-be-desired mundane appeal) is: BikeTrans
Progress continues to be made with geothermal heat pumps for occupied spaces heating and cooling. The ground has a high thermal capacity and is a deep source of heat flow and, as long as adequate measures are taken to limit wall and roof heat energy loss or gain, can be an efficient method of temperature control. Two associations that provide information on this technology are the International Ground Source Heat Pump Association and the GeoExchange. A supplier:
Compact fluorescent lights are now improved and four times as efficient as the same brightness of incandescents. Much can be said in their favor, including significant impacts on electrical power generation once adopted by homeowners and businesses everywhere. Several sites that describe them (now including dimmable varieties and the new "Mini-Spirals" that are as small or smaller than ordinary bulbs) are:
Sylvania also has the new Light Emitting Diode (LED) super bulbs.