Twin jet plumes on aircraft can couple, producing higher dynamic pressures in the internozzle region, which in turn can cause sonic fatigue of the external nozzle flaps. Thus, there is significant value in performing laboratory studies on twin supersonic jets that couple and screech at a discrete frequency. The coupling of rectangular jets is of particular interest because of the use of these jets in military aircraft, especially in situations requiring vectored thrust, stealth, or tailless flight. Considerable work was done in the late 1980's on twin jets at the NASA Langley Research Center and by the U.S. Air Force to alleviate problems with the B1-B and F15-E aircraft. However, the mechanism of twin jet coupling has remained far from being clearly understood.
Research being conducted by NASA Lewis Research Center's Nozzle Branch is focusing on fundamental mechanisms by assessing the steady and unsteady aspects of twin jet coupling. As shown in the preceding sketch, two rectangular nozzles are placed side by side, with their narrow dimensions parallel and their long dimensions in the same plane. A positioning apparatus keeps one of the nozzles fixed and moves the second one to achieve various internozzle spacings. Microphones mounted on the nozzles monitor the characteristics of the acoustic field, and a movable microphone measures the acoustic phase and amplitude distribution on the x - z and y - z planes. The sound pressure amplitude distributions on the y - z plane are shown in the following figure for two slightly different jet Mach numbers. The slight difference in operating condition caused completely different modes of jet coupling-antisymmetric and symmetric pressure waves.
While documenting the parametric range of nozzle spacing and Mach numbers over which the jets couple, we have been addressing the following questions:
So far, our results have revealed the following. The coupling occurs through the near acoustic field surrounding the jets. Particularly important is the "null" phase region surrounding the jets, where the phase of an acoustic wavefront (arriving from downstream) does not vary over a small radial distance. When the null regions of the two jets overlap, symmetric coupling occurs; when they do not overlap, the jets couple antisymmetrically. We use a parameter a as a simple test to determine the mode of coupling. Apparently, coupling switches from the antisymmetric to the symmetric mode because of an abrupt shift in the effective screech source from the third to the fourth shock, which in turn causes the null phase region surrounding the jets to grow abruptly and overlap. These results are summarized in reference 1.
Lewis contact: Dr. Khairul Zaman, (216) 433-5888,
Authors: Dr. Ganesh Raman and Dr. Ray Taghavi
Headquarters program office: OASTT
Programs/Projects: Propulsion Systems R&T, FQE, ASCOT
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Last updated April 15, 1998, by Nancy.L.Obryan@nasa.gov
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