Performance improvement through passive mechanics in jellyfish-inspired swimming

This computational investigation explores the effect that passively responsive components of a body can have on swimming performance. The swimmer is an articulated two-dimensional system of linked rigid bodies that is prescribed with a reciprocating shape change loosely inspired by jellyfish mechanics. The six constituent hinges can be either actively controlled by fully prescribing the kinematics, or passively responsive by substituting a torsion spring in place of an actuator. The computational solver is a high-fidelity viscous vortex particle method with coupled fluid-body interactions. The prescribed kinematic Reynolds numbers involved in this investigation fall within the range 70–700. Several configurations are explored, including cases with passively responsive hinges and cases in which pairs of the hinges were held in a rigid locked position. Certain choices of passive structure lead to optimal swimming speed and efficiency. This is elucidated by a simple model, which shows that optimal performance is obtained through a balance of maximized deflection of peripheral bodies and phasing that draws benefits from both reactive and resistive force mechanisms. A study is also made of an inviscid swimmer but, due to the reciprocating kinematics of the system, the swimmer is unable to achieve meaningful locomotion, showing that vortex shedding is essential to break the symmetry of the kinematics.