Strange nonchaos and crisis-induced intermittency in a forced self-excited jet*

We experimentally study the transition from order to chaos in a prototypical hydrodynamic oscillator, namely a self-excited low-density jet subjected to external harmonic forcing. On increasing the forcing amplitude at an off-resonance frequency, we find that the jet bifurcates through a complex sequence of nonlinear states: period-1 limit cycle $\rightarrow$ 2-frequency quasiperiodic torus $\rightarrow$ strange nonchaotic attractor (SNA) $\rightarrow$ crisis-induced intermittency $\rightarrow$ low-dimensional chaos. We verify the existence of the SNA through the spectral distribution, the correlation dimension, the 0-1 test, and the horizontal visibility graph. We find that the SNA emerges from a loss of smoothness in the quasiperiodic torus. We then verify the existence of crisis-induced intermittency through the scaling laws of the average and instantaneous SNA epoch durations as well as various recurrence measures. We find that the crisis-induced intermittency is caused by a collision between the SNA and a basin of chaotic attraction. Our measurements represent the first experimental evidence of an SNA and crisis-induced intermittency in an open shear flow, contributing to a better understanding of how chaos can arise in forced hydrodynamic oscillators.