Fauceted Wind Turbine, Reynolds number advice please

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Dart, Jul 14, 2019.

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  1. Jul 14, 2019 #1

    Dart

    Dart

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    Hello to all you amazing experts and passionate amateurs! I've found this forum to be an endless font of practical hard nosed wisdom, and creative, day dreaming CAD jockies and it's been very helpful. Thanks!

    In about 2010 I created a VAWT design that by my measurements, and the measurements of a hired engineer in 2012 (report is on my website), it is the worlds best VAWT in terms of swept area efficiency at Cp 0.31 (31% of the wind energy passing through it's swept area is converted to useful shaft power).

    It's true that some of the NREL Labs Flo turbines reached into 40%, but that was in scales where the turbine was over 100ft tall. In sizes of around 1m diameter, the independent testing I've looked at shows peak performance at between 11% and 17% for the thin bladed Darius (eggbeater), helical (Gorlov), or straight bladed, not much difference.

    It's generally understood that the maximum amount of energy collectable from the wind by a turbine is around 59%, and that very large HAWT turbines come close to that theoretical maximum. What's not generally known is that HAWT turbines of 30ft diameter or less max out at around 30%, and HAWT Turbines of less than 10ft diameter, 15-20%. This is why no large turbine manufacturer is trying to scale it's technology down, and most small HAWT manufacturers have gone out of business.

    I believe that a part of this poor performance for the scaled down thin bladed turbines is that they need high Reynolds number conditions to operate effectively.

    Here's a video of the largest model I made. It was never tested by a 3rd party, but my testing lead me to believe it was a bit better performance than the 1m diameter unit. You can find more video's of different size turbines on my youtube channel.



    Sadly I was terrible at running a company, and the technology ended up in the hands of a group of older wealthy folks who are rich and at least somewhat intelligent, but it turns out can't get along at all. I didn't realize wealth meant yo So it's stalled, and they own the patent and until at least a couple of them kick the bucket, I don't expect any movement.

    So after a lot of thinking, I combined some of the thinking from the previous design, with some new thoughts informed by testing, and created a new design (USPTO patent pending :)). While all this was going on, one issue I had with both the old and new designs is that they are large, complex, curving shapes, and quite difficult to fabricate. I saw the news that people are building Faucetmobile clones, and revisited what I could find about the design on the web. One interesting comment, I think from Barnaby Wainfan himself, when asked about flow separation on the upper surface around the skin bending points. What Barnaby seemed to say is that in the wind tunnel they reached a conclusion about the maximum brake angle that wouldn't cause separation, and their real world project seemed to prove it out. If I recall correctly, Barnaby made a comparison to the sharp transition that aileron's and rudders in some positions can take on, and pointed out that they don't cause flow separation (in normal flight conditions).

    A crossover between the Faucetmobile, and my turbine, is that they both have quite low Reynolds numbers. My turbine is a single helical blade, or can be thought of as two blades, joined at the thick trailing edges. When properly loaded and driven by the wind it's outermost edge is moving at just below the speed of the wind for a tip speed ratio of between 1 and .75. So from a simplistic point of view, the blade traveling away from the wind, see's almost 0 wind speed, and the side traveling into the wind is between 1-2 wind speed. For a 1m diameter turbine, in a 20km/hr wind, can anyone suggest what a the Reynolds number would be and how to calculate it?

    Here's a preview of the new faceted turbine
     
  2. Jul 15, 2019 #2

    proppastie

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  3. Jul 15, 2019 #3

    Dart

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    Thanks Proppastie, I've used calculators like this before, however in this case, I'm curious as to others opinions as to what chord length I should use, 1m or .5m, and with a 20km in speed, with the turbine spinning at TSR 1, the outer most part of the turbine, depending on which side you look at, is either at 40km/hr apparent wind speed, or 0 km/hr apparent wind speed. With the settings at 1m chord length, and 21km wind speed, it's Reynolds number is 422,000 aproximately. With chord at 0.5m, it's 210,000 aprox. If it's at TSR just under 1, and the chord is 0.5m then the apparent wind speed over the front edge is around 1km/hr, and the Reynolds number is around 10,000

    I'm not sure which applies, and I'd like to get a better understanding of how other people would think about this.

    As a side question, if I was calculating the Reynolds number of the Wainfan Faucetmobile, would I use the "foil", length at it's longest, shortest, or average or ???
     
  4. Jul 15, 2019 #4

    Mad MAC

    Mad MAC

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    I would take the chord length to be from edge to edge when sectioned.

    I guess typically most of the moment is generated post stall by drag so the value of exact shape is limited.

    Since post stall aerofoil CL curves are not normally discussed here, here's one for NACA 0015 which post 20 deg AOA is barely indistinguishable form a flat plate.

    [​IMG]
     
  5. Jul 15, 2019 #5

    Richard Schubert

    Richard Schubert

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    The Faucetmobile!! Aaucetmobile.jpg
     
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  6. Jul 15, 2019 #6

    BJC

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    That is an illustration of, among other things, why forward stick is used after the initial break in a snap roll.


    BJC
     
  7. Jul 15, 2019 #7

    BJC

    BJC

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    Facetmobile hood ornament.


    BJC
     
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  8. Jul 15, 2019 #8

    proppastie

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    wish I knew enough to answer you questions...however since you have a prototype perhaps you could run some numbers on all of those various cases you ask about and measure your prototype results to see which numbers best fit.
     
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  9. Jul 15, 2019 #9

    proppastie

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    an aircraft prop is up to 80% efficient a the proper rpm/angle (I think). So you claim much smaller numbers for windmills
     
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  10. Jul 15, 2019 #10

    Hot Wings

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    Toss some dirty oil on it and see what direction the resulting pattern says the air is flowing over the surface? From this you can then measure the actual chord as seen by the air.
     
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  11. Jul 15, 2019 #11

    Dart

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    I'm sorry if I miscommunicated, the 59.8% limit was calculated by someone far more knowledgeable than me, I think in the 1930's. Dr Betz, so it's called the Betz limit. No wind turbine has ever exceeded it, that I know of, and so it's still considered as fact by most wind turbine experts.

    The idea is that if you had a perfect turbine disk, consisting of an infinite number of perfect drag free blades, how much wind energy could you capture. The answer by Dr. Betz is 59.8%. Why not 100%? As I understand it the issue is that if you remove 100% of the energy in a moving fluid, it's no longer moving, and so new fluid trying to enter the turbine must use some energy to move out the static fluid. The counter intuitive problem in wind turbine design, is getting rid of the exhaust so that there is room for new flow.
     
  12. Jul 15, 2019 #12

    Dart

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    Thanks for the suggestion Hot Wings! I am going to start testing soon, and I will try this. I've heard of people using different paint and I think clay washes for this too. Any suggestions on where I can find recipes for mixes for this, or what you call the process? I'm also aiming to slap a gopro on the driven blade, and use a sheet laser to try to video smoke at different heights in the blade. However all that has to wait till I figure out what the power curve is and determine ideal rpm's at various wind speeds.
     
  13. Jul 15, 2019 #13

    BBerson

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  14. Jul 15, 2019 #14

    Dart

    Dart

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    Thanks, I agree it doesn't look like a conventional wing. One big difference between what I've found works, and anemometers, is that if you look at an anemometer, it has pretty much the same total frontal area, regardless of it's rotation position, if you look at what I'm doing you can see that it's actually quite thin. In the Blackwell report from the 1970's, when a fair amount of wind tunnel work was done on the historical Savonius design, they noted that while a 3 cup Savonius rotor could develop higher torque than a 2 cup, but it was at a much slower speed, and the best total Watts was from a 2 cup design.
     
  15. Jul 15, 2019 #15

    BBerson

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    Can't see much. Do you have a link?
     
  16. Jul 16, 2019 #16

    Dart

    Dart

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    This video is really just a preview, I'll make more and post more details once I've done some testing and can make a good comparison between this and my previous design. I've got some data on my website artinventing.com but not a lot. I do have a letter from the engineer who did some testing with me and then did his own analysis of it, but that's the pervious smooth version.

    I'm fairly confident that this design will outperform the last, at least in part because the freewheeling speed is quite high, but without a controlled load, and a data logger, it's interesting and fun, but not really able to be properly evaluated. That might take me a few months on the outside, or it could come together more quickly. I'm doing this on my own and often have to find ways to eat, so can't always give this the time it deserves.
     
  17. Jul 16, 2019 #17

    Mad MAC

    Mad MAC

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    The 59.8% limit isn't a measure of wind turbine efficiency directly compatible to propeller efficiency. Its a measure of how much energy can be extracted from the wind & as such 100% would actually mean you have stopped the wind which is a touch impractical.

    So has any one tried a saw tooth leading edge on these, not being drag sensitive the vortexes could delay the aerofoil stalling.
     
  18. Jul 16, 2019 #18

    Speedboat100

    Speedboat100

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    Errr...you might be surprised but at least 4 universities have counted that 59,3 % is only valid on HAWT propellers..VAWTs can exceed the limit with a hefty margin.

    https://iopscience.iop.org/article/...1RTtNO7s-BqjsTWhuND4H2BLs8dde4uPe7QihjfcNkKtg
     
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  19. Jul 16, 2019 #19

    Speedboat100

    Speedboat100

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    You are on the right track...only ANEW company in Poland claims 70% efficiency for the simple H-Darreus type...and they may be even correct.
     
  20. Jul 16, 2019 #20

    Speedboat100

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    Best source on this kinda turbine is the NREL data of the MCDonnell Aircraft Giromill tests in 1974-1978...during the oil crisis.
     
    Last edited: Jul 16, 2019

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