Windmill Blade Design: Learn Science & Math Behind Design for Tech Competition

[TheRyan]

I've searched the net and this site, and although I've found several sites and posts related to the idea of wind turbine / windmill design, I have yet to find anything explaining the science and math behind windmill blade design. I've entered into a technology competition which involves designing blades for a pre-made turbine. The fact that the motor / gears are the same for everyone makes the playing field level from the start, as well as the fact that we'll all be given the same supplies and tools.

We'll (my 2 person team) be given two hours to design the blades (and "nosecone" to mount them on), then test them. The design which performs best, in other words generates the most electricity wins. Can someone point me in the right direction for learning about windmill blade design? I'm guessing the materials will be composed of Styrofoam and cardboard sheets, possibly some others, and tools will include hot melt glue, scissors, tape, and a knife or other appropriate implements. It isn't specified whether the generator's axle must be either horizontal or vertical, but because it's called a "windmill blade design" contest I'm guessing it'll be horizontal. Any help would be great!

 

[mgb_phys]

The main difficulty in real-world wind turbine design isn't generating the most electricity at a given speed - it is making blades which will work at all across a range of wind speeds. An especially difficult problem is making the blades start from zero speed - although some sysems actually power the turbine to spin it upto a certain speed before using it as a generator.

 

[TheRyan]

The power source will be an electric fan, the turbine will be tested inside and so those components of the competition will be the same for all teams. I'm not sure if it would be possible to make an airfoil with the materials given, let alone make multiple identical airfoils (for as many blades as would be used). There is no need for the design to work at a range of wind speeds: only one will be used during testing.

 

[luben]

i have not tried such design before, but just want to share some thoughts, hope it helps...

it seems to me the competition uses an electric fan to blow your windmill model, which generates electricity through a connected motor. Given the motor being the same, the main concern is how to convert effectively the fluid(air) flow energy to the kinetic energy of the windmill. In principle, the energy in fluid is related to Bernoulli term $$\rho$$$$U^{2}$$. The pressure buildup across the blade is minimal(atmospheric surrounding), so the major transfer machanism should be the change of momentum of the incoming fluid by the blades(i.e. accelerating the fluid into a different direction), and create a reaction force to push the blade. The major factor of the blade design is, therefore, to turn the flow by nearly 80$$^{o}$$~90$$^{o}$$ and to maintain the speed of the leaving fluid.

The shape of the blade is the key, which have to prevent the loss of this Bernoulli term and to turn the fluid smoothly. How to do so? Often it is about streamlining the blades. A streamlined design with gradually changing curvature will turns the fluid direction by 90$$^{o}$$ without disrupting the flow. you can easily find turbine blade picture by some googling and get the idea. Your shape does not have to be very decent, a simple curvature in a card-board manner should capture most of the effects.

Another key factor should be the frontal blade area. I think a windmill having comparable size as the fan is effective enough. The weight of the blade also constitute some friction loss.

 

[frogman]

being you don't have a lot of time to make an air foil shaped blade. I think the pitch or angle of your blade will be the most important. I don't know about wind turbines, but windmills such as used to pump water, have fixed blades with a pitch between 15 and 30 degrees.

 

[TheRyan]

Thanks for the great ideas. By curving the blades, I'm assuming you mean concave towards the wind source, so the air moves "smoothly" as it changes direction as opposed to a flat, angled blade which would change direction suddenly and still leave some force against the blades in the direction perpendicular to the turbine's turning axis.

 

[luben]

Yes you got the idea. It is hard to calculate exactly which way of curving is optimal, but those ideas can at least help you create sub-optimal design. Try several designs first will be sort of fun i guess

 

[laurentien]

This competition is a very good idea, beware that they might retain the good ones and make a lot of money from it ;^)

In discussions within researcher fellows, we have noticed that the actual windmills that we see around Lancaster and the Lake District (UK) are all made with old fashion propellers with a design which is similar to the WWII war planes (Lancaser airplanes, for example).

It has been a while that aircraft designers such as Saab and Bombardier have been using six curved and twisted propellers drawn by CAD programs such as Catia. Have a look on the Q400 fo example. It is time to go that way. This allows to collect more power with available winds. Of course, if you had more propellers, the starting torque will be increased by the total mass. A question a compromised.

LHR

 

[TheRyan]

Thanks for the help, I passed it along to the person I would have been working with during the competition. However, I was unable to compete due to the re-scheduling of the testing/judging of another competition I was involved with. The other person on my team was able to get help in building his windmill blades from others at the competition from my school. His designs were based on research he had done on Wikipedia and other sites, with airfoils about 6" wide by 1.5" thick, made with 1/8" balsa ribs and 1/8" styrofoam covering them. He only made 3 blades, which looked really nice, but were extremely heavy.

I warned him that basing the design on those of very large windmills could produce uncertain results, but he followed through anyway, and barely produced any electricity as the blades could not spin very quickly. The winning team's design consisted of 6 rectangular pieces of styrofoam glued to wooden dowels and angled from the direction of the "wind" (electric fan). I think he learned the value of simplicity.