White Paper

THE PHYSICS OF TUBERCLE AIRFOILSand

THEIR APPLICATION TO WIND TURBINES

WhalePower Corporation

INTRODUCTION

Tubercles are bumps formed on the leading edge of an airfoil with inter-tubercular channels between each pair of bumps. The Tubercle Effect was first discovered during research conducted by Dr. Frank Fish1 into why humpback whales have tubercles (or bumps) on the leading edges of their flippers.

Dr. Fish's early studies of the whale flippers suggested that the bumps delayed stall and improved lift, contributing to the huge whale's remarkable agility.

1 Dr. Fish runs the Liquid Life Laboratory at West Chester University and is the founding president of WhalePower.

A NEW LEADING EDGE YIELDS A NEW KIND OF FLOW MANAGEMENT

Over the past decade, engineers and physicists from all over the world have studied tubercle airfoils and have concluded that they are more than a new design: They represent a new kind of fluid flow management which delivers a suite of aerodynamic improvements.

Tubercle airfoils always stall gradually leading edge stall angles can be dramatically increased higher pitch operation produces increased lift at higher angles of attack the drag penalty is significantly reduced span-wise pumping and tip stalling can be attenuated and even eliminated

completely. backpressure at the trailing edge is attenuated and Kármán vortex vibration

can be better managed

That combination of performance characteristics means that tubercle airfoils are hyper-stable, very efficient, very quiet and always stall gradually.

This Paper summarizes a number of the most significant studies published in peer reviewed engineering and science Journals. The findings define and explain the aerodynamic characteristics of tubercle airfoils. Combined with engineering research and development carried out by WhalePower Corporation this document explains the demonstrable advantages of tubercle airfoils as they apply to wind turbines including both fully engineered tubercle rotor blades and the application of retrofit elements to be attached to wind turbine rotors.2

2 These factors are also important to optimization of the aerodynamic performance of other rotational airfoil devices such as axial fans, compressors and pumps.

THE TUBERCLE EFFECT: A new technology inspired by nature

The early work on whale flippers by Dr. Fish led to the first significant scientific study of tubercle aerodynamics conducted at the US Naval Academy wind tunnel and published in the prestigious peer-reviewed scientific journal Physics of Fluids3.

This paper made it abundantly clear that the aerodynamic performance of tubercle airfoils is very different from conventional laminar airfoils and that tubercles delivered a suite of potential advantages. The authors concluded:

"We show, through wind tunnel measurements, that the addition of leading-edge tubercles to a scale model of an idealized humpback whale flipper delays the stall angle by approximately 40%, while increasing lift and decreasing drag.4”

The exceptional delay of the stall angle means that tubercle blades can operate at a significantly higher pitches than conventional laminar flow blades and that higher Angles of Attack ("AoA") make it possible for tubercle airfoils to produce more lift. Higher lift combined with lower drag is a formula for improved aerodynamic efficiency.

3 Miklosovic, D. S., Murray, M. M., Howle, L. E., and Fish, F. E.,“Leading-Edge Tubercles Delay Stall on Humpback Whale(Megaptera novaeangliae) Flippers,” Physics of Fluids, Vol. 16,No. 5, May 2004, pp. L39–42.4 Source ,” Physics of Fluids, Vol. 16, No. 5, May 2004, pp. L39–42.

This pair of conventional and tubercle blades were designed for the tests in the US Naval Academy Wind Tunnel. Superficially they appear to be very similar: both have the same surface area, both have approximately the same curvature, but there were major differences in the performance.

The tubercle airfoil delivered

dramatically better stall performance higher lift lower drag attenuation of span-wise flows and tip

stalling

CHANGING THE TURBINE BLADE DESIGN COMPROMISE

Every wind turbine and blade design is a compromise which must be capable of efficient operation despite changes in wind speed. Over the years, many high lift airfoils have been developed using conventional laminar airfoil principles.

In general, blades designed to operate at steeper AoA can produce more lift. But operating at higher pitch will also increase stalling when wind speeds drop. Worse a higher lift blade design typically increases drag as well. Beyond a threshold, drag rises proportionally faster than the increase in lift. This increase in drag imposes severe limitations on laminar airfoils because aerodynamic efficiency is proportional to lift divided by drag (l/d). Consequently as the Angle of Attack rises the laminar airfoil quickly reaches a point where the increase in lift cannot compensate for the impact of the rising drag5. In fact, as that ratio worsens, the net effect is that, despite the increased lift, the increased drag actually reduces the power that gets to the generator

The contrast with tubercle airfoil performance is striking: The Physics of Fluids study also found that while the stall angle increased drag was reduced by as much as 32%. Combined with increased lift, decreased drag pointed to increased aerodynamic efficiency.

5 This situation might be tolerable in jet fighters, because a jet fighter can compensate for high drag by essentially overpowering it with its thrust' However, if the engine stops working in such a maneuver the pilot usually has to eject before he crashes to earth. Obviously powered compensation is not a practical option for wind turbines.

PROOF OF CONCEPT TESTING OF TUBERCLE WIND TURBINE BLADES

Surface finishing retrofitted blade. Retrofitted blade ready to be installed at WEICan.

WhalePower retrofitted a conventional Wenvor 10-meter 2-blade 25 kW turbine by fabricating and attaching leading edge tubercle elements to produce the first tubercle wind turbine6.

The retrofitted turbine was tested for several months at a site near Guelph Ontario before the blades were transported to Prince Edward Island for third party testing by the Wind Energy Institute of Canada (WEICan). The results were very encouraging and the final WEICan report is attached as an the Appendix to this document, as is an analysis of its performance improvements prepared by Dr. Laurens Howle of Duke University7.

The results were very encouraging: Howle found that annual power production was increased by 20%. In addition, the notoriously noisy Wenvor turbine which is prone to much "tip chatter" due to tip stalling became very quiet, and responsiveness to rapid changes in wind speed was significantly improved.

6 As can be seen in the photos, these retrofit bumps can be engineered to transform a conventional blade.7 These documents are also available on our website http://www.whalepowercom

Obviously, a commercial 2 MW turbine has a diameter 10 times that of this test turbine but , nevertheless, there are a few interesting points of comparison. For example, the blade tip speed on this 10 meter diameter turbine turning at 120 rpm is fully 80% of the tip speed of a typical 100 meter diameter, two megawatt commercial wind turbine rotating at its normal 15 rpm.

SO WHY DO TUBERCLES WORK

Tubercles have been described as operating on the flow field in a manner comparable to that produced by aircraft strakes8. The Physics of Fluids paper noted that tubercles also appear to function in a manner similar to smaller vortex generators9. In effect, it appears that both vortex generators and tubercles energize the boundary layer flowing over the wing to keep it attached and in so doing maintain lift at higher angles of attack (AoA). Both improve lift by generating counter-rotating spiral flows which delay stall and energize the boundary layer. In effect, tubercles act as a flow management system and also act as a stall-control system10. This is can be seen in this photo of a hydrofoil with tubercle leading edges in a water tunnel. The flow clearly forms into counter-rotating vortices about half way up the sides of each inter-tubercular channel.

Photo: Dr. Jamid Johari

As with vortex generators the counter-rotating flow pattern for tubercles appears to be central to developing improved lift and the resistance to stall. However in this case, the Tubercles' counter-rotating spiral pattern is only coherent for about 40% of the chord behind the leading edge. it is not maintained as the flow

8 Strakes are large vortex generators.9 Most vortex generators are small vane shaped elements usually attached in pairs to the lifting surface of wings and rotors which increase lift and delay stall. 10 "On the Role of Leading-Edge Bumps in the Control of Stall Onset in Axial Fan Blades", Alessandro Corsini,Giovanni Delibra, Anthony G. Sheard, asme.org

progresses toward the trailing edge. The muddled flow at the bottom of the photo shows drag and the test measurements confirmed that interpretation.

Superficially, that looks like bad news for tubercles. Indeed, the paper by the same authors entitled "Effects of Leading Edge Protuberances on Airfoil Performance"11 makes clear that for some angles of attack and under some test conditions a conventional laminar airfoil will outperform tubercle airfoils. The graphic below summarises the test results.

Extended abstract presented to the 36th AIAA Fluid Dynamics Conference

Lift coefficient for the airfoils pictured above.

CONTRADICTORY SCIENCE? RESOLVING EARLY DOUBTS

11 Effects of Leading Edge Protuberances on Airfoil Performance, A. Levshin,† and H. Johari* and C.W. Henoch**

submitted to the 36th AIAA Fluid Dynamics Conference

The results from this flow tank test indicated that the combination benefits of increased lift and reduced drag found in the Naval Academy study did not appear in the water tunnel test. In fact when that was first reported about 8 years ago, it appeared to be a fundamental contradiction of the wind tunnel test at the Naval Academy. However, after further research it turned out that both studies are reliable.

The explanation of this apparent contradiction lies in the fact that the two experiments employed fundamentally different physical tests. The water tunnel test was conducted with the airfoil sections mounted wall to wall and with no space at either end. That means the test blades had no tip exposed to the flow of air or water. This test condition is generally referred to as a 2-D (two dimensional) test12.

For clarity it should be noted that 2-D tests are sometimes also referred to as "full-span" or "infinite" tests. The different names appear in different papers and sometimes even within the same paper. To make this perfectly clear: Full-span, infinite and 2-D tests are all the same.

By contrast, the original test in the Naval Academy wind tunnel had the blades mounted at the root only which left the blade tips free. That makes it a different kind of test, a 3-D test which is also known as a semi-span or finite test13.

TUBERCLES ARE 3-D AIRFOILS THAT ALWAYS STALL GRADUALLY

The full-span (2-D) water tunnel tests demonstrated that Tubercle airfoils don't work the same way as they do in semi-span (3-D) tests. Interestingly, at the same time both kinds of tests showed the remarkable ability of both 2-D and 3-D tubercle blades to continue to generate substantial lift after stalling. It should be noted that this performance characteristic is part of the evidence supporting the conclusion that tubercle airfoils always stall gradually.

12 "Effects of Leading-Edge Protuberances on Airfoil Performance", H. Johari∗ C. Henoch†Naval Undersea Warfare Center, Newport, Rhode Island and D. Custodio and A. LevshinAIAA JOURNAL Vol. 45, No. 11, November 200713 Here again there are three different names for the same type of test, (3-D, semi-span and finite tests) and once again they are all the same thing.

THE DEMONSTRATION OF INCREASED LIFT AND DECREASED DRAG IN 3-D In order to resolve the apparently contradictory findings between 2-D and 3-D tests a major study was conducted by Miklosovic, Murray and Howle14 in two wind tunnels at the US Naval Academy.

The 2-D wind tunnel proved that tubercle airfoils behaved very differently in 3-D tests. In fact, the semi-span 3-D test overcame the performance limitations displayed in the full-span (2-D) tests .

In summary, the vortical flow induced by the sinusoidal Leading Edgereduced the effectiveness of the full-span (2-D) wing by triggering earlyseparation but enhanced the effectiveness of the half-span (3-D) wing byinhibiting spanwise stall progression thus extending the operatingenvelope with minimal performance penalties15.

The characteristics discovered in the 2-D water tunnel tests were replicated in the 2-D wind tunnel test and the findings of the 3-D test agreed with the Naval Academy Wind Tunnel test. The authors concluded that tubercle airfoils are most applicable to 3-D operations, such as wind turbine rotors, because tubercles are inherently 3-D structures.

Though the aerodynamic mechanisms of the scalloped leadingedges are similar, the effects were vastly different between the

14 JOURNAL OF AIRCRAFT Vol. 44, No. 4, July–August 2007 "Experimental Evaluation of SinusoidalLeading Edges", David S. Miklosovic & Mark M. Murray U.S. Naval Academy & Laurens E. Howle‡Duke University,15 The study also reported that with "... the vortex-dominated flow over the wings with Leading Edge scallops... the maximum loss in prestall lift for the semispan (3-D) wing was 81% lower than the full-span wing; the poststall gains were 65% higher.... the maximum gain in prestall drag for the semispan wing was only 6% of the full-span drag increment. The poststall drag reduction was enhanced by almost fourfold. The maximum loss in prestall moment was 88% lower compared with the full-span wing and the poststall gains were 40% lower.

infinite (2-D) and finite (3-D) wings. The substantial loss in lift and increase in drag that accompanied the full-span16 results, but not the semispan17

results, means that the scallops18 had largely a 3-D benefit that is afunction of the planform shape and the Reynolds number. At prestallangles of attack, the trends were the same (decrease in lift andmoment, increase in drag) but the 3-D effects were much smaller inmagnitude. The poststall behavior showed the opposite trends as aresult of the scallops: increasing lift and moment, decreasing drag.19

The generation of vortices by the scallops was beneficial only to 3-Dplanforms in the range of Reynolds numbers tested. The authorsexpect that airfoils with tubercles might have utility on liftingsurfaces that are required to operate past their stall point, such as windturbine blades where power generation at lower wind speeds remainsa challenge. A flat poststall lift curve as seen in the full-span testresults might be beneficial when the blades are operating in theneighborhood of the stall angle for lower speed, unsteady winds.20

TUBERCLES ARE NOT JUST COMPLICATED VORTEX GENERATORS

It was noted earlier in this document that tubercle performance has some similarities to the operational mechanics of vortex generators and there are indeed similarities in the aerodynamic mechanisms at work with both tubercles and vortex generators. Both generate paired vortices, delay stall and therefore increase lift - but there was an even more important revelation in the results of the wind tunnel tests conducted by Miklosovic et al which demonstrated a fundamental difference between tubercle and vortex generator performance.16 That is 2-D or infiinite 17 That is 3-D or finite18 "scallops" are tubercles19 Emphasis added20 JOURNAL OF AIRCRAFT Vol. 44, No. 4, July–August 2007 "Experimental Evaluation of SinusoidalLeading Edges", David S. Miklosovic & Mark M. Murray U.S. Naval Academy & Laurens E. Howle‡

Duke University,

...the vortical flow induced by the scallops did not produce the same result as traditional vortex generators, which tend to increase both CL;max and CD;min21 together.

This finding is fundamentally important: Tubercle airfoils increase lift and lower drag. But vortex generators increase lift and drag together. That means that vortex generators can, at best, offer limited gains in aerodynamic efficiency22. However, unlike vortex generators, tubercle airfoils increase lift and lower drag. Since aerodynamic efficiency is a function of lift divided by drag (l/d) then tubercles pay a substantially lower drag penalty. The bottom line is that the combination of increased lift and lower drag is a formula which allows tubercle airfoils to outperform vortex generators.

Tubercle airfoils are inherently 3-D which is a natural fit for wind turbine blades which are inherently 3-D structures which are mounted at the root and have a free tip.

NUMERICAL ANALYSIS SUPPORTS THE EXPERIMENTAL RESULTS

A number of numerical studies have also concluded that tubercle airfoils hold great promise for wind turbine blades23.

The aerodynamic characteristics of a kind of bionic wind turbine blades with a sinusoidal leading edge have been investigated in this paper based on a three-dimensional Reynolds-averaged Navier–Stokes simulation. The calculated results show that compared with a straight leading-edge blade, the new-type blade has a great improvement in shaft torque at high wind speeds. The localized vortices shedding from the leading-edge tubercles, which can generate a much greater peak of the leading-edge suction pressure than that from the straight leading-edge case, are the physical essentials to enhance the wavy blade’s aerodynamic performances as the blade goes into stall.

21 CL;max and CD;min are respectively Maximum Coefficient of Lift and Minimum Coefficient of Drag 22 Since efficiency is proportional to lift over drag (l/d).23 Ri-Kui Zhang, (Peking University), and Jie-Zhi Wu, (U of Tennessee) "Aerodynamic characteristics of wind turbine blades with a sinusoidal leading edge" WIND ENERGY. 2012

This is a very interesting finding. In fact, the WhalePower Wenvor retrofit study found that the tubercles generated greater power in higher speed winds24. However, while this numerical analysis suggests a substantial benefit for tubercles on wind turbine blades the mathematical model25 employed in this study does not include all of the potential for improvement for wind turbine performance.

ATTENUATED SPANWISE PUMPING: ANOTHER SIGNIFICANT ENHANCEMENT

The combination of increased lift and lower drag alone is probably enough to recommend the application of tubercles for the improvement of the performance of wind turbines, but there is more.

[Tubercle leading edges] enhanced the effectiveness of the half-span wing by inhibiting spanwise stall progression26 thus extending the operating envelope with minimal performance penalties27.

This finding is important because every turbine rotor suffers performance penalties due to detachment of air which causes spanwise pumping, spanwise stall progression, and, tip stalling. These losses have been "givens" which engineers must work to minimize even though they cannot be eliminated. Consequently, these factors have stood as inescapable problems for all previous airfoils designs for rotating aerodynamic machines like wind turbines.

TUBERCLES CHANGE THE RULES FOR SPANWISE PUMPING AND TIP STALL

Spanwise pumping and tip stall impose significant penalties on the aerodynamic efficiency of any wing or rotor blade. On laminar flow blades, spanwise flows begin when attached air detaches freeing it to flow toward the tip. As they move toward the tip they not only contribute little or no lift, they routinely actually disrupt some of the attached flows as they pass and increase drag. This is a 24 Please refer to the analysis conducted by Dr. Laurens Howle (Duke) attached to this paper. Note that the tubercle retrofit blades generated maximum power at a wind speed of 12.1 m/s vs a speed of 15 m/s for the stock blades. It is also notable that a wind speed of 12.1 m/s the power available is only slightly over half of that available at 15 m/s because the power in the wind is proportional to the cube of the wind speed. That pattern is central t o delivering gains.25 3-D mathematical studies are substantially more complex than 2-D and engineers in the wind industry have only recently moved to incorporate them in their development process.26 Emphasis added27 Miklosovic et al in Journal of Aircraft.

complex patter because some of these flows reattach momentarily and therefore recapture some of the lift potential, but on balance, this mechanism lowers efficiency and degrades performance28.

The losses as spanwise pumping disrupts lift mechanisms are significant but the impact becomes even more damaging when it leads to tip stall. Tip stall literally throws some of the power generating potential in the airflow off the tip of the blade. It is worth noting that even an airplane wing can suffer from spanwise flows and loss of attached flows off the tip. In aircraft, tip stalling reduces aerodynamic efficiency and increases fuel consumption. That is why modern aircraft are frequently fitted with winglets29 at the blade tips.

Obviously, suppressing the progression of spanwise flows and tip stalling are important goals for the development of efficient wind turbine blades. Rotating machines like wind turbines and fans suffer more serious spanwise pumping and tip stall than do straight airfoils because, as they rotate, centrifugal forces literally throw the partially detached air toward the tip where it frequently detaches completely in a tip stall. The impact of a tip stall on performance is immediate. The graphic on page 6 shows how lift plummets when a conventional airfoil stalls. Similarly, a tip stall causes the very substantial lift being generated near the tip to plunge. That same graphic shows that tubercle airfoils continue to generate substantial lift beyond the stall point. The bottom line is simple: all rotating aerodynamic blades - turbines, compressors, axial pumps and fans are made less efficient by spanwise flows and tip stalling30. Conventional airfoil designs have very limited ability to attenuate spanwise pumping and tip stall. Tubercle airfoils present a new range of superior options that conventional laminar airfoils can't match.

28 In number of experiments and studies at NREL in the USA, engineers have noted that a portion of the stalled air pumped from root toward the tip can actually increase power generation in the mid-span region by recapturing part of such detached flows and contemporary blade designs work to enhance that effect. However, inevitably there is a reduction in total aerodynamic efficiency.29 Winglets are aerodynamic devices usually mounted at the wing tip at an angle approximately perpendicular to the span. In effect, they serve as a wall blocking flow off the tip and sending it back onto the blade where it can potentially reattach. It is comparable to a fan shroud and, while winglets can improve aerodynamic efficiency, winglets suffer from the same limitations, further detachment and reattachment and disruption of invicid flows leading to momentary dramatic rises in drag.30Enercon, a leading turbine manufacturer has been experimenting with winglets on its rotor tips with some success over the past decade.

TIP STALL DRIVES BOTH NOISE AND BLADE DAMAGE

Tip stall involves more than a simple reduction of lift. The detachment of vortices near the tip is not a quiet process: The physical force of that abrupt detachment effectively plucks the blade and generates blade vibrations which generate both audible noise and, on longer blades, very low frequency subsonic noise, (sometimes referred to as "infra-sound"). Engineers at the US National Renewable Energy Laboratory (NREL) conducting the various wind tunnel studies of unsteady blade aerodynamics estimated that tip stall was responsible for generating in the range of 85% of turbine blade noise31.

In wind turbine blades the tip stall noise is significant enough to interfere with wind farm siting, requiring large turbines to be installed at greater distances from homes, etc. The associated vibrations are powerful enough to contribute to or cause blade degradation, laminate de-bonding and even breakage.

CONVENTIONAL METHODS TO REDUCE SPANWISE FLOWS AND TIP STALL

Shrouds have been employed for over a century to block air flowing off the tips. This wall forces some of the air back onto the blade where it can reattach (at least momentarily) to generate more lift. However such shroud blockage itself has complex characteristics (sometimes called "wall effects") which typically increase the level of instability of these spanwise flows. As this destabilized flow is deflected back onto the blade only a portion of it will in fact reattach momentarily before being detached and stalling again.

The net effect is usually a somewhat limited gain in efficiency accompanied by a substantial increase in noise and potentially damaging vibration. This noise is due to the additional stall events and the associated blade vibrations that they cause. In some cases these vibrations are known to degrade the blade structure itself

In sum, laminar flows are inherently unstable and, even when modified by shrouds and winglets, have serious limitations when applied to rotating machines.

But tubercle airfoils are fundamentally different.31 Private communication with James Tangler, formerly senior NREL engineer on the studies focusing on Unsteady Aerodynamics.

THE TUBERCLE DIFFERENCE MAKES A DIFFERENCE

There is a consensus among researchers that tubercle leading edges attenuate spanwise flows without requiring winglets. For example Mark W. Lohry, David Clifton and Luigi Martinelli have noted this important feature.

We have also shown that a variation in thickness along the span, which creates channels of sorts along the chord, can be used to break up the separation regions and create spanwise fences, which can increase Clmax. (Maximum Coefficient of Lift) 32

There is no question that these virtual "fences" are generated by Tubercular flows around the bumps and through the channels and this creates a complex of 3-D aerodynamic effects. One WhalePower experiment illustrates this performance very clearly.

The photo is a frame from a video made with the camera mounted on the hub of the 24-foot fan.33.

32 Characterization and Design of Tubercle Leading-Edge Wings - Seventh International Conference onComputational Fluid Dynamics - Mark W. Lohry, David Clifton and Luigi Martinelli.33 These video frames were shot by a camera mounted on the hub of a 7 meter fan with tubercles on the outer 60% of the span. The tape line on the left photo marks the point where tubercles begin. The photo on the right shows the outer 60% of the blade and shows that flow behind the tubercles has completely eliminated the spanwise flow.

WhalePower modified a prototype of the big Altra-Air 24 foot fan34 so that the inner 40% of a blade had a smooth leading edge and the outer 60% had tubercles. A tape line marked the spot where the tubercles began and streamers were added to help visualize the flow.

This fan was rotated at 55 rpm at an operating pitch of 22 degrees. That angle is sufficient to make the root portion of the blade (without tubercles) stall at which point spanwise pumping takes over and the air flows directly toward the tip.

The picture on the left also shows what happens when the spanwise flow reaches the tape line where the tubercles start. The spanwise pumping is abruptly stopped and in a short space the flow becomes perpendicular to the span and attaches again. The still on the right shows that this pattern of properly aligned flow continues all the way to the tip35. It is important to note that there was no indication of tip stalling and fan noise was reduced dramatically36.

No such study of the wind turbine blade was conducted but the test turbine was exceptionally quiet and the tip chatter that is normally produced by the stock Wenvor turbine virtually disappeared.

It is important to re-emphasize that only part of these gains delivered by tubercle blades are accomplished through higher lift due to higher angle of attack and 34 This fan was engineered by WhalePower and licensed to Envira-North Systems who market it worldwide.35 A short selection of the video is available at ________36 Noise reduction for the tubercle blades vs. laminar blades was dramatic. The hub and motor with no blades at 55 rpm produced 54 db. With normal blades installed the noise level rose to 64 db at 55 rpm moving about 200,000 cfm. The WhalePower tubercle blades produced only 56 db at 55 rpm while moving 25% more air (250,000 cfm) and consuming 20% less power. The additional noise was mostly due to motor load and airflow.

reduced drag: The control and/or elimination of span-wise pumping and elimination of the shedding of tip vortices provides further contributions to efficiency gains and operational stability.

TUBERCLE AERODYNAMICS CHANGE THE ESSENTIAL DESIGN COMPROMISE

Wind is an inherently unstable energy resource37. The complexity of the fluid flow imposes a number of design constraints with respect to aerodynamic performance as well as serviceability. In practice, every wind turbine design is a study in compromise which must balance one characteristic against the others.

Wind swirls, dips, shoots up or down and even forms cyclonic spirals, large and small. Most fundamentally, the wind changes speeds - rapidly - and that impacts both efficiency and stability. For example, a drop in ambient wind speed from 5 meters per second (5 m/s) to 4 m/s almost cuts the power available in the wind in half38. The rotation of the turbine rotor itself makes the effective wind speed encountered by the blade different at every point on the span. The effective wind speed is a function of both the ambient wind and the rate at which each portion of the blade is rotated through its arc. For example, a point at the mid-span encounters a much lower effective wind speed than that at the tip or the root. These considerations are compensated for in each blade design with different planforms each with different twist and taper along the blade. That compromise has grown ever more complex as turbines have grown larger because, for example, the wind speed encountered at the top of the rotation can be very different from that at the bottom of the rotation.

Conventional laminar airfoils have inherent problems with stability and many design modifications have been developed to curb these instabilities. Active pitch

37 For that matter, so are most energetic fluid flows such has tidal flows, ocean currents, low head water power, etc. and in real world applications the intake fluid flows for axial fans, pumps and compressors are also inherently unstable. 38 The power available i the wind is proportional to the cube of the wind speed. The ratio for the comarison of 4 and 5 m/s flows is 125 to 64 (5x5x5 /4x4x4).

control manages the blade's angle of attack at different speeds, optimizing output to a degree and preventing damage and extra wear in higher winds. The best of these systems can fine tune the pitch at different points in the rotation and, when linked with LIDAR (or equivalent high resolution wind speed monitors), can even tailor pitch adjustments to optimise performance as the speeds of the various currents in the arriving wind change.

THE COMPETITION: ENHANCEMENT ADD-ONS

Not all of the conventional modifications which have been applied require active control systems. Passive blade elements like vortex generators can enhance lift by delaying stall, permitting operation at higher angles of attack and generating more lift. Vortex generator designs are optimized as much as possible to minimize their drag penalty as well. Other additions can moderate vibrations caused by the wake. However, all of these approaches have limits and one enhanced characteristic frequently conflicts with another.

SUMMATION: THE UNCOMMON COMPATABILITY OF TUBERCLE BENEFITS

To summarize: Tubercle airfoils offer several distinct performance improvements in a deceptively simple, hyper-stable and passive package:

1. INCREASED LIFT: Tubercles generate higher lift by energizing the boundary layer in a way that is comparable to the way that vortex generators operate. But, as a rule, tubercles can outperform vortex generators.

2. REDUCED DRAG: When tubercle airfoils operate at higher pitch to generate more lift, they do not incur as great a drag penalty. In fact, Tubercle airfoils may incur the lowest drag penalty of any airfoil type as pitch increases.

3. ULTRA-STABLE OPERATION: Tubercles always stall gradually and even when they partially stall they continue to generate more lift than any other airfoil type. This makes the tasks of dynamically managing blade pitch in the incredibly complex environment of outdoor wind simpler, easier and much less critical. For turbines which do not have the most advanced active pitch control and wind speed monitoring that is even more important39.

4. VORTEX CONTROL: There is evidence that Tubercle flows mitigate some undesirable vortices40 and reduce aeroelastic instabilities in their wakes;

5. ATTENUATION OF SPANWISE FLOWS AND TIP STALLING: Tubercles direct flows between the bumps into the associated inter-tubercular channels which generate accelerated chordwise steams41. These streams act as virtual fences to block spanwise flows and tip stall and thereby limit reductions in aerodynamic efficiency. This blocking of spanwise flows results in the reattachment of those flows and can even produce a significant net gain in efficiency. (This is somewhat comparable to what is accomplished by shrouds around fans and turbines.)

6. VIBRATION AND NOISE REDUCTION: It is important to note that tip stalling is a major driver of noise and vibration on turbine blades. Turbo machines fitted with real fences, winglets and shrouds are typically too noisy and prone to

39 Systems which use LIDAR and sophisticated individual blade pitch control are more efficient but at the moment it is not yet clear how much of the gains is eaten up by the additional costs to purchase and maintain these systems. There is no question they will generate additional power but it is unclear how economically beneficial they will be over the life of the turbine.40 The unique and complex flow generated by tubercle pumps and inter-tubercular channels reduces the tendency for air to detach off the trailing edge of the blade in a manner that produces Kármán vortices and associated noise. 41 The inter-tubercular channels have relatively wide and deep inlets which grow narrower and more shallow until they merge back into the lifting surface at about 40% of chord. In effect, this shape is comparable to a Venturi chamber with an air cap. This shape accelerates the flow and sends the resulting stream toward the trailing edge with its momentum essentially concentrated. These accelerated streams essentially form virtual fences, blocking stalled spanwise flows from flowing toward the tip.

vibration. In wind turbines the vibration caused by tip stalls is also responsible for a good deal of blade wear and tear and damage, frequently resulting in composite de-bonding and material fatigue. Tubercle streams through the inter-tubercular channels serve as "virtual" fences but the difference between these flows and real fences are significant. Tubercles do not incur wall effects. The reduction of, and, in some cases, the elimination of spanwise flows presents a valuable new option for the attenuation of tip stalling. As a result, in addition to improved power production this vibration reduction makes the blades quieter. Further, these virtual fences don't simply block the air back onto the blade for sporadic attachment and detachment as solid fences and winglets do. Instead, these virtual fences are in the form of accelerated inter-tubercular streams which have enough momentum to redirect the detached spanwise flows back into position to reattach and can maintain attached flows longer. In the case of Tubercle airfoils that means the enhanced lift and lowered drag generated by tubercular flow stays attached longer. In combination this new kind of flow management generates more lift delivering a quiet boost in power. In addition, these inter-tubercular streams produce further stable flow modification as they progress across the blade and this has been shown to reduce Kármán vortices off the trailing edge and their associated noise and vibration42.

Kármán vortex

42"Wake Control Based on Spanwise Sinusoidal Perturbations" Dobre, Hangan, Vickery, AIAA Journal Oct 2005.

THE BOTTOM LINE

Both numerical studies and physical tests concur: Tubercle airfoils have many unique and valuable advantages over other airfoils but beyond question the greatest advantage is that all of the gains come together in a stable suite of features which complement each other. As a consequence, tubercle blades can deliver greater lift and lower drag which delivers greater efficiency combined with an ultra-stable performance envelope. It seems reasonable to conclude that this harmonious combination will prove to be ideal for the blades on wind turbines and also for conventional blades which have been retrofitted with leading edge elements which modify the shape of a conventional airfoil into a blade with tubercle leading edges.

In closing it should be noted that each wind turbine rotor type has its own unique planform and tubercle retrofits will have to be engineered to fit the individual shape of each blade design but once that is accomplished fabrication and installation costs can be minimized.

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Prepared by Stephen DewarVP Operations WhalePower Corporation

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