Locomotion performance of an axisymmetric 'flapping fin'.

Inspired by the jet-propulsion mechanism of aquatic creatures such as sea salps, a novel locomotion system based on an axisymmetric body design is proposed. This system consists of an empty tube with two ends open. When the diameters of the front and back openings are changed periodically, the forward-backward symmetry is broken so that the system starts swimming. Viewed within a cross section, this system resembles a two-dimensional flapping fin with its leading edge located at the front opening and the trailing edge at the back opening. The feasibility of this system has been proven via numerical simulations using a fluid-structure interaction model based on the immersed-boundary framework. According to the results, at relatively low Reynolds number (O(100)), this simple locomotion method can easily achieve a mean swimming speed of 2 to 3 body lengthes per deformation period. Further simulations illustrate the following characteristics: 1) Within the chamber, the hydrodynamic interactions among different parts of the body leads to a performance-enhancing mechanism similar to the ground effect; 2) Reducing the diameter of the body can strengthen this effect so that both the swimming speed and the energy efficiency are improved; 3) For better performance the amplitude of diameter oscillation at the trailing edge should be larger or at least equal to the one at the leading edge. .