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
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 Navier-Stokes equations with slip boundary condition .
Principle of action of a wing: Potential flow (upper left) with zero lift/drag modified by low-pressure counter-rotating 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 non-maximal 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.