Abstract
When a helix is rotated by a torque along its helical axis, a propelling force is generated as a reaction of the force exerted on the surrounding fluid by the helix. In this study, we numerically investigated the optimal shape of a helix which maximizes the ratio of the propelling force to the torque. The most efficient helix has the filament's diameter of a tenth of the wavelength, the pitch angle of 30 degrees, and the number of turns of around 1. With increase in the number of turns, increase in the propulsive force decays, while torque increases almost proportionally. Analysis of force distribution on the helix reveals that shear stress on the helix of inside half, having the opposite direction to the propulsion, increases more than the pressure force to propel. Shear force has partially the same direction to the propulsion on the outside half of the helix.
