Self-supporting string loops through air drag Hydrodynamics / fluid-structure interactions

with Juliette Courson, Margaux Abello, Wladimir Toutain & Bruno Andreotti

dispositif experimental
Superposition of snapshots of a light string loop propelled in the upper-right direction by the system of counter-rotating pulleys visible on the left. With increasing velocity air drag causes the loop to rise from its hanging shape to a self-supporting spindle shape.

A string propelled by counter-rotating pulleys exhibits, when tied into a loop and in steady state, 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. We found that this loop of light string is not suspended in the air due to inertia, but through the hydrodynamic drag exerted by the surrounding fluid, namely air. Moreover 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.

We have studied this system while preparing for the French Physicists’ Tournament 2019, and subsequently published our results in the Journal of Fluid Mechanics. Two other reports were published simultaneously, one by the leaders of the ENS Lyon team, another in Emergent Scientist by a collaboration of students from several FPT2019 teams including ours (Université Paris-Sud, ENS Ulm, Université de Paris).

In the press…

Last modified: 18 Jul 2020