Spatiotemporal intermittency of global helical modes in low-density jets

In previous experiments, we demonstrated that low-density jets issuing from long injectors can exhibit global instability in the form of self-excited helical oscillations. We found that decreasing the non-dimensional injector length $L_t^{-1}$ can cause the jet to transition from a global axisymmetric mode to a global helical mode. In the present study, we quantify the statistical properties of this transition by modelling the jet as a superposition of axisymmetric and helical eigenmodes whose amplitudes vary independently in time. Depending on $L_t^{-1}$ and the density ratio $S$, we find that the joint probability distribution of these modal amplitudes shows a statistical preference for axisymmetric or helical modes. We present evidence of intermittency using scaling laws for the ‘laminar’ phases and for the spectral density. We also demonstrate how the intermittency varies with spatial location, $L_t^{-1}$, and $S$. Finally, we use dynamic mode decomposition to extract the dominant frequencies of the axisymmetric and helical modes, yielding a universal frequency scaling for these modes in low-density jets.