What specific aerodynamic phenomenon necessitates stall strips on offshore wind turbine rotor blades?

Stall strips are added to offshore wind turbine rotor blades to address a specific aerodynamic issue related to blade stall. Blade stall occurs when the angle of attack, which is the angle between the incoming wind and the blade's chord line (an imaginary line from the leading edge to the trailing edge of the blade), becomes too high. This excessive angle of attack causes the airflow over the blade's surface to separate, creating turbulence and a significant reduction in lift, therefore reducing the turbine's power output and increasing loads on the blade. In offshore environments, the inboard sections of the rotor blades tend to operate at higher angles of attack due to lower relative wind speeds near the hub. Because the blade moves faster at the tip than it does closer to the hub, the inboard sections experience a lower relative wind speed and thus a higher angle of attack. This can lead to early stall in these inboard sections, disproportionately impacting overall performance and increasing stress. Stall strips, typically small triangular or rectangular pieces of material fixed to the blade's surface near the leading edge, are designed to promote controlled turbulence. By creating a small amount of turbulence, the stall strip energizes the boundary layer (the thin layer of air directly adjacent to the blade surface) which helps to delay or soften the stall. In essence, they trip the airflow slightly, allowing it to remain attached to the blade surface at slightly higher angles of attack than would otherwise be possible. This ensures that the entire blade, especially the inboard sections, continues to generate lift more efficiently, resulting in increased power production and reduced fatigue loads on the turbine structure, especially during high wind events or periods of increased turbulence common in offshore conditions.