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I'm currently having a bit of an issue selecting the type of aerofoil I would want to use in my ridge mounted wind turbine honours project. I want to select a high lift generating aerofoil at low speeds, but I am overwhelmed with the options.

The blade will be fixed and have no moving flaps, the wind speeds will be controlled with a variable stator upstream of the blades in order to provide a wind speed as close as possible to the optimal speed for maximum lift generation which would allow for a higher rpm and thus more power generation.

I know the parameters of the wind tunnel I will be testing in and the wind speeds I could encounter in real world applications. Through my research it seems that liebeck aerofoils are the way to go, if I'm right which one? If I'm wrong, what questions should I be asking and what should I be doing differently?

Are there any online sources I should read through before I make my choice or should i just pick a standard blade with a high CL/CD? I would have liked to have two choices in order to evaluate which would be the preferred design.

In real world application wind speeds will range from 1 m/s to 25 m/s. Therefore, I will replicate these speeds in the wind tunnel.

Henry
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  • An essential information is missing: At which Reynolds number (speed and size are the important parameters here) will the airfoil be operated? – Peter Kämpf Jul 11 '23 at 11:48
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    Does this answer your question? airfoil choice for wind turbine – sophit Jul 11 '23 at 11:48
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    You need to consider your manufacturing technique. Many of the extreme high lift airfoils end up with a concave bottom surface. This can be a challenge for several manufacturing techniques.

    You need to size your blades to achieve high Cl/Cd at your expected operating point -- it isn't enough to have a high max L/D, you have to be able to achieve it.

     – Rob McDonald Jul 11 '23 at 14:41
  • @PeterKämpf ive added the wind speeds expected with the Reynolds numbers being quite low. – Henry Jul 12 '23 at 10:03
  • @sophit The qblade is quite interesting thank you it definitley helped. – Henry Jul 12 '23 at 10:04
  • @RobMcDonald I will be 3D printing the design and applying epoxy over the surface for a smooth finish, I don't think the manufacturing process is too much of an issue in that regard as I have the universities resources at my disposal – Henry Jul 12 '23 at 10:05
  • If the Reynolds number is going to be so small I wouldn't care of the surface finish, as taught by the golf ball – sophit Jul 12 '23 at 10:17
  • @sophit The golf ball analogy is the wrong one. The dimples on the golf ball force turbulent flow -- which delays separation and reduces drag on a sphere. An airfoil is a streamlined shape designed to avoid separation anyway. At low Re, you may be able to have laminar flow, which would have lower skin friction drag than by forcing turbulence. Surface finish is important. – Rob McDonald Jul 12 '23 at 16:38
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    @RobMcDonald: at those low Re numbers, the laminar boundary layer is so sensitive that, as soon as the pressure starts to recover over the rear part of the airfoil, it separates from the surface making the rear part of the airfoil virtually stall and causing a big increase in pressure drag. So, depending on the needs, sometimes it's better to trade skin friction with pressure drag i.e. sometimes it's better to trip a earlier turbulent boundary layer (increasing the skin friction) but retarding the stall on the rear part (decreasing even more the pressure drag)... – sophit Jul 12 '23 at 18:13
  • ... so what you state is correct for flat plates and airfoils at very low AoA, but as soon as the AoA increases then the rise in pressure drag might not be acceptable anymore and tripping the change from laminar to turbulent boundary layer might actually decrease the total drag. This trick is widely used in model aircrafts. – sophit Jul 12 '23 at 18:16
  • @sophit Fair enough -- point is, it comes down to some details. We don't know the Reynolds number for sure. He states the expected airspeeds, but we don't know the chord. He plans on matching speed in the tunnel, but would probably be better off running the tunnel fast (by the ratio of chords) to match the Reynolds number. He is far from any compressibility effects. – Rob McDonald Jul 12 '23 at 21:08
  • @RobMcDonald: agreed – sophit Jul 12 '23 at 21:25
  • @RobMcDonald The golf ball is indeed misleading, and you are right insofar that roughness is only needed along a chord line to trip the boundary layer. But sophit is correct, at low Re you need to force the turbulent transition shortly after the pressure rises again. On the golf ball you don't know the flow direction, so you put dimples all over it. On the airfoil you do know it, so roughness is only needed along one carefully selected line. – Peter Kämpf Jul 13 '23 at 05:24

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Given the low end of your very wide speed range, your airfoil should have very low thickness and high camber, similar to a bird wing's airfoil.

Thickness helps when a wide range of angle of attack needs to be tolerated and of course when structural strength is required.

The SG6043 in the answer linked to by @sophit checks those marks. The S807 also mentioned is a root airfoil for the blade of a wind turbine and has its high thickness for structural reasons.

Peter Kämpf
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