Aerodynamics of modern wind turbine

[DaveC426913]

I've been thinking about the wind turbine (horizontal turbine - like an airplane propellor.) that I pass on the way downtown and I'm realizing I've been taking for granted how it works.

Seems to me it's exactly opposite of an airplane wing. Rather than forward speed generating lift, you have lift generating forward speed. (It is the wind attacking the blade rather than the blade attacking the wind)

The wind hitting the blades should create lift on the downwind side, pulling the blade in a perpendicular direction.

But it seems to me, the blades have an awfully high angle of attack - they are almost flat in the plane of the rotor, yet the wind's angle of attack is parallel to the axis. The result is that the blades attack the wind at a 45+ degree angle. Also, it seesm to me, as the wind picks up, the rotors move faster, which means they partially cancel the effectiveness of the oncoming wind. You'd want to raise the angle of attack to compensate, rather than lower it, like you would in a plane as speed increases.

 

[AlephZero]

Seems to me it's exactly opposite of an airplane wing. Rather than forward speed generating lift, you have lift generating forward speed. (It is the wind attacking the blade rather than the blade attacking the wind)

If you think about the air motion relative to the blade (or wing) the two are basically the same.

But it seems to me, the blades have an awfully high angle of attack - they are almost flat in the plane of the rotor, yet the wind's angle of attack is parallel to the axis. The result is that the blades attack the wind at a 45+ degree angle.

You need to draw a diagram of the velocity if the air relative to the rotating blade. You should find that the angle of attack relative to the air flow is fairly small - if it was large, the blade would tend to stall instead of working efficiently as an aerofoil. The "angle of attack" relative to the axis of the turbine might well be large. For a large turbine, the tangential velocity of the blade at the tip may be higher than the axial wind velocity.

Many large wind turbines run at constant speed (or two different speeds in light and strong winds) and the blade angle of attack is adjusted according to the wind speed and the electrical power being generated. A large AC electrical machine connected to the grid is constrained to operate in sync with the grid frequency - it will operate either as a generator or a motor, to keep turning at costant RPM. Clearly running a wind turbine as a motor isn't very useful, but if the wind speed drops suddenly that will happen till the blade angle of attack is changed and it starts to generate lift in the correct direction again.

Small turbines tend to run at variable speed, generate variable amounts of power, and use an electrical frequency converter to connect to the electricity grid.

FWIW, compare this with the fan blades on a large jet engine. At the blade tips the blade angle relative to the engine axis is (VERY approximately - no proprietary design secrets here!) 45 degrees because the axial air velocity (i.e. the forward speed of the plane) is the same order of magnitude as the tangential blade velocity). At the blade root the blades are aligned almost along the engine axis (and curved) because the axial velocity higher than the tangential. The blades are fixed, not adjustable, and so some large jets can't generate maximum power when the plane has no forward speed - the throttle is opened as the speed builds up during the takeoff run and the engine thrust can increase to its max value, without stalling the fan blades.

 

[russ_watters]

Seems to me it's exactly opposite of an airplane wing. Rather than forward speed generating lift, you have lift generating forward speed. (It is the wind attacking the blade rather than the blade attacking the wind)

There is no need to think of it as being fundamentally different from an airplane wing. Starting with the turbine as stationary, there is some amount of lift created by the airflow over the blades. As the blades start to move, you need to start factoring in the relative wind and adjust the pitch accordingly. That's exactly the same as how a plane works at takeoff. You'll see planes climbing at, say, 30 degrees nose-up angle, but that doesn't mean the angle of attack is 30 degrees. The aoa may be 5 and the other 25 is due to the relative wind of the plane being in a climb. When you are actually lifting off the runway, the pilot eases back on the stick, being careful not to raise the nose too fast before the plane can accelerate upward.