Self-sustained oscillations in a low-viscosity round jet

When a jet of fluid emerges into an ambient medium of the same density but higher viscosity, the dominant mode of instability transitions from axisymmetric at low viscosity ratio M to a helical mode at high M. It is shown that these helical modes are unstable global modes associated with enhanced mixing and the emergence of a single dominant frequency observed everywhere in the near field. These frequencies align well with predictions of absolute instability from spatiotemporal linear stability theory.