Jet engines are tested in so-called test cells, where the desired sea-level or altitude 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 can be observed in such facilities. The intensity of these fluctuations reaches, in some cases, nearly 170dB in the exhaust system, which is beyond the acceptable limit for the installations. In general, the frequency of the fluctuations is close to that of an acoustic normal mode of the facility, typically that of the exhaust-diffuser. There is to date no means to predict whether this high-amplitude resonance will occur for a given set of testing conditions. So far these resonances have been suppressed by trial-and-error procedures. A complete understanding of this phenomenon is still sought.
The aim of the present work is to study the high-amplitude resonance phenomenon observed in test cell facilities from a fundamental point of view. To this end, a simple, two-dimensional model of a jet thorough a duct is simulated numerically for different jet regimes. The use of numerical simulations allows us to visualize in detail the flow variables as they go through a resonance cycle. This allows for identification of the features involved in the acoustic loop and characterize the acoustic-jet coupling better. The numerical method that is used to solve the Navier-Stokes equations, a high-order finite difference compressible flow solver on a staggered mesh, is presented. The method features shock capturing capabilities based on hyperviscosity. A study of boundary closures allows for the selection of high-order boundary schemes and the implementation of characteristics-based boundary closures on the staggered mesh.
The results of selected test cases are presented and the main features of resonant and nonresonant regimes for this model problem are discussed. The implications of the results obtained in the model problem on the resonance phenomena in test cell facilities are also discussed.