What is the fundamental difference in the energy extraction mechanism between an axial flow turbine and a cross-flow turbine in a tidal stream?

The fundamental difference in the energy extraction mechanism between an axial flow turbine and a cross-flow turbine in a tidal stream lies in how they interact with the water flow: axial flow turbines extract energy primarily through lift forces acting on rotating blades aligned with the flow direction, while cross-flow turbines extract energy using a combination of lift and drag forces as water passes through the turbine perpendicular to its axis of rotation. Axial flow turbines, similar to airplane propellers or wind turbines, have blades that rotate around an axis parallel to the direction of the water flow. The blades are designed with an airfoil shape, which generates lift as the water flows over them. The lift force causes the turbine to rotate, extracting energy from the tidal stream. Cross-flow turbines, also known as vertical axis turbines, have blades that rotate around an axis perpendicular to the direction of the water flow. The water flows through the turbine, interacting with the blades on both sides. The blades are typically curved or shaped to capture energy from the flow. The energy extraction mechanism in cross-flow turbines is more complex than in axial flow turbines, involving a combination of lift and drag forces. As the water enters the turbine, it exerts a force on the blades, causing them to rotate. The blades on the downstream side of the turbine also experience a force from the water, which helps to sustain the rotation. The fact that axial flow turbines use lift force efficiently makes them usually more efficient in constant stream flows. The omnidirectional capacity of cross-flow turbines becomes more beneficial in complex flow conditions. Therefore, the key difference is the primary force used: lift for axial flow, and a combination of lift and drag for cross-flow, leading to different performance characteristics and suitability for various tidal stream conditions.