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#Post#: 5287--------------------------------------------------
Re: Newbie questions....turbine design
By: maximoney1 Date: January 28, 2015, 8:03 pm
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Cheers lynx. ;D
#Post#: 5295--------------------------------------------------
Re: Newbie questions....turbine design
By: maximoney1 Date: January 29, 2015, 8:07 am
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Elaborate on 'solidity' problem please.
#Post#: 5298--------------------------------------------------
Re: Newbie questions....turbine design
By: maximoney1 Date: January 29, 2015, 8:34 am
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@CG
Fantastic mate, so the centre shaft through the airfoils to the
single pivot point, is the 'lesser evil' trade off of between
better dynamic balancing(or at least less impact) versus the
disrupted airflow to the downwind blade.
Really appreciate this education.
Thanks
#Post#: 5326--------------------------------------------------
Re: Newbie questions....turbine design
By: maximoney1 Date: January 30, 2015, 7:26 am
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@lynxwind
I have read your work on solidity, first class, but i want to
look at aspects in isolation.
From what you have said previously, power = torque x speed. If i
wanted to experiment/analyse, purely the speed aspect(ignoring
torque and start up problems etc etc), what %solidity should i
be looking at, purely to max the TSR? I realise dropping the
diameter down helps speed things up, (down to whatever, my
experimentation indicates is practical ). What is the approx
maximum TSR one could hope to achieve(aim for), with this type
of design?.
Any 'ballpark' figures or pointers to where i might want to
start looking would be greatfully received.
Cheers
Jim.
#Post#: 5327--------------------------------------------------
Re: Newbie questions....turbine design
By: lynx wind Date: January 30, 2015, 7:42 am
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If you want high speed drop the solidity. It will hurt startup
and the point at which the VAWT goes into high speed. I have
one that has a diameter of 20" with 3.25" blades that runs into
the 1000-1500 rpm range only when winds are 12mph and over.
It's a fairly tight range.
48-50% solidity = high speed
55-60% solidity = nice startup and good low wind TSR
50-55% might be a good compromize
There is talk of 6-7 TSR I have never seen that. I have seen
3+ with low solidity designs but torque is lower.
One hint would be to have very little angle off the
circumference line. Startup suffers but it helps with high
speed. Perhaps TE off circumference line by 3 degrees. The
German guys were doing this and just playing with airfoil. The
foils they were using were close to NACA 0020, but with tweaks
to the TE to compensate for the low angle. The circumference
line ran right through the chord, but the TE turned back toward
the center just slightly.
An advantage to high speed is you get higher voltage. The
tradeoff is you need much higher precision and a heavier design.
Designing these VAWTs is like trying to shoot a cannonball
through a forest and not hit anything.
#Post#: 5330--------------------------------------------------
Re: Newbie questions....turbine design
By: maximoney1 Date: January 30, 2015, 8:05 am
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Fantastic Lynxwind,
Just what i needed, many thanks.
I'm presuming most of the airfoil shapes tend to be more
'symmetrical' either side of the chord as opposed to the
'normal' aircraft type shape, simply due to the angles etc of
the downwind blades? I take it the german guys tweaks were to
simulate the aircraft 'flaps' for better lift at lower
velocities.
#Post#: 5333--------------------------------------------------
Re: Newbie questions....turbine design
By: maximoney1 Date: January 30, 2015, 9:44 am
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@lynxwind
Ignore previous post comments on airfoil shapes...
I found this which answers it nicely...
"Traditionally airfoils that are used for VAWTs are symmetrical
airfoils eg NACA0020. The main drawbacks with these types of
section are their minimum or negative torque generation for the
lower TSR. As can be seen from the result, all the torque values
are positive for lower TSR for the modeled NACA2415 which
implies camber airfoils have great potential in self staring of
VAWTs. However, the coefficient power (Cp) result shows a
reduced efficiency as compared to the conventional symmetrical
airfoils of eg NACA 0020 which are not self starting."
Thus if i have another method of self starting, which has no
bearing/influence on the runability of the rotor once it is up
to speed, then i would be better off with the symmetrical shape.
The other thing mentioned in the article, was the impact of the
centre shaft on performance co efficients, thus if dynamic
balancing can be mastered, there might be gains to be had.
#Post#: 5334--------------------------------------------------
Re: Newbie questions....turbine design
By: lynx wind Date: January 30, 2015, 10:17 am
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Some of that info is academic crap. Self starting is not an
issue and symmetrical blades can have tremendous torque. Camber
would come into play if you subscribe to lift theory as taught
in Kindergarten. If you observe more closely you will see
Coanda effect and angle becomes more the driver.
#Post#: 5337--------------------------------------------------
Re: Newbie questions....turbine design
By: maximoney1 Date: January 30, 2015, 11:11 am
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Thanks Lynxwind,
Experience over theory everytime ;)
Found this a while ago and thought it interesting....a bit long
mind.
HTML https://www.youtube.com/watch?v=XWdNEGr53Gw
#Post#: 5346--------------------------------------------------
Re: Newbie questions....turbine design
By: maximoney1 Date: January 30, 2015, 7:54 pm
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This was also interesting, Doug Mclean Michigan Aircraft
engineer, mentioned coanda effect(not required) at 21:19
HTML https://www.youtube.com/watch?v=QKCK4lJLQHU
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