Flight Effects on Supersonic Jet Noise from Chevron Nozzles

Currently tactical aircraft supersonic jet noise studies are conducted at near static conditions motivated by the need to develop nozzle concepts for mitigating aircraft carrier launch crew noise levels. Effects of forward flight on supersonic jet noise source mechanisms need to be included as the next step to demonstrating concept benefits for community noise reduction during training scenarios where aircraft operate from land based airfields. Hence, the current study was directed at the flight effects on far field noise from round and chevron nozzles in model scale experiments conducted in the United Technologies Research Center (UTRC) open jet acoustic wind tunnel. The objective was to document the statistical characteristics of the noise at tunnel Mach numbers up to Mt =0.4 since there is limited generic data for military engine supersonic exhaust conditions under such forward flight scenarios. The study focuses on the noise characteristics in the aft quadrant which dominates the tactical aircraft noise signature. A generic chevron nozzle, developed for noise reduction, was included in the evaluation as part of tracking the noise reduction consistency with increasing forward flight. The chevron geometry consisted of the baseline shock-free round nozzle with 12 chevrons attached at the nozzle exit. Exit diameter corresponded to 2” for both nozzles. Static temperature ratios ranged from isothermal to Tr =2. The impact of chevrons on impulsive signature skewness reduction was tracked in the acoustic far field at near static conditions. Both the round and chevron nozzle were also operated at off-design conditions with over and underexpanded nozzle pressure ratios (NPR) for the skewness study.