1,721,072 research outputs found
Acoustic scattering by a spliced turbofan inlet duct liner at supersonic fan speeds
No full textFan noise is one of the principal noise sources generated by a turbofan aero-engine. At supersonic fan speeds, fan tones are generated by the “rotor-alone” pressure field. In general, these tones can be well absorbed by an inlet duct acoustic liner, apart from at high supersonic fan speeds. However, in practice inlet duct liners contain acoustically hard longitudinal splices which cause scattering. This leads to acoustic energy being scattered into a range of different azimuthal mode orders, similar to the modal content resulting from rotor–stator interactions. The effectiveness of an inlet duct lining is reduced because acoustic energy is scattered into modes that are poorly absorbed by the liner. In this article, the effect of this acoustic scattering is examined by three-dimensional finite-element simulations of sound transmission in a turbofan inlet duct. Results include predictions of the sound power transmission loss with different splice widths, and at different supersonic fan speeds. These results demonstrate how acoustic scattering by liner splices can adversely affect fan tone noise levels at low supersonic fan speeds, but have little adverse affect on noise levels at high supersonic fan speeds. The potential noise benefit that could be achieved by manufacturing thinner splices is also examined
Calculation of modes in azimuthally non-uniform lined ducts with uniform flow
No full textIn a recent article, a method was proposed to calculate the mode scattering by an azimuthally non-uniform impedance liner section inserted in an infinite duct. The method allowed the problem to be formulated as a two-dimensional Helmholtz eigenvalue problem, which could be solved with general purpose software rather than custom written codes, but appeared to be limited to ducts without flow. In this short communication, the relevant system of equations is reformulated so that problems with flow can also be treated. The resulting eigenvalue calculation shows good agreement with a well-tested one-dimensional solver when applied to a circular duct section with constant impedance
On the prediction of "buzz-saw" noise in aero-engine inlet ducts
No full text"Buzz-saw" noise radiated from an aero-engine inlet duct occurs when the relative speed of the inlet flow impinging on the fan blades is supersonic. The pressure field attached to a supersonic ducted fan, in a direction normal to the shock fronts, closely resembles a sawtooth waveform. The non-linear propagation of a high-amplitude sawtooth waveform spiralling around a duct is calculated by two numerical simulation models. The models and their validation are discussed critically. Results are presented comparing the numerical simulations with experimental data. Overall there is good agreement comparing the results of the simulations with the experimental data, and in particular, the "Buzz-saw" noise in a hard-walled aero-engine inlet duct is successfully predicted
On the prediction of "buzz-saw" noise in acoustically lined aero-engine inlet ducts
No full textAero-engines operating with supersonic fan tip speeds generate an acoustic signature containing energy spread over a range of harmonics of the engine shaft rotation frequency. These harmonics are commonly known as the "buzz-saw" tones. The pressure signature attached to a supersonic ducted fan will be a sawtooth waveform. The non-linear propagation of a high-amplitude irregular sawtooth upstream inside the inlet duct redistributes the energy amongst the buzz-saw tones. In most modern aero-engines the inlet duct contains an acoustic lining, whose properties will be dependent on the mode number and frequency of the sound, and the speed of the oncoming flow. Such effects may not easily be incorporated into a time-domain approach; hence the non-linear propagation of an irregular sawtooth is calculated in the frequency domain, which enables liner damping to be included in the numerical model. Results are presented comparing noise predictions in hard-walled and acoustically lined inlet ducts. These show the effect of an acoustic liner on the buzz-saw tones. These predictions compare favourably with previous experimental measurements of liner insertion loss (at blade passing frequency), and provide a plausible explanation for the observed reduction in this insertion loss at high fan operating speeds
Far-field sound radiation due to an installed open rotor
No full textFuture single rotation propeller and contra-rotating advanced open rotor concepts promise a significant fuel efficiency advantage over current generation turbofan engines. The development of rotors which produce a minimum level of noise is a critical technical issue which needs to be resolved in order for these concepts to become viable aircraft propulsors. Noise and emissions are subject to stringent legislative requirements, thus accurate models are required in order to predict the noise radiated from aircraft engines. In this article, the development of a theoretical model to predict noise levels of an installed open rotor is reported. First a canonical problem is examined: how to predict the pressure field produced by a rotating ring of point sources adjacent to a rigid cylinder. Analytic expressions for the far-field pressure from a rotating ring of single-frequency monopole and dipole point sources, located near an infinitely long rigid cylinder, immersed in a constant axial mean flow, are explicitly formulated. Illustrative results show how the far-field pressure is affected by varying the source rotational direction, source location and source radius. Next the solution of the canonical problem is utilized to formulate a more advanced model to predict the noise due to an installed open rotor. In this model, the rotor noise sources are represented by a distribution of rotating sources. The adjacent aircraft fuselage is modeled by the rigid cylinder, and the effect of the fuselage boundary layer and other steady distortions are neglected. Also neglected is the scattering from other surfaces such as the pylon, wing and centerbody. This distributed source model can be used to calculate the effect of scattering of open rotor noise by an adjacent cylindrical fuselage. The model can be used to calculate both rotor-alone tones and tones produced by periodic unsteady loading on the rotor blades. Practical examples are provided which show how the effect of blade rotational direction and propeller location relative to the fuselage affect the sound produced by the interaction of a pylon wake with a rotor in a pusher configuratio
A weak-scattering model for turbine-tone haystacking outside the cone of silence
No full textWe consider the scattering of sound by turbulence in a jet shear layer. The turbulent, time-varying inhomogeneities in the flow scatter tonal sound fields in such a way as to give spectral broadening, which decreases the level of the incident tone, but increases the broadband level around the frequency of the tone. The scattering process is modelled for observers outside the cone of silence of the jet, using high-frequency asymptotic methods and a weak-scattering assumption. An analytical model for the far-field power spectral density of the scattered field is derived, and the result is compared to experimental data. The model correctly predicts the behaviour of the scattered field as a function of jet velocity and tone frequency<br/
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