Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127
Posted: 04:37am 11 Feb 2009
Copy link to clipboard
Print this post
There seems to be some interest in this area.
perimeter rings, tiplets etc have been used to reduce wasted air exiting the blade tips without passing over the blade surface to perform useful work.
Passenger jets use them and I found tiplets provided additional lift on model gliders.
I would imagine a non optimum blade may show improvement with tiplets etc, but think that a well designed blade will not be improved much.
This is poking a rod into the fire.
Gordon.become more energy aware
SparWeb Senior Member Joined: 17/04/2008 Location: CanadaPosts: 196
Posted: 05:49pm 11 Feb 2009
Copy link to clipboard
Print this post
Place your BETTS, everyone, this could either go up in flames, or up in smoke.
Sorry for the tease, Gordon, but it's spelled "Betz". Not so important, but the likelihood of turning up the thread title in a Google search has already shrunk to zero. Perhaps an admin could fix the spelling and then I could delete this comment.
On with the show...
The Betz limit assumes no geometry in the rotor disk itself, other than it is planar. Even "circularity" isn't assumed in the derivation of the Betz limit. Betz worked out the simple fact that there is only a finite amount of energy per volume of air passing through the rotor plane. Only a portion of that can be captured. Attempting to collect any more energy creates a situation where the air simply cannot get out of the rotor plane fast enough to let new air in.
In the real world, I've been able to measure moments where the Betz limit is exceeded, but obviously a gust has passed, and the windmill was not in a state of equilibrium. Any moment exceeding Betz limit must be followed by an equal or longer period of time below it.
Putting fancy things on the tips doesn't change the Betz limit. If your question has more to do with the Cp, then you can build an argument. Whatever argument you can make, probably be backed up by carefully collected test data. Data logging from your backyard windmill doesn't count. "It works for me" doesn't count either. You can cut a plastic pipe and get wind power, for example. A comparative test must have wind-tunnel controlled conditions, multiple models, and pitot-static measurements upwind and downwind of the model, to demonstrate that any tip modification (tiplet) has an beneficial effect.
If you're interested, research was done on these topics, starting in the 1930's, in the USA and Europe. Much of the old test data is available on the internet ("NACA" reports being the most famous and readable). Research is still on-going, so maybe you could look at the NREL, UUIC, and Sandia for some reports.
There is no audience on an internet forum for analysis of vortex-shedding, induced drag, nor the calculus and computational methods that are needed. I'm nowhere near being able to work that stuff out, and I got two years of it in college. I don't think anyone else would take an interest.
The general consensus is that the complexity outweighs the benefits, and the only use typically given to tip devices is a braking/speed control aid for industrial turbines that pivot the outer 5% of the tip.
Putting tip devices on home-made windmills begs the question: How are you going to bolt that thingy in place so it won't fly off and hit me in the head?
Steven T. Fahey
niall1 Senior Member Joined: 20/11/2008 Location: IrelandPosts: 331
Posted: 06:46pm 11 Feb 2009
Copy link to clipboard
Print this post
hi all
i think the winglet thing is interesting and hopefully will be trying these out later on ,(to cold at the minute) their roughly based on glider winglets (epoxied to the tips)
.these props are at my size limit so i think any add ons are worth trying
i wont be able to measure any results but less noise might be an indication ,if they have the opposite effect .....its pretty easy to knock them off , part of a winglets mission in life is to find something to break itself off on
edit... the room has since been redecorated ..
niallEdited by niall1 2009-02-13niall
GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127
Posted: 10:42pm 11 Feb 2009
Copy link to clipboard
Print this post
Thanks sparweb,
I thought the spelling was wrong, but once I created the thread I found I could not change it. Maybe Gizmo can help out here.
I agree with you about the difficulty of proving any upward change in the Betz limit. There are other ways of changing the direction of airflow along the blade as well. I have made glider wings with a sweep forward design. These produced more lift, but the aircraft was more difficult to control.
I am sticking to typical designs for the moment.
Gordon.become more energy aware
vawtman Senior Member Joined: 14/09/2006 Location: United StatesPosts: 146
Posted: 01:44am 12 Feb 2009
Copy link to clipboard
Print this post
Edited by vawtman 2009-02-13
RevUpWind Regular Member Joined: 03/05/2008 Location: AustraliaPosts: 60
Posted: 03:16am 12 Feb 2009
Copy link to clipboard
Print this post
Sparweb QUOTE: "There is no audience on an internet forum for analysis of vortex-shedding, induced drag, nor the calculus and computational methods that are needed. I'm nowhere near being able to work that stuff out, and I got two years of it in college. I don't think anyone else would take an interest."
Regarding Betz or "Betts" law/ limit or whatever Let's think outside the circle; Literally.
Betz law assumes a non real world cylinder of air which passes through the turbine blades and cannot be slowed down to 100% to achieve a maximum power generation. (assumed from the law)
In the real world this cylinder of air does slow down but it also compresses and ignoring the inner part of the turbine for the moment, the wind spirals out towards the tip of the blade being an area of least resistance where it speeds past in a vortex and rapidly returns behind the blade to fill the partial vacuum behind.
Betz law assumes that if you present a flat disc or object to the wind and stop the whole column of wind that no rotation of the disc or object would be observed.
This is true. However I may still be able to make use of the wind in the real world that accelerates around the edge of our disc under pressure before being being sucked back in.
E.G. Put the flat disc in place as your turbine and mount trailing turbine blades at right angles to the disc behind the outer edge but fully within the limits of the cylinder of wind projected at right angles fore and aft of the disc and see what happens. Yes it will turn and make limited power!
This is the portion of wind that I am experimenting with to boost the performance of wind turbines. (small ones at ths stage.)
The movement of the incident wind in this area is highly complex and I would imagine would be difficult to observe at high wind speed, even in a wind tunnel.
Some assumptions that I have arrived at (and I welcome input and refuting arguments) are.
1/ Wind pressure increases towards the tip'
2/ Blade turning causes the incident win to begin to spiral due to upwind feedback effect as the wind is slowed. The exhaust wind spirals at a greater speed immediately behind the blades.
3/ The wind falling into the rear of the turbine blade forms a doughnut shaped vortex at high speeds as it falls in to the low pressur area.
4/ Fore and aft solution are potentially realisable.
One of the rationalizations behind the idea of investigating various methods of tip enhancement is the fact that the effect of wind at the tip is likely to be a cubed factor increase over the inner portions of the turbine. So any benefits obtained at the tip are benefits indeed.
Laws are made to be broken. That is true science. (Betz limit remains true within the theoretical bounds of the law. So it is a law until .
Because thee are other actions ocuring in the real world doesn't mean that it is possible to atain more than 60 whatever percent of the posible power available from the wind. However this does not mean that improvements may not be achieved in even the best of straight blades as assumed in the theory.
If however tip improvements (tiplets) did provide a result that was proven to be in exess of the Betz imit this would not diprove Betz law.
Dealing with losses occuring in the middle is another subject.
Russ....River Heads - Australia
The wind blows wherever it wishes; you hear the sound it makes, but you do not know where it comes from or where it is going.
GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127
Posted: 07:43am 12 Feb 2009
Copy link to clipboard
Print this post
This is not the case for all designs of wind turbine blades. A blade can be made where the lowest pressures are developed close to the root hub.
I would think that rpm, will decrease on the way towards an optimum energy extraction point. Any benefit gained from better blades may require redesigning of the alternator afterwards.
It is possible that adding tiplets will make the blade tip more efficient at the expense of the performance of the rest of the blade. The profile of the whole blade may need to be taken into consideration when modifying the tips.
Good luck with your testing.
Gordon.become more energy aware
Jarbar Senior Member Joined: 03/02/2008 Location: AustraliaPosts: 225
Posted: 11:11am 12 Feb 2009
Copy link to clipboard
Print this post
It may also prove advantages that if modified tip theory has merit,so too might fins placed at different positions along the length of the blades.I have seen something akin to this on the surface of airplane wings.These appear to direct airflow across the surface to optimize effiencey and I presume lift.And I imagine would reduce wind pressure slipping up along the blade.
Anthony"Creativity is detirmined by the way you hold your tounge".My Father
"Your generation will have to correct the problems made by mine".My Grandfather.
Jarbar Senior Member Joined: 03/02/2008 Location: AustraliaPosts: 225
Posted: 11:25am 12 Feb 2009
Copy link to clipboard
Print this post
It would be interesting to see what pattern is produced on blade surface when under a strong constant breeze.If a spray can (aerosol) was held in a way that created minimal disruption to normal wind flow.By spraying a mist of paint towards spinning blades the paint residue and where it adheres may give great insights to what is actually happening as air flows over spinning blades.
Anthony"Creativity is detirmined by the way you hold your tounge".My Father
"Your generation will have to correct the problems made by mine".My Grandfather.
wind-pirate Senior Member Joined: 01/02/2007 Location: CanadaPosts: 101
Posted: 04:02pm 12 Feb 2009
Copy link to clipboard
Print this post
Hi
One might be able to solve this in a wind tunnel.
Add some "smoke maybe" slow motion, that might help to see what is happening. Anyone have a Tunnel?
THE Pirate.
stealing wind & solar energy is fun
Jarbar Senior Member Joined: 03/02/2008 Location: AustraliaPosts: 225
Posted: 10:31pm 12 Feb 2009
Copy link to clipboard
Print this post
The idea of paint spray was to gain some insight without a wind tunnel.Another method might be to paint the blade surface with a water soluble paint and spray a mist of water that would dissolve the the paint and leave a trail of the airflow over blade.It could be seen if air is leaving blade at right angles or otherwise.On my VAWT I use wool tell tails to better visualise the airflows.Or throw some dust or talcum like material through it.But the VAWT is rotating more slowly admittedly.
Anthony."Creativity is detirmined by the way you hold your tounge".My Father
"Your generation will have to correct the problems made by mine".My Grandfather.
GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127
Posted: 11:51pm 12 Feb 2009
Copy link to clipboard
Print this post
This may give uncertain results. Any water droplets formed will travel towards the tip from centrifugal forces acting on them, and not necessarily from airflow.
Gordon.become more energy aware
niall1 Senior Member Joined: 20/11/2008 Location: IrelandPosts: 331
Posted: 12:46am 13 Feb 2009
Copy link to clipboard
Print this post
would fins then not counter act the blade throwing as much air down the blade to the tip , which might give the blade a bit more grunt ....the fin could be back and front of the blade Edited by niall1 2009-02-14niall
GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127
Posted: 01:07am 13 Feb 2009
Copy link to clipboard
Print this post
The fins are usually called strakes, and are used to guide air flow over a surface. Most passenger jets have these types of wing additions as well at strategic points.
Gordon.Edited by GWatPE 2009-02-14become more energy aware
niall1 Senior Member Joined: 20/11/2008 Location: IrelandPosts: 331
Posted: 01:08am 13 Feb 2009
Copy link to clipboard
Print this post
i,m wondering if the only purpose in life for the inner 40 per cent of a blade is nothing more than to provide something for the outer powerfull speedy bit to hang on to ....less wine then niall
GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127
Posted: 01:11am 13 Feb 2009
Copy link to clipboard
Print this post
The inner portion provides torque that helps with blade acceleration, during startup and windspeed transients.
Gordon.become more energy aware
niall1 Senior Member Joined: 20/11/2008 Location: IrelandPosts: 331
Posted: 01:28am 13 Feb 2009
Copy link to clipboard
Print this post
makes sense ...an ideal root angle helps this even more ..
but i,m thinking ...a blade probabely spends just as much time in its life slowing down as it does speeding up ...now maybe the wide root angle section mightened help at all
niall Edited by niall1 2009-02-14niall
GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127
Posted: 02:51am 13 Feb 2009
Copy link to clipboard
Print this post
I don't know what to say!.
I hope the weather warms up so you can fly those blades.
Gordon.become more energy aware
Jarbar Senior Member Joined: 03/02/2008 Location: AustraliaPosts: 225
Posted: 03:18am 13 Feb 2009
Copy link to clipboard
Print this post
Yes Gordon,and so Niall you can get those Strakes on and and report back as to their effect.Go for it I reckon.A little extra weight for a massive increase in torque.It worked for the Americas cup Ben Lexan ect.But maybe I'm dreaming.
Anthony "Creativity is detirmined by the way you hold your tounge".My Father
"Your generation will have to correct the problems made by mine".My Grandfather.
RevUpWind Regular Member Joined: 03/05/2008 Location: AustraliaPosts: 60
Posted: 03:37am 13 Feb 2009
Copy link to clipboard
Print this post
When a blade is spinning and the wind suddeny decreases the blades may go into negative relative pitch and really put the brakes on.
Blades in general are designed to take spiral slices out of the wind and slow it down to no more than around 2/3 when making max power. This is why blades are not too fat especially towards the tip. At the required power making tip speed ratio the blades are narrow enough across the cord to allow about 1/3 of the wind to swish through without blade contact before the next blade comes around. IE the wider the blades the more you will be likely to come up aginst Betz limit before max power is achieved. Which is why you don't see too many blades with a shape like the radiation warning symbol.
The slowing of the wind is caused by the wind making angular contact with the effective cross sectional area of the blades as well as impedance caused by the stuff in the centre of your mill. This increases the pressure at the front of the blades causing the air that is not making power to swish thru faster.
A relief area is usually provided just out from the middle stuff to allow pressure relief at the centre to allow Betz law to not be a problem by over slowing the wind.....River Heads - Australia
The wind blows wherever it wishes; you hear the sound it makes, but you do not know where it comes from or where it is going.
Page 1 of 3
Print this page
The Back Shed's forum code is written, and hosted, in Australia.
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
Print this post
.....its pretty easy to knock them off , part of a winglets mission in life is to find something to break itself off on
the room has since been redecorated ..
