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Linear stability analysis is applied to the mean flow of an oscillating roundjet with the aim to investigate the robustness and accuracy of mean flowstability wave models. The jet's axisymmetric mode is excited at the nozzle lipthrough a sinusoidal modulation of the flow rate at amplitudes ranging from 0.1% to 100 %. The instantaneous flow field is measured via particle imagevelocimetry and decomposed into a mean and periodic part utilizing properorthogonal decomposition. Local linear stability analysis is applied to themeasured mean flow adopting a weakly nonparallel flow approach. The resultingglobal perturbation field is carefully compared to the measurements in terms ofspatial growth rate, phase velocity, and phase and amplitude distribution. Itis shown that the stability wave model accurately predicts the excited flowoscillations during their entire growth phase and during a large part of theirdecay phase. The stability wave model applies over a wide range of forcingamplitudes, showing no pronounced sensitivity to the strength of nonlinearsaturation. The upstream displacement of the neutral point and the successivereduction of gain with increasing forcing amplitude is very well captured bythe stability wave model. At very strong forcing (>40%), the flow becomesessentially stable to the axisymmetric mode. For these extreme cases, theprediction deteriorates from the measurements due to an interaction of theforced wave with the geometric confinement of the nozzle. Moreover, the modelfails far downstream in a region where energy is transferred from theoscillation back to the mean flow. This study supports previously conductedmean flow stability analysis of self-excited flow oscillations in the cylinderwake and in the vortex breakdown bubble and extends the methodology toexternally forced convectively unstable flows.

Mean flow stability analysis of oscillating jet experiments