Wind energy - Aerodynamics

I just noticed a pattern.

Doing 120 Things in 20 years has taught me a few things about myself. One is that whenever I need to  discuss aerodynamics, my medium of choice gravitates toward crayons and my seemingly limitless ream of old dot matrix printer paper rather than animation or 3D design tools. Interestingly this was my preferred media for everything when I was a five year old.

I have no idea what that means.

A modern wind turbine blade has a few improvements over the old water pumping windmills. It's blades don't just get moved out of the way by the wind, the blades interact with the wind in a much more amazing way.

When the wind passes over a wing shape, it has to travel further over the top than the stuff that passes under the wing. The path of wind lump "B" is much longer than the path of wind lump "A". This means the air has to stretch out. And that means you get a lower pressure above the wing than you do below it. Put all that together, and you get lift. This means the wing wants to fly up to the sky.

Yep, that's right, to the sky.

An old bi-plane might have a thick, high lift wing on an angle to the wind like "A" . (I think that angle is called "angle of attack")

A fast modern jet might have a shape, and angle of attack more like "B"

*Do not use these crayon drawings to build a modern, high speed jet. It's possible they may have not been as rigorously tested as they should be.

If we put the bi-plane wing on the jet, it would slow the thing down too much, and would geneate hilarious flight antics, and many spilled coffees. If we put the jet wing on the bi-plane we would have a funny shaped, propeller powered car, with wing looking things sticking out the sides. It would never get off the ground, because at bi-plane speeds, it would never generate enough lift.

Another interesting thing has to do with the speed different points on a wind turbine travel at, relative to each other.

Point "A" travels only about a quarter of the distance point "B" travels.

If both point "A" and "B" do one revolution, but point "B" travels 4 times the distance, then point "B" must travel a stack faster.



What all this means is that as the turbine is spinning, Point "A" sees the windspeed of the day, PLUS the extra speed it sees because the turbine is spinning. Point "B" sees the windspeed of the day, PLUS AN ABSURD EXTRA SPEED, seen because the windmill is spinning.

Absurd!

This extra windspeed that different points on a blade see, is called "apparent wind". As far as point "B" is concerned the wind speed might feel like 6 times the windspeed someone not standing on the blade might feel. Now, interestingly, we need to take this apparent wind speed into account if we want to use "lift" to power our wind turbine. If we present a blade so that the bottom is flat and point it into the wind, it will lift. If one end of the blade is attached to the centre hub of a wind turbine, it will lift, and rotate. But now the thing is rotating, there is some apparent wind. This means that the wind is no longer hitting the blade from the right angle. It's now hitting more from the top down. We have to rotate the blade so that its interacting with the APPARENT windspeed not the windspeed of the day. Because the blades will be rotating quite fast, most of the wind will present itself almost on the plane of rotation, so we adjust the blades so they are flatter to the direction of spin. I need a model to show this better.

"Why are you telling us this?" I hear you ask.

It turns out very clever people in lab-coats use all this stuff to design really efficient blades.

An ideal blade would be like a bi-plane wing at the centre, where the apparent wind speed is slowest, a jet wing at the tip, where the apparent wind is fastest, and everything else in between.

The more accurately we set all these varying angles the better our wind turbine will work. But we can also get a turbine to work inefficiently. It's up to us. I'll find some nice comfortable spot in between once I've explored a few different ways to go about making a set of blades.

And a model. I'll make a model.