Unsteady flow characteristics in vacuum ejectors with convergent-divergent primary nozzle

This study investigates the unsteady flow characteristics of a vacuum ejector equipped with a convergent-divergent (C-D) nozzle using both experimental and computational approaches. Unsteadiness is quantified across various flow conditions, and the primary sources of unsteadiness are identified as screech tones, acoustic duct modes, and their harmonics. Computational fluid dynamics (CFD) analysis confirms that disturbance waves generated as part of the feedback mechanism responsible for screech significantly influence the mean pressure in the secondary chamber of the vacuum ejector. Coherent flow structures and their associated frequencies are extracted using proper orthogonal decomposition (POD) of density field from CFD, revealing jet oscillations in both longitudinal and lateral directions, which are associated with symmetric and asymmetric tones, respectively. Vorticity-based POD analysis further reveals the formation of screech through asymmetric vorticity structures near the nozzle lip. Additionally, acoustic mode analysis successfully captures both symmetric and asymmetric duct modes, which show good agreement with experimental data. Overall, the study provides a detailed understanding of the mechanisms contributing to unsteadiness and their modal characteristics in vacuum ejector.