Various fluid turbine systems and methods are described. The turbine can be a vertical axis wind turbine configured to generate power from wind energy. The turbine system can have a blade assembly. The blade assembly can have a plurality of blades rotatable about an axis. The turbine system can have a concentrator positionable upwind and in front of a return side of the blade assembly. The concentrator can define a convex surface facing the wind. The turbine system can also have a variable concentrator positionable upwind of a push side of the blade assembly. The variable concentrator can be adjustable between a first position and a second position, the variable concentrator being capable of deflecting more wind toward the turbine in the first position than in the second position.