Numerical study on vortex structure of undulating fins in stationary water

The Gymnarchus Niloticus fish(GNF) with long dorsal-fin undulating generally cruises with high efficiency and extra-ordinal maneuverability while keeping its body for straight line. In the paper the unsteady flow field of undulating fins in stationary water is calculated by Large Eddy Simulation and dynamic grid technique with diffusion-based smoothing model. The thrust produced by undulating fins is measured at wave amplitude of 850, and the unsteady flow field around undulating fins on middle cross section at wave amplitude of 850 at a series of phase angles is measured by phase-locked PIV in the stationary water. The thrust calculated by numerical simulation is in good agreement with hydrodynamics experiments. The streamwise vortex structure and its evolution on cross sections calculated by numerical simulation qualitatively agree well with PIV experiments. The numerical results at wave amplitude 300 indicates there are three kinds of vortexes in the flow field, i.e. streamwise vortex, crescent vortex and toroidal vortex. Vortex characteristic and its temporal and spatial evolution is described, and the vortex structure and its relationship with thrust, heave force is analyzed. It is interesting that the Gymnarchus Niloticus fish(GNF) swims with long dorsal-fin undulating with high efficiency and extra-ordinal maneuverability while keeping its body for straight line. There are few researches on vortex structures to discover propulsion mechanisms. The highlights is as follows,1.In the paper the unsteady flow field of undulating fins in stationary water is calculated by Large Eddy Simulation and dynamic grid technique with diffusion-based smoothing model. The parameters of undulating fins are set as: L = 800 mm, h/L = 0.05625, L/λ = 2,f = 2.0Hz, Φ max = 30 deg., where L is fin length, h is fin height, λ is wave length, f is wave frequency, ϕm is wave amplitude.2.Vortex characteristic and its temporal and spatial evolution is described, and the vortex structure and its relationship with thrust, heave force is analyzed. The results indicates there are three kinds of vortexes in the flow field, i.e. streamwise vortex, crescent vortex and toroidal vortex. The crescent vortexes mainly induce high speed axial jet, which produces the main thrust. The streamwise vortexes mainly induce the vertical jet, which generates the main heave force.3.The simulated thrust of undulation fins at wave amplitude 850 generally agrees well with the hydrodynamics experiments. The