Experimental investigation of the far-field noise due to jet-surface interaction combined with a chevron nozzle

Abstract

High-speed jets exhausting from engines, especially during takeoff, remain one of the main sources of external aircraft noise. Regulations on aircraft noise levels in regions close to airports are very strict and are forcing the aerospace industry to develop quieter aircrafts. Mixing enhancement devices at the exit of nozzles, such as chevrons, have assisted in the reduction of noise levels for some time. However, installation effects are becoming more intense due to highly integrated aircraft-engine configurations, which are currently employed by the aviation industry as an attempt to increase the aerodynamic efficiency and to reduce noise. Consequently, these installation effects may interfere with the noise reducing performance of such devices. This paper reports experimental investigations involving the combined effect of a flat plate integrated with nozzles, both with and without chevrons, exhausting cold subsonic jets. Assessment of the far-field noise was conducted for different Mach numbers and several configurations formed by relative positions between the nozzle and the plate. In general, it was observed that the acoustic benefit of chevrons, as found in isolated jets, remains effective in most of installed configurations. Nevertheless, for highly integrated configurations the low-frequency noise reduction provided by the chevron nozzle becomes almost negligible, whereas the increase of high-frequency noise becomes substantial and the shielding effect is observed to decrease.