Often basic research on supersonic jet noise is conducted using converging-diverging nozzles that typically have smooth contours determined by the method of characteristics (MOC). Full-scale nozzles for tactical fighters, on the other hand, generally have straight conical and divergent sections, which often have a short divergent section. This study compares the supersonic jet noise produced by sub-scale converging-diverging (C-D) rectangular nozzles designed using MOC with non-contoured converging and diverging sections that are just straight ramps, referred to here as the non-MOC nozzles. Only one wall of these C-D nozzles actually converges and diverges. Two sets of nozzles, each of aspect ratio of 1.5 and equivalent diameters of 0.75 in. and 2 in., were tested. The data presented includes farfield noise, source location using a beamformer, flow visualization acquired using a highspeed video camera, selected PIV measurements, and thrust measurements of both the MOC and the non-MOC C-D nozzles. Jet-noise scaling of the noise data for the two non-MOC nozzles is also developed. For reference, selected results are also compared with a converging rectangular nozzle of the same equivalent diameter and aspect ratio (= 1.5) as the C-D nozzles tested. It is shown that the non-MOC nozzle has shocks propagating from inside the nozzle, which appear overlaid on the shock diamonds shed from the nozzle lip. This is seen to be present whether the nozzle is operated at design or off-design conditions. Acoustic measurements also show that, for the non-MOC nozzle, shock-associated noise is generated even at the design condition. Examination of the curves of OASPL as a function of fully-expanded Mach number shows no minimum at or near the design Mach number as seen for the MOC nozzle. As expected, the non-MOC nozzle produces less thrust compared to the MOC nozzle, which is about 4%. It seems that the jet-mixing noise region of the farfield narrow-band spectrum for both the MOC and the non-MOC nozzles can be scaled well using the D3 law for the same operating nozzle pressure ratio or the fully expanded Mach number. The observed results match well those published in the literature for round nozzles with and without contoured nozzle geometry.
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