How do wind turbines do their thing?

[DaveC426913]

TL;DR Summary

I mean, I know how wind turbines work, but I realize I don't know how they actually work, y'know?

We have wind turbines like this in the city on top of buildings:

I asked about standard wind turbines before but this design raises even more questions.

1. I assume all turbines have resistance because that’s how they generate energy. They always look like they’re free-spinning, but on a calm day I assume if I could get up there and push it by hand I would meet a lot of resistance as it is pushed the rotor through the magnetic field. This should go without saying but I'm just sayin' it.
2. Those little inner scoops. They are locked tot eh outer blades - the whole thing spins as one. I assume they become effective under different conditions, say, low wind speeds.
3. I assume the bowed shape here has a number of practical functions - makes them more compact, safer (eliminates bird-clobbering blade tips), and reduces tip vortices.
4. I also assume there is a significant efficiency cost to have most the blade surface at an oblique angle. Ideally, you’d want the blade surface to be parallel to the axis, so that lift is perpendicular. A blade section that’s at a 60 degree angle is wasting 50% of its energy, right?
   
   
5. Finally, - and this is something that I have never understood - you want the blades to generate lift from their passage through wind. Surely, the direction of the lifting force should match the direction of rotation, right? So why do these blades seem to be flat (i.e their surface are tangential to the rotation, therefore zero angle of attack)?
   
   
   

 

[berkeman]

Paging our wind turbine expert @cjl

 

[Lnewqban]

Those surfaces induce drag rather than lift, extracting kinetic energy from the horizontal airflow.
From a vertical perspective, one half of the rotor has higher coeficient of drag than the opposite one.

 

[.Scott]

From the Darrieus Wind Turbine wiki article:

When the Darrieus rotor is spinning, the aerofoils are moving forward through the air in a circular path. Relative to the blade, this oncoming airflow is added vectorially to the wind, so that the resultant airflow creates a varying small positive angle of attack to the blade. This generates a net force pointing obliquely forwards along a certain "line of action". This force can be projected inwards past the turbine axis at a certain distance, giving a positive torque to the shaft, thus helping it to rotate in the direction it is already travelling in. The aerodynamic principles which rotate the rotor are equivalent to that in autogiros, and normal helicopters in autorotation.

 

[DaveC426913]

From the Darrieus Wind Turbine wiki article:

When the Darrieus rotor is spinning, the aerofoils are moving forward through the air in a circular path. Relative to the blade, this oncoming airflow is added vectorially to the wind, so that the resultant airflow creates a varying small positive angle of attack to the blade. This generates a net force pointing obliquely forwards along a certain "line of action". This force can be projected inwards past the turbine axis at a certain distance, giving a positive torque to the shaft, thus helping it to rotate in the direction it is already travelling in. The aerodynamic principles which rotate the rotor are equivalent to that in autogiros, and normal helicopters in autorotation.

Thank you. Now I have a name I can look up.

 

[DaveC426913]

Found this, which shows diagramatically the forces of wind and lift:

(The original diagram is irritatingly illustrated backwards which does not help with comprehension, so I have flipped it for consistency.)

 

[MontufarServidone]

For a Darrieus-style VAWT (which this looks like), each blade passes through a full rotation, and the apparent wind hits it from different angles the whole time. This is why the turbine needs to move for lift to build — it relies on a kind of cyclical lift generation, and some parts of the rotation generate negative torque (a net energy loss), but overall, the average torque is positive.

Here’s a good analogy: it’s like a cyclist pedaling in a circular motion — each leg goes through ups and downs in force application, but the net force drives you forward.

 

[DaveC426913]

it relies on a kind of cyclical lift generation

Sounds a little like heading up into the wind when sailing. You can't do it right from a stand still, but the faster you go, the more apparent wind you generate, so the faster you go.

 

[sophiecentaur]

Found this, which shows diagrammatically the forces of wind and lift:

The diagram would be improved a lot if it showed say three positions of the rotor with respect to the wind. This point has already been made but it would be nice to compare the effect at different angles. Obviously the effect in the 'upwind downwind' position would be to produce torques in nearly opposite directions producing a nearly net zero resultant.

There are two basic designs for vertical axis wind turbine, the Darrieus and the Savonius. One uses lift and the other uses drag. (see the Wiki search result)
Neither is very efficient, compared with a horizontal axis turbine, which is why you don't see many of them around.
Most of that style of turbine have several blades and each blade would be in the preceding blade's disturbed air. That would mean the angle of the face of the blades would need to be different from that of a two bladed turbine. I'd suggest that a suitable cam system could also improve the performance by changing the pitch according to the angle to the wind. BUT that would take away the one advantage that a simple vertical axis turbine doesn't need to be 'pointed' at the wind.

 

[A.T.]

Found this, which shows diagramatically the forces of wind and lift:
View attachment 359873
(The original diagram is irritatingly illustrated backwards which does not help with comprehension, so I have flipped it for consistency.)

I tried to find a better vector diagram online, but most of them are plain wrong. The shown tangential force that turns the rotor is often inconsistent with the total aerodynamic force (lift + drag). Seems like they draw too much drag, and then simply cheat to get the right direction of the tangential propulsive components.

The best I could find is Fig. 2 in this paper (The legend is on page 1):

https://link.springer.com/article/10.1007/s40095-014-0129-x

It doesn't show the propulsive tangential force Ft, but you get it by projecting the total aerodynamic force R* onto the velocity wR.

Note that Ft can be opposed to wR for a certain range of angles, so you get negative propulsion (braking) from the blades in that range. But what matters is that the average over an entire cycle is positive.