Blade Element Momentum Theory 3

[bd411]

Hi all,

I am new to this forum and have to say am really happy to have found such an extensive knowledge database online

I am a university student studying a bachelors in Electrical and Electronic Engineering in Europe, in my final year. My final year project is to do with optimising rotor designs for a miniature wind turbine, for use in energy harvesting for wireless sensor networks.

In order to complete my project to an excellent standard, my supervisor and I agree that it would be beneficial for me to develop an understanding of aerodynamics. In particular I will need to have a robust understanding of BEM (Blade Element Momentum) theory and any fluid mechanics / aerodynamics leading to this.

Therefore, my question is, can anybody recommend a course of study, or any resources in particular that I can use to help myself understand these concepts? I have to say I am a complete beginner to fluid mechanics / aerodynamics and the last time I studied mechanics was back in high school.

Thanks in advance - all advice is greatly appreciated.

 

[rb1957]

MIT's opencourseware

Quando Omni Flunkus Moritati

 

[GregLocock]

Also, I think you have received bad advice. Unless you are going to design a blade airfoil from scratch then you need to understand how a prop selection chart works, about 1/2 day at most, rather than the details of momentum theory.

Cheers

Greg Locock


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[KENAT]

So, to get anything like a detailed understanding of aerodynamics you'd have to take at least a quarter, possible more, of my entire course at university on an Aerospace Systems Course.

That said, home builders and model builders manage to get by so maybe you don't need that much.

Make it interesting, look into Vertical Axix Wind turbines and not just the 'drag' type.

Do you have any aero or mechanical professors in your school you could talk to? Chances are the library may have some relevant resources.

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[bd411]

Thank you very much for all your responses. I have managed to delve into online resources and think I've managed to get a reasonable insight into BEM. Kenat, thank you for mentioning VAWT's, I'll be sure to look into this as well.

I do have a rather basic question that I hope somebody could answer, having had a look at basic aerofoil mechanics, I understand that the increased velocity of the fluid on top causes a decrease in pressure compared to the underside, causing lift. I have read about the "fallacy of the equal transit theory', where one claims that air molecules must meet each other at the other end. My question is therefore, why does the velocity of the fluid increased on the top side of the aerofoil?

Many Thanks in Advance.

 

[KENAT]

Ha, ha. Well that chestnut was revealed to me as the culmination of the first year fluids course at my uni. I think it would do you good to do some digging in real aero text books rather than online - where the websites of even nominally reputable organizations sometimes are at best misleading on this topic.

While I won't go into detail, [at least as I was taught by this guy] it's all to do with the effective angle of the aerofoil to the air stream setting up a vortex system which leads to a bound vortex on the 'aerofoil' which adds to the velocity on top of the wing and slows it below.

The explanation you find should probably mention Kutta condition, maybe even Kutta-Zhukovsky.

Be warned there are some folks out there that will debate this (it has happened on this site), & I got lost trying to follow the guy at "secret of flight" so I'm not gonna claim to be smart enough or have spent enough time delving into it to help you much more.

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[rb1957]

my 2c (and i don't think it's worth that !) ...

"what causes the reduced pressure above the airfoil ?" ... the increased airspeed,
"what causes the increased airspeed ?" ... the reduced pressure.
it's a "chicken and egg" problem.

i think the current theory is based on "Prantdl lifting line". there are several "contenders". like Kenat i've given up getting to involved in them. airfoil lift is due to lower pressure above the wing and to a lesser extent higher pressure below. Airspeeds change courtesy of Bernoulli. It's worked pretty well for 100 years.

the "equal time of travel" is a simplification that tries to rationalise decriptively the faster and slower airpseeds but is flawed because ...
1) it doesn't involve pressure
2) why do the two particles have to join up again (after traversing the wing) ?

Quando Omni Flunkus Moritati

 

[3DDave]

There is an excellent series of videos on fluid dynamics on Youtube posted by Barry Belmont. They are numbered 1.title, 2. title and so forth and were produced by MIT, Cambridge, et al. They are excellent at combining explanation with demonstration, so there is no question about the effects. I've only gotten to 9.1 on Vorticity, which is part of what mathematically is used to evaluate subsonic wings, so I know the video that shows flow around wing sections is in the first 8.

 

[Sparweb]

BD411,
It sounds to me like you have been given a ridiculously complicated assignment for a very simple task. Like studying General Relativity for a year before answering the question "how long does a ball take to fall from a 3 storey building to the ground?"

If you are studying Electrical Engineering, I also can't see how an in-depth knowledge of BEM will advance your thesis. Any BS calculation will impress your EE review team. Surely you must be more concerned with the energy harvest in the micro-generator to which the blades are attached?

If you define the problem as: "Given a certain rotor speed and torque, the input power of the rotor will be converted into electrical power by this generator, thusly..."
then you can dispense with all of the aerodynamics.

In your orignal posting:
"... a miniature wind turbine..." So the cost of the rotor is trivial...
"...energy harvesting for wireless sensor networks..." whose energy requirements are also trivial...
"I am a university student..." I'm supposed to red-flag this. Forum rules.

Anyway, go look up the BEM papers about wind turbines funded by Sandia National Labs (USA) under the name of Lissaman and Wilson

http://wind.nrel.gov/designcodes/papers/

As we say in Canada, fill your boots!

STF

 

[bd411]

Thank you very much for all your help and advice.

SparWeb, I will need to be designing rotor blades for the energy harvester and analysing its performance, hence the need of BEM. I very much appreciate the resource you've given me and will use to to good effect. It is odd to be going this far into another discipline, however I find it far more interesting than anything I learnt in EE and therefore am very enthusiastic about it.

Furthermore, apologies about breaking the forum rules. I'll be more prudent next time.

Kind Regards,

BD411

 

[3DDave]

I'd give him a student pass on the grounds that he's not asking others to do his homework, which is more professional than some "professional" requests I've seen.

bd411, have you watched the series on YouTube by the National Committee for Fluid Mechanics posted by Barry Belmont?

They discuss a variety of topics concerning liquids and gases. Some of them may prove helpful in either understanding the material or explaining it to others, particularly flow visualization.

 

[GregLocock]

"however I find it far more interesting than anything I learnt in EE and therefore am very enthusiastic about it."

Ah, the force is strong in this one. Always pursue hard things that interest you if you can afford the time. Your posts are quite welcome, if anyone argues tell them you know the secret handshake, and refer them to this thread.

I still think that in terms of your project you could easily get by without it, but that's OK.



Cheers

Greg Locock


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[bd411]

Thank you very much everyone !

I will be sure to check out the National Committee for Fluid Mechanics posted by Barry Belmont videos. I have heard they are fantastic.

Currently I am enjoying the lecture series on Wind Energy by IIT. Such is my interest, I have begun to entertain the thought of studying masters in the mechanical / aero discipline. My supervisor says this is possible even with my EE background. My main interest is in the Automotive and Motorsport (e.g. Formula 1) sectors, does anyone have an opinion on whether this would be a good move ?

Regards,
BD411

 

[KENAT]

Well, Formula 1 teams booked out many of our larger wind tunnels etc. at university while I was there in the late 90's.

At the start of my aero course the department head (I think) said something along the lines of 'Aero is one of the most demanding courses because you'll have to cover most of the stuff that mechanical, electrical & materials engineers cover...'. I suspect this was a gross exaggeration but there's some truth to it.

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[Sparweb]

bd411,
I think everyone caught on that I was testing you on the student posting stuff. I didn't actually hit the red flag on you. Since you stood your ground and you certainly sound eager to study these subjects, then I want to feed you more, probably more than anyone can absorb:

http://energy.sandia.gov/?page_id=3057

http://www.nrel.gov/wind/

A lot of the Sandia work is theoretical, and much of the NREL data is done in the field. Sure there's always a mix of theory and practice, but as a rough guideline that's how I look at them. The NREL Small wind testing results are probably a great way for you to see summarized in one place how the year-long testing and performance measurement of a few types of wind turbine really pan out in the real world, and most applicable to your project, I expect. I strongly recommend following the links to the Mariah Energy Windspire, where you will discover a detailed report on a device that failed multiple times during testing. One of the best documented cases of rushing a product to market that I've ever seen.

There is also a lot of public-domain data available from the organizations that promote wind energy in various countries. Of particular use are the AWEA (american) and Windpower.org (danish).

Oh and there's this guy:

http://www.sparweb.ca

who takes an interest in building small-scale wind turbines, does testing occasionally too.

STF

 

[mohr]

Hi bd411:

My reply and suggestion may be late....try with WIND ENERGY HANDBOOK, Burton,Sharpe,Jenkins,Bossanyi...it is a very complete book.


Cheers

 

[bd411]

Hi all,

I'm pleased to say that my project is progressing well and have found everyone's advice very helpful indeed!

I was however wondering if anyone could help me with a couple of questions. I have been given some BEM MATLAB code to help simulate the rotor designs that I come up with and speaking with my supervisor, we both agreed that if I could further the model in some way it would be really beneficial. In particular I was asked to look at "wake rotation".

Therefore I wonder if anybody would be able to point to sources or have some knowledge of including the phenomena of wake rotation in my code, currently it implements the classical BEM theory. I am also interested in implementing "Prandtl's tip loss correction factor", but haven't been able to find any literature on this for the case of shrouded turbines. Does anyone know if there is literature relating to this?

Any other opinions / thoughts are, as always, most welcome.

BD411

 

[Sparweb]

Prandtl worked on numerous lifting surface analysis models, from fixed-wing aircraft, to rotorcraft, to propellers. I didn't think he got involved in wind turbine research, but maybe he did or maybe his work was "adapted" to use in wind turbines. Prandtl's work is easily found at the NACA technical report server.

http://naca.larc.nasa.gov/

His work was so influential that you may have trouble finding what you want through all the cross-references.

...wait a second...

What's wrong with doing a google search of Prandtl's tip loss correction factor??? LMGTFY?
First hit.

STF

 

[moon161]

Hugh Piggott has written a few books on small scale wind power.

http://scoraigwind.co.uk/

Probably longer on pragmatism than calculus.

 

[IFRs]

See if your college has a Formula SAE group - lots of fun and learning there!