Vortex Ring Formation through Flexible Bio-Inspired Circular Orifices

Vortex rings are found over many scales across nature, including in the efficient locomotion of jellyfish, squid, and cuttlefish. In each of these marine organisms, the animal actively contracts their body to expel fluid through a flexible orifice, producing a series of vortex rings. This study focuses on the fluid-structure interactions of vortex rings (L/D=3) produced by a piston-cylinder arrangement through circular orifices of varying stiffness (varying flexibility). It is expected that the use of a passively flexible orifice will impart elastic energy to the flow, increasing the hydrodynamic impulse of the vortex ring. Four piston-profile velocity programs and two Reynolds numbers are considered. Particle image velocimetry (PIV) is used to quantify thrust via hydrodynamic impulse. Finite-time Lyapunov exponent (FTLE) fields are calculated from PIV vector fields and are used to determine the rearrangement and merging of vorticity throughout vortex ring formation. Edge tracking is used to determine the flexible orifice deformations throughout fluid ejection from the different velocity programs. A relationship is found between orifice flexibility and fluid acceleration to maximize thrust from a single vortex ring.