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Slowing of vortex rings by development of Kelvin waves.

We study experimentally the slowing of viscous vortex rings. In particular we do so using the concept of drag coefficient, which is a bulk coefficient which aims to capture the various mechanisms of slowing that can occur. At early times of flight the ring slows at a certain rate. After some time, instabilities (which we refer to as Kelvin waves) begin to form on the ring and there is a transient increase in the measured drag coefficient. After this brief transient the rings enter another regime with constant drag coefficient, which is larger than the early time one. In particular, our data illustrate that there appears to be a direct link between the number of waves that form on the ring and the relative increase in drag coefficient.