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Wind turbines use aerodynamic forces to extract energy from the wind. The two main aerodynamic forces are drag and lift. Drag applies a force on the body in the direction of the relative flow, while lift applies a force perpendicular to the relative flow.
Aerodynamics determines how much lift and drag forces are generated by the blades, which affect the torque and power output. When wind flows across the blade, the air pressure on one side of the blade decreases. The difference in air pressure across the two sides of the blade creates both lift and drag. The force of the lift is stronger than the drag and this causes the rotor to spin.
Wind turbine blades are shaped so that the air molecules moving around the blade travel faster on the downwind side of the blade than those moving across the upwind side of the blade. This is based on Bernoulli's Principle, or the Bernoulli Effect, which tells us that faster moving air has lower pressure.
Periodic aerodynamic forces caused by wind shear, off-axis winds and rotor rotation and randomly fluctuating forces induced by turbulence and dynamic effects are the source of fatigue loads and are a factor in the peak loads experienced by a wind turbine.
Related: Renewable Energy Practice Test: Wind Turbine Siting
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