Global Helical Modes in Low-Density Jets

It is well known that low-density jets can exhibit global instability in the form of self-excited axisymmetric oscillations. It is less well known, however, that such jets can also exhibit global instability in the form of self-excited helical oscillations. The existence of such helical modes has been predicted before in the local linear spatiotemporal stability analysis of Coenen, Sevilla, and S\'anchez \cite{Coenen_Sevilla_S\'anchez2008} for long circular injectors with thick shear layers, but they have yet to be found in experiments. We report experimental evidence that low-density jets can exhibit global helical modes. We characterize the dynamics of the jet with time-resolved Schlieren imaging and quantify the departure from axisymmetry with the phase angle between diametrically opposite shear layers. On decreasing the non-dimensional injector length $L_t^{-1}=(Re_j a)/l_t$ (where $Re_j$ is the jet Reynolds number, $a$ is the jet radius, and $l_t$ is the injector length), we find that the global instability switches from an axisymmetric mode to a helical mode, with the latter being weaker than the former. This shows that it is possible to create self-excited helical modes in laboratory low-density jets, provided that the shear layer at the nozzle exit is sufficiently thick.