Why are Darrieus turbines less efficient in highly turbulent tidal flows compared to axial flow turbines?

Darrieus turbines are less efficient in highly turbulent tidal flows compared to axial flow turbines due to their inherent design limitations in handling fluctuating flow directions and velocities. Darrieus turbines, also known as vertical axis turbines, rely on the lift force generated by the flow of water over their curved blades as they rotate around a central axis. These turbines are omnidirectional, meaning they can accept flow from any direction without needing to be actively oriented. However, this omnidirectional capability becomes a disadvantage in turbulent flows. In turbulent flows, the water's direction and velocity change rapidly and unpredictably. Axial flow turbines, which resemble propellers, can be actively oriented to face the incoming flow, maximizing energy capture even when the flow direction changes. Darrieus turbines, on the other hand, experience fluctuating angles of attack on their blades as the flow direction varies. The angle of attack is the angle between the blade's chord line (an imaginary line from the leading edge to the trailing edge of the blade) and the relative water flow direction. When the angle of attack deviates significantly from its optimal range due to turbulence, the lift force decreases, and drag increases, reducing the turbine's efficiency. Additionally, the complex flow patterns around the rotating blades in a Darrieus turbine create significant drag, particularly when the flow is turbulent. The changing flow directions cause the blades to stall more frequently, which means the water flow separates from the blade surface, creating eddies and reducing lift. Axial flow turbines are designed with streamlined blades that are less susceptible to stall, allowing them to maintain higher efficiency in turbulent conditions. Therefore, the inability to adapt to changing flow directions and the increased susceptibility to stall make Darrieus turbines less efficient than axial flow turbines in highly turbulent tidal environments.