Hello Magnet Juice,
I've got what might be some pretty controversial idea's around high solidity VAWT's, especially how they function at very low Reynolds numbers where viscosity is much more important than inertia.
Quite a few years back I learned a lot about alternator design here, posting as oneirondreamer, however I've lost access to the email that I used back then, so wasn't able to reactive the account. I see now that with my new account I can't post links, which is too bad as I've got some good youtube video's and instructables. In youtube you can search Andrew Rokeby-Thomas.
My suggestion is that you first consider the classic NREL funded research by Blackwell in the 70's, but I suggest first reading a paper by Ian Ross, "wind tunnel blockage correction factors for high solidity VAWT's" or something close to that. You should be able to find links to them on my site ...artinventing.... though I haven't even looked at my site in a while.
After reading those you will likely recognize that the classic savonius design failed because it's likely between 5-15% efficient, and the 15% is probably quite optimistic. There are no independent measurements, of non wind tunnel results, that suggest higher than 10%, that I've been able to find.
That said, Blackwell reported that a 3 bucket rotor had higher peak torque, but at a significant loss of rpm vs the 2 bucket, with the 2 bucket having a reported Cp of somewhere around 0.21 and the 3 bucket having around half that.
So, if you are determined to use a Savonius for startup of your Darius, then a two bucket is likely better than a 3.
I've recently published my construction notes for my very unusual looking Savonius type turbine on the instructables website, along with my plans for a 3D printed Axial Flux alt (150mm magnet circle, 8 poles, 6 serpentine windings in 3 sets of two, wired in series).
It's inspired by Barnaby Wainfan's faucetmobile , an unusual flying wing made from a series of flat planes, as well as my previous research into high solidity VAWTS.
I have not tested it yet, and hope to be doing that soon. I'll be posting an instructable on the testing.
A previous VAWT of mine 1 m did by 2.25m tall, based on similar idea's was tested by a consulting engineer, paid by potential investors, and found to be likely 24-29% efficient (Cp 0.24-0.29) as measured by Proney Brake dynamometer on a moving platform test bed (not wind tunnel), and stationary in measured wind on the same platform. You can find the letter from the engineer on my website artinventing . I haven't found a credible 3rd party report of another VAWT of similar sized doing anywhere as well. Darius of similar sizes (around 1m diameter), like the wind spire, and UGE/Gorlovs in testing by 3rd parties top out at around 17% in air (much better in water due to higher Reynolds numbers). It's true that big Darius turbines, as reported by the Sandia Labs "A VAWT retrospective" found Cp's up to around 44%, but at small sizes, the Reynolds numbers are so poor, and dynamic stall such a large factor at that cord length, 11-17 is as good as commercial manufacturers are getting.
My old website was ....artturbine... Sadly that company was taken over by some investors who couldn't agree on anything other than they could turn it into a product without me. It imploded about a year later, after a few years of big bills from an engineering company that was happy to jump in over their heads.
I've got what might be some pretty controversial idea's around high solidity VAWT's, especially how they function at very low Reynolds numbers where viscosity is much more important than inertia.