Physics

[Blinx]

For my Physics coursework I did an experiment on how Windspeed affects the output voltage of a six bladed turbine. It worked well and I got good results and I've been able to explain them.
My only quibble is explaining the theory about it. Now I know how the properties of Lift and drag work on say a wing (greater flow underneath, decrease in pressure = lift), but how about a windturbine that did not curve and was titled at an 80 degree angle?

Should I draw a diagram of Lift? I don't think I should because what is really happening is more turbulent airflow caused by the windturbine blade impeding the wind.

So should I draw a diagram of how turbulent flow is generated? Fair easy, the wind passes around the object and forms vortexies behind it. Do these vortexies generate the lift required to turn the turbine? Or is it something else?

Thanks

[thomasp]

When you say the wind turbine was tilted at an 80deg angle and "did not curve", do you mean there was no camber on the aerofoil cross section (http://en.wikipedia.org/wiki/Camber_%28aerodynamics%29) and it was at an 80 degree angle of attack to the wind? If so, that very fact of an 80deg AoA will generate lift for reasons that still confuse me :p

And airflow is faster over the "top" surface of the aerofoil, not the bottom. The bottom has slow flow, high pressure, the top has low pressure, fast flow.


Not really sure why you want to go down the turbulent flow route - wind turbines are surely exactly the same as helicopter rotors or aircraft propellors, apart from they're not being driven by a motor. The fact that you have air flowing over an aerofoil cross section will generate lift. This will try to pull the blade "up", but since it can only rotate, it will therefore rotate around the centre. And since lift is proportional to velocity squared, the more airspeed (wind) you have the greater the lift, so therefore the faster the rotational speed.

Similarly with helicopters, the faster you turn the rotors, the more lift you get since the air is essentially flowing faster over the blades.

[Blinx]

What I mean is that the Blade wasn't a proper curved aerodynamic blade. It was a flat bit of yellow plastic that if you left it flat, facing straight towards the windsource, it did not turn.
So by twisting each of the "blades" 10 degrees (sorry, I meant 80 degrees from as if it was flat) then it begun to turn.

My problem with my coursework was that I explained the incorrect thing. I explained it well, but I explained how a proper windturbine would work.

This is the diagram for the experiment.
http://img111.imageshack.us/my.php?image=windturbinejf4.jpg

Effectively what I did was use a cooling fan with different speed settings. Connected the windturbine to a voltmeter, and then proceed to take readings at intervals of 10cm, 20cm and 30cm away from the fan. I'd then up the speed and do it again.

[thomasp]

What I mean is that the Blade wasn't a proper curved aerodynamic blade. It was a flat bit of yellow plastic that if you left it flat, facing straight towards the windsource, it did not turn.
So by twisting each of the "blades" 10 degrees (sorry, I meant 80 degrees from as if it was flat) then it begun to turn.

That's because by turning it you've essentially given it "camber" (not in the conventional sense of an aircraft wing), so it can now generate lift. Therefore, it'll work on the same principle as a wind turbine.

If you keep turning the blade it'll generate more and more lift (ie spin faster for a given airspeed) until you reach the point at which that aerofoil (yes you can have rectangular aerofoils - they're just not very efficient :p) stalls and the flow separates from a large portion of the blade - lift will then decrease quite dramatically.


If I'm being a bit too technical let me know - I'm thinking more in terms of aircraft, since that's what I learnt this theory for!

[Blinx]

Hehehe. It's no problem. I understand it I think. :)

"The physics behind how the wind speed will affect the voltage output is linked to lift. Lift is generated when air passing over and under the blades cause alterations in pressure. Due to the curve and angle of the blade the wind flows more easily underneath the blade than it does over the top. This ease of flow generates lower pressure underneath the blade compared to the pressure on top. This low pressure lifts the blade up and therefore causes rotation. The greater speed at which the flow occurs, the greater the amount of lift that is generated and therefore the faster the blades will turn, so long as the blades are of an angle that allows it to do so. This will then generate more electricity. "

That is what I typed in my physics report. Seems to be correct with what you say. :) But I haven't mentioned the camber. I will do so now. :) Thanks man.

[thomasp]

Hehehe. It's no problem. I understand it I think. :)

"The physics behind how the wind speed will affect the voltage output is linked to lift. Lift is generated when air passing over and under the blades cause alterations in pressure. Due to the curve and angle of the blade the wind flows more easily underneath the blade than it does over the top. This ease of flow generates lower pressure underneath the blade compared to the pressure on top. This low pressure lifts the blade up and therefore causes rotation. The greater speed at which the flow occurs, the greater the amount of lift that is generated and therefore the faster the blades will turn, so long as the blades are of an angle that allows it to do so. This will then generate more electricity. "

That is what I typed in my physics report. Seems to be correct with what you say. :) But I haven't mentioned the camber. I will do so now. :) Thanks man.

Sorry to be pedantic, but it's the pressure differential that generates lift, and that lift induces rotation due to the blades being fixed at their root but able to rotate.

[Blinx]

Oh.

Balls. Right. I should change that then.

Thanks for pointing that out!