Chaotic advection in 3D unsteady flows: not the tori way*

Theoretical and numerical studies revealed that 3D unsteady flows with invariant spheroids follow a route to chaos upon (weak) perturbation that is fundamentally different from that of the classical case of invariant tori. Weak perturbation of the invariant spheroids results in the formation of intricate coherent structures consisting of thin spheroidal shells connected by tubes via a mechanism termed "resonance-induced merger" (RIM). These structures enable a first global dispersion of tracers and constitute the precursor to unrestricted 3D chaos emerging from perturbation of systems accommodating invariant spheroids. Recent studies provided first experimental evidence of RIM yet direct validation remained outstanding. The present study conclusively demonstrates RIM and the associated coherent structures by way of experimental 3D Lagrangian studies using 3D Particle Tracking Velocimetry.