Compressor Noise Contribution to Inlet Noise

Abstract

Turbofan engine inlet noise is composed of tonal and broadband noise components. Since the inlet noise is typically dominated by noise generated by the fan, the inlet noise is frequently only separated into fan tone and fan broadband noise. Fan tone noise is composed of the fan blade passing frequency (BPF) and its harmonics. These tones are typically well above the broadband noise and can easily be identified. The “fan broadband noise” component is frequently not true broadband noise but also includes all the other tone noise components that occur at frequencies other than the fan BPF and its harmonics. These other tones include the compressor tones and the sum and difference tones that result from the scattering of the compressor tones as they propagate forward through the fan. The significance of these tones can be easily missed if the data is available in one-third octave bands or if the narrowband data bandwidth is too coarse. If these tones are lumped into the fan broadband noise component they may make a significant contribution to this so-called broadband noise component. This paper evaluates the effects of compressor noise on the airplane noise metrics, e.g., PNL, and approach EPNL. Full-scale test results (engine static tests) are compared with model scale fan test results (research fan rig without low pressure compressor) and suggest: 1) compressor tone and broadband noise may be significant components of the inlet noise; 2) the high order BPF harmonics may be dominated by the rotor wake/core stator interaction and not by the fan rotor wake/ outlet guide vanes (OGV) interaction.