  Linear stability analysis is applied to the mean flow of an oscillating round
jet with the aim to investigate the robustness and accuracy of mean flow
stability wave models. The jet's axisymmetric mode is excited at the nozzle lip
through a sinusoidal modulation of the flow rate at amplitudes ranging from 0.1
% to 100 %. The instantaneous flow field is measured via particle image
velocimetry and decomposed into a mean and periodic part utilizing proper
orthogonal decomposition. Local linear stability analysis is applied to the
measured mean flow adopting a weakly nonparallel flow approach. The resulting
global perturbation field is carefully compared to the measurements in terms of
spatial growth rate, phase velocity, and phase and amplitude distribution. It
is shown that the stability wave model accurately predicts the excited flow
oscillations during their entire growth phase and during a large part of their
decay phase. The stability wave model applies over a wide range of forcing
amplitudes, showing no pronounced sensitivity to the strength of nonlinear
saturation. The upstream displacement of the neutral point and the successive
reduction of gain with increasing forcing amplitude is very well captured by
the stability wave model. At very strong forcing (>40%), the flow becomes
essentially stable to the axisymmetric mode. For these extreme cases, the
prediction deteriorates from the measurements due to an interaction of the
forced wave with the geometric confinement of the nozzle. Moreover, the model
fails far downstream in a region where energy is transferred from the
oscillation back to the mean flow. This study supports previously conducted
mean flow stability analysis of self-excited flow oscillations in the cylinder
wake and in the vortex breakdown bubble and extends the methodology to
externally forced convectively unstable flows.
