Abstract
We explain how the flow of air around the rotor blades of a wind turbine creates a lift force, which turns the rotor around its axis and drives a generator of electric energy.
Rotor Blade Acts Like a Wing

...the air flow over the rear side must have a higher velocity…Greater velocity produces a pressure drop on the rear side of the blade, and it is this pressure drop that produces the lift.
Lift and Drag of a Wing
Mechanism
Sideview of velocity and pressure, and topview of streamwise vorticity of Naca0012 wing at aoa = 14. Observe the turbulent streamwise vorticity emanating from top separation, as sketched above. Computed solution of the NavierStokes equations with slip boundary condition [1].
Principle of action of a wing: Potential flow (upper left) with zero lift/drag modified by lowpressure counterrotating rolls of streamwise vorticity from instability mechanism at separation (upper right), switching the pressure on rear wing (bottom) to give both lift and drag (H high, L low pressure).
Lift/Drag Ratio L/D
We see (larger figs) that for a symmetric airlfoil like a Naca0012, L ~ 2.5 is maximal for aoa = 20 with L/D ~ 3 small, while for unsymmetric airfoil like a sail, L/D > 6 for aoa = 20 at maximal lift. This means that a sail works efficiently at maximal lift for aoa = 20, while a symmetric airfoil has a satisfactory L/D only for aoa < 15 with nonmaximal lift (as discussed in more detail in Why It Is Possible to Sail). A propeller normally is rather thin and is more similar to a sail than a symmetric airfoil.