China Ship Scientific Research Center, Wuxi, China
As one of MPF modes(median／paired fin modes),the torsional wave propulsion by flexible long fin offers several advantages including good propulsive efficiency, great maneuverability at low speeds. The Gymnarchus Niioticus fish(GNF) cruises generally with high efficiency with long dorsal-fin torsional wave propulsion while keeping its body for the straight line.
The flow field of torsional wave propulsion is numerical simulated by Large Eddy Simulation and dynamic grid technique with diffusion-based smoothing model. Then the vortex spatial structure and its evolution are analyzed intensively by combining the vortex structure with pressure distribution on fin surface and unsteady force.
The results indicate that there are mainly four kinds of vortexes in the flow field, i.e. streamwise vortex, crescent vortex, toroidal vortex and wake vortex. The crescent shape vortexes mainly induce high speed axial jet, which produces the primary thrust. This is hydrodynamic mechanism of torsional wave propulsion. The streamwise vortexes mainly induce the vertical jet, which generates the primary heave force.