  Pulsatile jet propulsion is a highly energy-efficient swimming mode used by
aquatic animals that continues to inspire engineers of underwater vehicles.
Here, we present a bio-inspired jet propulsor that combines the flexible hull
of a jellyfish with the bivalve compression of a scallop to create individual
vortex rings for thrust generation. Similar to biological jetters, our
propulsor generates a non-linear temporal exit velocity profile and has a
finite volume capacity. The formation process of the vortices generated by this
jet profile is analysed using time-resolved velocity field measurements. The
transient development of the vortex properties is characterised based on the
evolution of ridges in the finite-time Lyapunov exponent field and on local
extrema in the pressure field derived from the velocity data. Special attention
is directed toward the vortex merging observed in the trailing jet. During
vortex merging, the Lagrangian vortex boundaries first contract in the
stream-wise direction before expanding in the normal direction to keep the
non-dimensional energy at its minimum value. The circulation, diameter, and
translation velocity of the vortex increase due to merging. The vortex merging
takes place because the velocity of the trailing vortex is higher than the
velocity of the main vortex prior to merging.
