Intracycle Angular Velocity Control of Cross-Flow Turbines
  Cross-flow turbines, also known as vertical-axis turbines, have numerous features that make them attractive for wind and marine renewable energy. To maximize power output, the turbine blade kinematics may be controlled during the course of the blade revolution, thus optimizing the unsteady fluid dynamic forces. Dynamically pitching the blades, similar to blade control in a helicopter, is an established method. However, this technique adds undesirable mechanical complexity to the turbine, increasing cost and reducing durability. Here we introduce a novel alternative requiring no additional moving parts: we optimize the turbine rotation rate as a function of blade position resulting in motion (including changes in the effective angle of attack) that is precisely timed to exploit unsteady fluid effects. We demonstrate experimentally that this approach results in a 79% increase in power output over industry standard control methods. Analysis of the fluid forcing and blade kinematics show that maximal power is achieved through alignment of fluid force and rotation rate extrema. In addition, the optimized controller excites a well-timed dynamic stall vortex, as is found in many examples of biological propulsion. This control strategy allows a structurally robust turbine operating at relatively low angular velocity to achieve high efficiency and could enable a new generation of environmentally-benign turbines for wind and water current power generation. 
Benjamin Strom, Steven L. Brunton, and Brian Polagye
physics.flu-dyn