Research on Fan Noise Generation

Abstract

Different aspects of multiple-pure-tone (MPT) "broadband" noise generation in high-speed fans are discussed. An analytical model for multiple-pure-tone generation is presented. Experimental evidence indicates that the MPT generation does not seem to be a function of the presence of shock waves. It is also shown that fan "broadband" noise is not broadband in character and could be attributed to the modulation of the blade-passing-frequency tones, their sub-and higher harmonics. INTRODUCTIONThe subject of fan noise has received a lot of attention since the advent of high-bypass turbofans. A considerable amount of theoretical and experimental research has been carried out with the dual purpose of defining the fan noise-generation mechanisms and developing adequate means to suppress fan radiated noise. This effort has led to a relative satisfactory understanding of the generating mechanisms of blade-passing-frequency (BPF) tones and their higher harmonics (see, for example, Ref. 1). It is evident, however, that as the BPF noise is brought under control, more attention should be directed to understanding the rest of the spectrum. Subharmonic tones (sometimes called multiple pure tones) as well as broadband noise components of the fan spectrum can no longer be ignored if further fan noise reductions at the source are required. This paper will therefore present the results of some analytical and experimental work on the definition of multiple pure tones and broadband noise-generation mechanisms in high-speed fans. I. MULTIPLE PURE TONESThe presence of discrete tones other than harmonics of the blade passing frequency has been noted in the inlet spectra of turbofan engines since the advent of high-tip-speed fans and compressors. The main research effort was directed towards the understanding and reduction of the BPF noise and little work was directed to subharmonic tones. It is now evident that the importance of low-frequency tones can no longer be ignored.A good understanding of the multiple-pure-tone (MPT) phenomena is therefore required if greater progress is to be achieved in the noise control of transonic fans. Two questions should eventually be answered: (1) What phenomena are responsible for the MPT generation? (2) Under what circumstances do the MPTs propagate to the farfield? Physical observations of high-speed-fan data indicate that MPTs have the following characteristics.(1) They are detected only in the inlet duct.(2) They propagate only when the rotor-tip Mach number is greater than 1.(3) The strongest MPTs exist in general at frequencies below the rotor blade passing frequency and occur at integral multiples of engine shaft revolution frequency.(4) The higher-frequency MPTs are the first ones to appear as the fan speed is increased. Figs. 1 and 2, which represent probe measurements taken respectively in the inlet and exhaust duct of a fan at a supersonic tip speed. While Fig. 1 indicates the presence of MPTs in the inlet, Fig. 2 shows only the blade passing frequency superimposed on a broadband noise backg...

Three-dimensional visualizations of open fan noise fields

“…(3) and (4), the resulting equations become identical to the Statistically Optimal Nearfield Acoustical Holography (SONAH) description as Hald presented in Ref. 11.…”

“…These noise components are usually present below the Blade Passing Frequency (BPF) defined as the multiplication of the rotation frequency and the number of blades. Benzakein observed that these noise components are generated at the fan leading edges in the form of shock waves and propagate only at supersonic blade tip speeds 3 . In landing conditions where the blade tip speed is subsonic, the fan noise mainly consists of the BPF and its harmonic components as well as broadband frequency components.…”