Duct resonance and its effect on the performance of high-pressure ratio axisymmetric ejectors

Abstract

The effect of duct resonance on the performance of high-pressure ratio axisymmetric ejectors was investigated in a series of experiments involving analysis of the emitted noise and schlieren photography of the ejector duct flow. The thrust characteristics obtained with changing blowing pressure were found to contain irregularities, and these were found to be associated with certain tones of acoustic radiation which tuned to the transverse modes of the mixing duct. When varying the blowing pressure for a given ejector the discrete tone fundamental frequency showed a staging behavior, and the frequencies at the discontinuities separating the stages coincided with the calculated modes of the duct. For a given frequency discontinuity, the Helmholtz number (based on duct diameter), was found to be the same for all ejectors tested. For each family of ejectors having the same duct-length to diameter ratio, the blowing pressure at which certain discontinuities occurred varied linearly with duct to nozzle area ratio, and the thrust irregularities assumed similar linear relationships. Increasing the duct length greatly improved thrust augmentation in the relatively high-pressure range and shifted thrust irregularities and the corresponding frequency discontinuities to lower pressure ranges. The schlieren photographs showed that the primary jet spread, and its mixing with the entrained secondary stream, were evidently enhanced by duct resonance. The jet appeared to be surrounded by interspersed helical eddies and to be oscillating in a spinning mode.

A Theoretical and Experimental Investigation on Ejector Acoustics and Ejector Silencer Design

“…There is no reason to believe that in the subsonic case another acoustic source could not couple with the duct modes in a similar discontinuous fashion. Reference [31] finds that at lower frequencies, the tone emitted by ejectors coincides with the symmetric natural modes of the throat. Schlieren photography, a technique where local differences in fluid density caused by flow can be visualized and photographed, is employed on ejectors emitting discrete tones.…”

“…Schlieren photography, a technique where local differences in fluid density caused by flow can be visualized and photographed, is employed on ejectors emitting discrete tones. Reference [20] from 1995 investigates the generation of discrete tones in supersonic ejectors -in much the same way as [31] -but with a rectangular ejector and an emphasis placed on understanding the mechanism of tone generation. The tones are found to be as much as 20 dB greater than the broadband noise and as the area ratio between the ejector and primary nozzle is decreased (smaller ejector throat for fixed nozzle size) more tones appear, with the additional tones being related to the duct modes.…”

“…The Schlieren photos taken by reference[31] of a resonant (left) and stable (right) ejector flow, adapted from[31] …”