Estimation of the relative balance between the rotor and the OGV broadband noise

Abstract

This paper investigates the measured noise data in the engine intra-stage, between the rotor and the outlet guide vanes (OGV), to predict the dominant broadband (BB) noise source in the fan-stage. Unlike the engine intake and the bypass, the intra-stage is particularly suitable for estimating the relative contribution of the fan and the OGV BB noise sources as well as to provide valuable insights into their respective source generation mechanisms. This was the motivation behind the test expedition carried out in March 2018 on a scaled turbofan engine (ACAT1 fan) at AneCom AeroTest (ACAT) in Germany within the framework of the TurboNoiseBB project. The present work has focussed on the intra-stage measurements of the ACAT1 fan at the approach condition, where the fan BB noise is a dominant component of the aeroengine noise. The source separation of the BB noise data into the rotor and the OGV contributions has been accomplished by predicting the total downstream and the total upstream modal levels using appropriate phased array techniques that are compatible with the given intra-stage instrumentation. It has been shown here that the BB noise from the OGV dominates the rotor noise by around 6-8 dB across the complete frequency range of interest. This is the first time that an intra-stage noise measurement, involving both the rotor and the OGV, has demonstrated a clear preponderance of the OGV noise over the rotor noise. The results presented here have also corroborated and elucidated the key features of the fan BB noise in the intra-stage that are documented in the existing literature. An important component of the present work is the establishment of the in-duct directivities of the dipole source mechanisms of the rotor and the OGV BB sources in the engine intra-stage. The analysis has revealed that the characteristics of the BB modal distributions in the intra-stage do not comply with the uniform modal energy distributions of equal energy per mode or the equal energy density per mode, which have been considered to be appropriate for the engine intake and the bypass sections.