Experimental and theoretical fluid dynamics of spherical Savonius turbines operated in pipe flows

The performance of Savonius turbines driven by flow in a pipe is experimentally investigated. The turbine is manufactured to have a spherical outline based on the pipe cross section at a small clearance. The torque and power of the turbine are obtained from the time derivative of the rotational speed measured using a high-speed camera and an equation of rotational motion. We find that the idling tip-speed ratio of the turbine exceeds 5, which is much greater than that of a turbine operating in an open free stream. This proves the dominance of pulsatile flow through the gap between two hemispherical blades in torque generation. Widely varying the gap (i.e., the overlap ratio O_R of the Savonius turbine) reveals that a turbine with O_R = 30 % has the highest power coefficient. The output efficiency exceeds 50 % for a tip-speed ratio of approximately 3. These experimental results are supported by fluid dynamics theory and computational fluid dynamics simulation, which clarify the driving mechanism of the turbine in a pipeline.