Formation and propagation of a semi vortex-ring connected to a free surface

The vortex rings are torus-shaped vortices existing in many different situations in fluid mechanics. They are involved in the locomotion of numerous animals like birds and fish. The understanding of their formation process is essential to optimize the propulsion in engineering. They also play a major role for blood pumping in left ventricle and are considered as an index of cardiac health in the human heart. As proof of the phenomenon diversity, the presence of charged vortex ring with a quantized circulation has also been identified in superfluid helium.
The first part of this talk will give a short overview about interest and properties of the vortex rings. In a second part, I will present an experimental study on a semi vortex ring connected to a free surface, notably involved in the locomotion of water striders. This U-shaped object is here generated by the circular motion of a flat circular disc in water. Digital particle image velocimetry provides velocity fields and vortex properties. The formation process highlights not only the existence of a classical rolling up at the rear of the disk, but also a shedding phenomenon on the leading edge of the disc. The created small vortices affect final properties of the semi vortex ring.