Jet engines are tested in so-called test cells, in which the desired environmental conditions are controlled. The jet plume is exhausted out of the test cell, usually through a diffuser. Under certain testing conditions, high-intensity pressure fluctuations (170 dB) can arise in such facilities. Their frequency is close to that of an acoustic normal mode of the exhaust-diffuser, but the mechanisms driving the resonance are unclear. In the present work the resonance phenomenon is studied using numerical simulations of a model configuration, which shares the key features of actual facilities: an underexpanded supersonic jet, a finite-length solid-wall shroud surrounding the jet, and the receptivity of acoustic disturbances to excite jet instabilities at the nozzle. For this configuration a high-amplitude resonance, qualitatively similar to that of experiments in actual facilities, is observed for a symmetric overexpanded Mjet = 1:2 jet. This strongly resonant case is contrasted with a nonresonant ducted Mjet = 1:5 jet in the same geometry, as well as with a free jet at Mjet = 1:2. The hydrodynamic mechanism of this resonance is studied using linear stability analysis. The presence of excited acoustic modes of the duct is revealed by the Fourier analysis of the data. A numerical experiment shows that slight artificial damping of just the most excited acoustic mode suppresses the resonance.
ASJC Scopus subject areas
- Aerospace Engineering