Aero-acoustic-structural interactions and noise control in the fan-ducted system

Abstract

In a fan-ducted system such as the air-conditioning system and the turbofan engine of the aircraft, the noise attenuation of the fan noise at low blade-passage frequency is still a challenge. The flow effect is a major element which influences the silencing efficiency of the noise attenuation approaches. The effects of the flow field on the acoustic performance of the silencer with flexible structures can be classified into two categories: (1) change of acoustic characteristics of the silencing device due to the vortex formation; (2) change of acoustic response of the flexible structure due to the existence of unsteady flow. Micro-perforated panel (MPP) is commonly utilized as the acoustic liner in the aircraft engine. The engine noise at high intensity sound changes the acoustic properties of the perforations due to the process of jet formation. The effect of orifice nonlinearity on the acoustic response and absorption performance of the promising MPP absorber array in parallel arrangement is investigated. The MPP absorber array is constructed by three parallel-arranged MPP absorbers with different depths of cavity. A finite element model is established to simulate the acoustic response of the MPP absorber array under normal incidence with high sound intensity by adopting the nonlinear impedance model. The results show that the absorption mechanism of the MPP absorber array is highly subjected to the incident sound pressure. The moderate acoustic pressure excitation enhances the effect of the neighboring MPP absorber compound. If the sound pressure level increases continuously, compared with the MPP absorber array in the linear regime, the spectral peaks on the absorption curve are smoothed out and the absorption bandwidth with high absorption coefficient (α>0.8) is wider due to the added-mass effect given by the non-resonating sub-cavities. The effect of the panel vibration on the absorption performance of the MPP absorber array is also considered in the proposed model. The measured normal absorption coefficients of a prototype MPP absorber array compare well with the numerical prediction in both linear and nonlinear regimes. The silencing performance of the MPP silencer for controlling the grazing incidence sound wave at moderate intensity in duct is studied. A two-dimensional finite element model is established to access the effect of the nonlinear feature of the orifice on the sound suppression mechanism of the MPP silencer. The silencing performance of the MPP silencer is achieved as a combination of sound reflection induced by panel oscillation and acoustic absorption given by the perforations. The performance of the silencer is improved by the effect of orifice nonlinearity when sound absorption mechanism is dominant. The broader stopband is achieved in the nonlinear regime due to the additional absorption in between the second and third stopbands which is given by the nonlinear effect of perforations. The fan noise in flow duct can be controlled by using the membrane housing device at the source position directly. This present study investigates the noise reduction mechanism of a tensioned membrane housing device that directly controls the sound radiation from the doublet which is enclosed in the infinitely long duct with presence of a point vortex. The time dependent sound radiation mechanism and the vibro-acoustics coupling mechanism of the systems are studied by adopting the potential theory and matched asymptotic expansion technique. The silencing performance of such passive approach depends on the amplitude and phase of the sound field created by the doublet and the acoustic pressure induced by the membrane oscillation in order to achieve sound cancellation. Results show that the response of membrane vibration is strongly associated with the flow field induced by the grazing uniform flow and also the fluid loading generated by the inviscid vortex. The geometrical property of cavity and the mechanical properties of the flexible membranes play important role of controlling the performance of the proposed device. In this study, the flow effects on the acoustic responses of the silencing device are both examined at low Mach numbers.