The "string shooter" experiment uses counter-rotating pulleys to propel a closed string forward (for a video, see [*]). In its steady state, the loop exhibits a transition from a gravity-dominated regime at low velocity towards a high-velocity regime where the string takes the form of a self-supporting loop. I will show that this transition cannot
be explained by inertial effects alone, but depends on the hydrodynamic drag exerted by the surrounding fluid, namely air. The string loop presents a critical point in its lifted state, analogous to a hydraulic jump, separating a supercritical zone where the wave velocity is smaller than the rope velocity, from a subcritical zone where waves propagate faster than the rope velocity. This critical point is also a geometrically singular point both in experiment and theory. I will discuss potential regularisation mechanisms of the string dynamics in the vicinity of this singularity.
References:
[*] Video of the experiment at: https://www.youtube.com/watch?v=rffAjZPmkuU
Paper at: http://dx.doi.org/10.1017/jfm.2019.631
Accès Salle des séminaires FAST-LPTMS (Bât. 530, salle C.120, 1ER)