Unsteadiness in vacuum ejector and their sources

This study investigates unsteadiness in the secondary chambers of a vacuum ejector during its transient startup phase. Experiments reveal unsteadiness at various frequencies across different nozzle pressure ratios. These frequencies are categorized as symmetric (in-phase) or asymmetric (counter-phase) through cross-spectral phase analysis. Dynamic mode decomposition, using time-resolved schlieren images, elucidates the corresponding flow structures and acoustic fields. The mode shapes display alternating density gradients along both the longitudinal and lateral axes, which explain the oscillatory behavior of the jet. These oscillations arise from instabilities in the jet's shear layer and acoustic disturbances due to the jet being confined within a duct. Using empirical relations, spectrogram analysis, time-resolved schlieren imaging, and classical wave theory, the sources of unsteadiness in the secondary chamber are identified as jet screech, its harmonics, and modes due to duct acoustics (symmetric and asymmetric). Additionally, the screech feedback loop in the vacuum ejector is found to be distinct from that in open jets, and the effect of screech on duct-mode acoustics is also observed.