Flow Visualization and Performance Measurements of a Flagellar Propeller

A new type of propeller that is optimized for low Reynolds numbers is required to propel a small object in a medium where the flow is dominated by viscous rather than inertial forces. A propeller in the shape of a bacterial flagellum seems an appropriate choice for driving a small object. Accordingly, in this study, we visualized the velocity field induced by a spring-like propeller inspired by the Escherichia coli flagellum, using a macroscopic model and applying stereoscopic particle image velocimetry. We also experimentally evaluated the effect of pitch and rotational speed on the performance of this flagellar propeller. Silicone oil, which has a kinematic viscosity 100,000 times that of water, was used as the working fluid to generate a low Reynolds number for the macroscopic model. Thrust, torque, and velocity were measured as functions of pitch and rotational speed, and the efficiency of the propeller was calculated from the measured results. We found that the flagellar propeller reached a maximum efficiency when the pitch angle was approximately 53. Compared to pitch, rotational speed had a relatively small effect on the efficiency, and the pitch altered the flow pattern behind the rotating propeller.