A Near -Field Investigation of Chevron Nozzle Mechanisms

A detailed investigation into the effect of chevron nozzles on the near -field acoustics of a separate flow exhaust system was conducted in the University of Cincinnati Anechoic Test Facility. The nozzle configurations included a baseline conic fan and core nozzle as well as three different chevron core nozzles. Chevrons with varying numbers of lobes and levels of penetration were selected to provide insight into the effects of these geometric parameters on the acoustic near -field. Tests were conducted at two different nozzle operating conditions and the chevrons were shown to produce substantial modifications to the near field over a range of frequencies. The chevrons were most effective at low frequencies where the peak noise region was reduced by 5 – 7dB an d dramatically reduced in size. The near -field showed relatively little sensitivity to the chevron geometry at the lower frequencies. At high frequencies, the chevrons were shown to generate increased noise near the nozzle lip, which is quite sensitive to the chevron geometry and nozzle operating condition. The nozzle penetration was shown to have the largest effect, particularly on the size and intensity of the increased noise region near the nozzle lip. More subtle differences were seen with respect to th e number of chevron lobes, with the largest differences being confined to the higher frequencies. All of these observations are consistent with trends seen in a previous study on the nozzle far field acoustics using the same nozzles. In addition to validat ing some of the conclusions from this previous study, the current near -field study was able to provide substantially more insight into the effects of the chevrons on the near -field sources and noise generation mechanisms.