Interactions between screech tones and ejector performance

The thrust augmentation and the dominant acoustic frequency of a family of simple ejectors have been measured at primary stagnation pressures up to 6 atm. The convergent primary nozzle induced atmospheric air into a selection of constant diameter ducts that provided length-diameter ratios between 1 and 11 and an inlet area ratio around 25. The thrust-augmentation data tended to follow the monotonic reduction in performance that theory predicts with increasing pressures, except at certain pressures where the trend reversed. At these pressures, the acoustic frequency tuned to a transverse resonant mode of the mixing duct. It is speculated that resonance intensifies the large-scale vortices that mix the two streams and, at the same time, give rise to screech tones by interacting with the shock structure in the underexpanded jet.