Public reporting burden for this collection of information is estimated to average 1 hour per response, including gathering and maintaining the data needed, and completing and reviewing the collection of information. Send collection of information, including suggestions for reducing this burden, to Washington Headquarters Services Davis Highway, Suite 1 204, Arlington, VA 22202-4302, and fcV-tW 0Vf*si?of Management and Budget, Paper« AGENCY USE ONLY (Leave blank)2. REPORT DATE 11/29/00 REPORV-AFRL-SR-BL-TR-00-öürtl i. mm UM I i:a lUVbHbU 04/01/98-'09/30/00 TITLE AND SUBTITLE(U) A Numerical Investigation of Sound Generation in Supersonic JET Screech AUTHOR(S)Ted A. Manning and Sanjiva K. Lele FUNDING NUMBERSF49620-98-1-0355 The noise : i supersonic jet flows is due in part to the interaction between jet instability waves and the jet shock-cell structure. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)DEPARTMENTThe emitted shock-cell noise re-excites certain instability wave modes at the nozzle lip and causes resonant feedback to occur. This feedback resonance, known as supersonic jet screech, causes the jet to flap violently at discrete frequencies and generate very strong, narrow-banded tones. Jet screech is a source of acoustic fatigue in the tail and nozzle structures of supersonic aircraft. It is therefore important that methods for predicting the screech amplitude be developed.In the present research we investigate the screech sound generation process, particularly for high amplitude instability waves. We isolate the interaction of an unsteady shear layer with a single oblique shock. To obtain an overall understanding of the phenomenon with fewest simplifications, we study this problem through the numerical solution of the Navier-Stokes equations. We then consider idealizations to obtain a similar but wider range of results with specially linearbed Euler equations. These results motivate the use of geometric acoustics to describe the screech generation process. The Navier-Stokes and Euler simulations have revealed important details about the interaction process, how the acoustic field results, and why the screech is so loud. AbstractThe noise of supersonic jet flows is due in part to the interaction between jet instability waves and the jet shock-cell structure. If no counter-measures are taken, the emitted shock-cell noise will re-txcite certain instability wave modes at the nozzle lip and cause resonant feedback to occur. This feedback resonance, known as supersonic jet screech, causes the jet to flap violently at discrete frequencies and generate very strong, narrow-banded tones. Jet screech has been shown to be a source of acoustic fatigue in the tail and nozzle structures of supersonic aircraft. It is therefore important that methods for predicting the screech amplitude be developed. While comprehensive screech models will require taking all elements of the feedback loop into consideration, a basic understanding of each element in isolation will also be necessary. Screech sound generation is one such ...
Recent work has described screech noise from a supersonic jet as being due to leakage of a wave that is otherwise trapped in the jet's interior. In that work, the simplest of many techniques used is ray tracing for a single shear-layer modeled as a row of Stuart vortices. In the present work, a lower row of vortices is added to form a plane jet. Instead of plotting ray paths, a technique of visualization analogous to streaklines is used that better corresponds to instantaneous density fields as observed, for instance, by the Schlieren method. This produces striking images that show leakage of waves at each internal reflection resulting in a row of acoustic sources as envisioned since the 1950s. However, the sources are not isotropic and each has a zone of silence in the downstream direction. Leakage creates a fold in the wave pattern internal to the jet which leads to fine scale features. Reported experiments have also observed fine scale features (described as splitting) in the shock-cell pattern; they may be related to those observed here. Internally reflected rays also undergo a diffusive process as they propagate down the jet. In particular, each successive internal reflection at an unsteady shear-layer scatters rays along a wider range of wave angle and makes them more susceptible to leakage at the next reflection. It also causes more downstream directivity for the more downstream sources. An important result is that as the Mach number Mj is varied, maxima in leakage rate and mean acoustic amplitude occur at (near) resonances between the Mach-wave and shear-layer periods. Maxima in sound pressure level versus Mj have also been reported for laboratory round jets. Finally, as the shear-layer thickness is increased, a minimum in the rate of leakage (correlated with a minimum in radiation amplitude) occurs due to the competing effects of increased shear-layer penetration versus reduced eddy passage frequency.
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