Investigations on vortex structures for undulating fin propulsion using phase-locked digital particle image velocimetry

The Gymnarchus niloticus fish can swim in surging and heaving directions only with a long undulating ribbon fin while keeping its body along almost straight line.These features substantially inspire the design of underwater vessels with high maneuverability and station keeping performance,which is characterized by peculiar vortex structures induced by undulating fin propulsion.To reveal the propulsion mechanism under the evolution of these complex vortex structures,the variation of velocity field with the undulating fin’s wave phase on cross section and mid-sagittal plane at wave amplitude of 85°is investigated by phase-locked digital particle image velocimetry(DPIV).Through experimental flow field images,two typical vortex structures are clearly identified,i.e.,streamwise vortex and crescent vortex,which is further explained by supplemental numerical simulations using large eddy simulation.Vortex characteristic and its evolution on cross sections and mid-sagittal planes is investigated,and its relationship with thrust,heave force is also analyzed.It is found that the two kinds of vortexes induce the main hydrodynamic forces in two directions synchronously,which brings the undulating fin propulsion an extra-ordinal maneuverability.The research will be useful for understanding the potential mechanism of this novel propulsion and is of great application prospect in designing more maneuverable underwater vehicles.

Braking Performance of a Biomimetic Squid-Like Underwater Robot

In this study, the braking performance of the undulating fin propulsion system ofa biomimetic squid-like underwater robot was investigated through free run experiment and simulation of the quasi-steady mathematical model. The quasi-steady equa- tions of motion were solved using the measured and calculated hydrodynamic forces and compared with free-run test results. Various braking strategies were tested and discussed in terms of stopping ability and the forces acting on the stopping stage. The stopping performance of the undulating fin propulsion system turned out to be excellent considering the short stopping time and short stopping distance. This is because of the large negative thrust produced by progressive wave in opposite direction. It was confirmed that the undulating fin propulsion system can effectively perform braking even in complex underwater explorations.