Fauceted Wind Turbine, Reynolds number advice please

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Dart

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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/10.1088/1742-6596/753/2/022056/pdf?fbclid=IwAR2KZbeLkfs1F01RTtNO7s-BqjsTWhuND4H2BLs8dde4uPe7QihjfcNkKtg
Thanks so much for this link! I've been saying that the Betz is not properly applied to VAWT's since about 2006.

I'm not really confident in their numeric simulation in this paper, which is the "proof" they have that VAWT's may have a higher limit. I've talked with experts in CFD flow simulation, at UVIC, and 2 years ago with one of the research heads at Canada's largest wind tunnel, they don't believe that CFD is capable of good modelling of a VAWT with 3D flows, yet. That's not really relevant to the question of should Betz be applied to VAWT's when the calculations are based on the conventional HAWT.

I'm not sure the limit is higher, or lower, just that Betz is a useful tool that doesn't fit in this application.

There's a paper by Gorlov which numerically "proves" a limit of 35% (by conicidence the exact number Gorlov claimed to reach with his helical darius). Since then Gorlov's work has been shown to be problematic. I worked with an independent group who licensed the Gorlov design and bought test turbines from Gorlov. The turbines did not survive testing, too fragile and never reached more than 20% (Cp 0.20). A straight blade Darius was tested in it's place which reached 25%, using the same blade profiles.
 

Dart

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Near Nelson B.C.
Best source on this kinda turbine is the NREL data of the MCDonnell Aircraft Giromill tests in 1974-1978...during the oil crisis.
I agree, and the easiest access to that Data is to google "A VAWT retrospective" published by NREL in maybe 2005, where they go over all their data and results. In short, they had tropenskine Darius turbines over 100ft tall, that regularly reached 40%+ efficiency. However to get that level of efficiency they had to use very high aspect ratio blades that had very little support, and those blades were so resonant in so many directions (the failure mode analysis section is very interesting) that no common material could withstand the fatigue. In one case while the aluminum blades were being craned off, a blade actually just fell right apart. Another issue is that the 2 blade tropenskine was the only version to hit high efficiency, but it wouldn't self start. The final kicker is that the only way they could even get the short lifetime they did, was by using a control system to speed up or slow down the turbine to get it away from rpm's that would cause resonance.

To me though the realization is that this is much more about reynolds numbers and scale. No one that I'm aware of, and I'd be happy to look at the polish version, has tested a small Darius type VAWT (helical, straight bladed, gyromil, or tropenskine) and got anything over 20% (Cp0.2). Small wings don't work like big wings.
 

bmcj

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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.
Stick with the oil. It offers easy flow and visibility. It might be best if you can apply the oil while it is spinning and look at it while it is still spinning (high speed camera or stroboscopic lighting), but if not, you can stop it and snap some photos immediately afterward.
 

Speedboat100

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I agree, and the easiest access to that Data is to google "A VAWT retrospective" published by NREL in maybe 2005, where they go over all their data and results. In short, they had tropenskine Darius turbines over 100ft tall, that regularly reached 40%+ efficiency. However to get that level of efficiency they had to use very high aspect ratio blades that had very little support, and those blades were so resonant in so many directions (the failure mode analysis section is very interesting) that no common material could withstand the fatigue. In one case while the aluminum blades were being craned off, a blade actually just fell right apart. Another issue is that the 2 blade tropenskine was the only version to hit high efficiency, but it wouldn't self start. The final kicker is that the only way they could even get the short lifetime they did, was by using a control system to speed up or slow down the turbine to get it away from rpm's that would cause resonance.

To me though the realization is that this is much more about reynolds numbers and scale. No one that I'm aware of, and I'd be happy to look at the polish version, has tested a small Darius type VAWT (helical, straight bladed, gyromil, or tropenskine) and got anything over 20% (Cp0.2). Small wings don't work like big wings.

Yes their system was heavily flawed, but their conclusions are very interesting. Also the solidity was very low...you can see the difference here in the ANEW company product which claims 70% efficiency in converting wind energy to electricity.

https://www.anew-institute.com/vertical-wind-turbine.html
 

Speedboat100

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Thanks so much for this link! I've been saying that the Betz is not properly applied to VAWT's since about 2006.

I'm not really confident in their numeric simulation in this paper, which is the "proof" they have that VAWT's may have a higher limit. I've talked with experts in CFD flow simulation, at UVIC, and 2 years ago with one of the research heads at Canada's largest wind tunnel, they don't believe that CFD is capable of good modelling of a VAWT with 3D flows, yet. That's not really relevant to the question of should Betz be applied to VAWT's when the calculations are based on the conventional HAWT.

I'm not sure the limit is higher, or lower, just that Betz is a useful tool that doesn't fit in this application.

There's a paper by Gorlov which numerically "proves" a limit of 35% (by conicidence the exact number Gorlov claimed to reach with his helical darius). Since then Gorlov's work has been shown to be problematic. I worked with an independent group who licensed the Gorlov design and bought test turbines from Gorlov. The turbines did not survive testing, too fragile and never reached more than 20% (Cp 0.20). A straight blade Darius was tested in it's place which reached 25%, using the same blade profiles.
I think the H-Rotor ( cycloturbine/H-Darreius ) is not ready yet...I mean the design is still not 100% perfect. That is why so many investigate it.

Many have tried and failed, but Pinson and Buhler seemed to conquer something:

Roman Buhler broke a wing at 120 G:s.
 
Last edited:

Dart

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Joined
Apr 9, 2015
Messages
38
Location
Near Nelson B.C.
Yes their system was heavily flawed, but their conclusions are very interesting. Also the solidity was very low...you can see the difference here in the ANEW company product which claims 70% efficiency in converting wind energy to electricity.

https://www.anew-institute.com/vertical-wind-turbine.html
Looking at the data it seems they are using a different set of measurements to create the 70% number, and it's interesting that they don't have any engineer signing off on their data. Instead of using the standard swept area section, they are using a cylinder. They seem to be selling 3 different units, and yet they haven't bothered to have any independent testing done. I've never sold a turbine and I only had to pay about 5,000$ to get an engineer to inspect my setup, come along for a set of tests, and produce a report. That they haven't attempted any independent testing, or released it, speaks pretty loudly to me.

It's sad, but the VAWT history is full of massaging of the metrics, to outright lying. Another issue is that historically, at least up to 2010, testing VAWT's in wind tunnels was most often done using standard blockage factors that produce incorrect results. This is true of the historical blackwell report from NREL, where they found a peak of 21%, when real world results find closer to 10%.
 

Speedboat100

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Messages
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Location
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Looking at the data it seems they are using a different set of measurements to create the 70% number, and it's interesting that they don't have any engineer signing off on their data. Instead of using the standard swept area section, they are using a cylinder. They seem to be selling 3 different units, and yet they haven't bothered to have any independent testing done. I've never sold a turbine and I only had to pay about 5,000$ to get an engineer to inspect my setup, come along for a set of tests, and produce a report. That they haven't attempted any independent testing, or released it, speaks pretty loudly to me.

It's sad, but the VAWT history is full of massaging of the metrics, to outright lying. Another issue is that historically, at least up to 2010, testing VAWT's in wind tunnels was most often done using standard blockage factors that produce incorrect results. This is true of the historical blackwell report from NREL, where they found a peak of 21%, when real world results find closer to 10%.
Polish company is 30 years old. It seems to me that their turbine stalls very early and actually cannot utilize the 70% efficiency for more than a brief period at wind speeds below 10 m/s...where there is very little energy in the first place.
 
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