  Playing a role in the locomotion of some animals such as the water strider,
the formation and dynamics of a semi vortex ring connected to a free surface
are experimentally investigated. This semi vortex ring is generated by the
circular motion of a flat circular disk in water. Digital Particle Image
Velocimetry provides velocity fields and vortex properties. We show how in a
broad range of Reynolds numbers, the properties of the semi vortex rings are
related to the disk characteristics. In particular, our results highlight a
formation process more complex than for a complete vortex ring produced by a
piston stroke. In addition to the classical rolling up at the rear of the disk,
a shedding phenomenon occurs on the leading edge, producing secondary vortices.
The Strouhal number related to this shedding process reveals that it comes from
the free-shear instability of the boundary layer identified in the near wake of
a cylinder by Bloor. Finally we show that the disk thickness affects the final
properties of the semi vortex rings through emission frequency of secondary
vortices.
