1,720,990 research outputs found
Application of frequency-domain linearised Euler solutions to the prediction of aft fan tones and comparison with experimental measurements
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A review of engine noise source diagnostic methods for static engine tests, with phased array and polar correlation techniques
No full textThis paper reviews the state of the art of engine noise source evaluation
using microphone arrays and identifies issues that are currently not well
understood. It is found that most methods currently assume a model of
uncorrelated point sources. However it will be shown that this is not a
necessary assumption and images of the complete source CSD can be
obtained, but that the effect of source convection cannot be readily obtained
from the source image. It is also shown how source images can be
improved by frequency averaging across the far field CSD matrix. This
approaches reduces aliasing and improves the source image especially at
high frequencies
CAA methods applied to liner splices and other circumferential impedance non-uniformities in cylindrical ducts with flow from CFD to CAA
No full textJet mixing noise: a review of single stream temperature effects
No full textThe work of Viswanathan and others in recent years has raised seriousquestions about jet noise databases such as that of Tanna - the two underlyingconcerns being noise contamination and the effect of Reynolds number - and asa result it has been argued that the effects of temperature have beenmisunderstood in the past. In this paper, we have compared the Tanna datawith that of QinetiQ [1983], which has a similar nozzle size and hence Reynoldsnumber to that of the data of Viswanathan and although we find similardifferences, we argue there are other reasons for this. Apart from extensive rigvalidation, Tanna & Morris repeated their static tests with a simulated flightstream and a substantial flight effect was observed. This would not have beenpossible if there had been significant noise contamination. In fact, thesedifferences are more likely due to some other effect related to comparing datafrom two different facilities and/or two different nozzles, as shown by Bridges &Brown and discussed in depth by Harper-Bourne at this conference. It isconcluded the available experimental evidence still strongly indicates thepresence of an additional dipole source due to heating that scales on the sixthpower of jet velocity. A brief review of recent flow-acoustic interactioncomputations based on the Lilley-Goldstein equation has confirmed that flowacousticinteractions cannot be the main reason for the effects of heating at lowvelocity ratios even though the quadrupole source in sheared flow is clearly‘amplifying’ the sound radiation at low frequencies. Finally convectiveamplification effects are briefly considered in both isothermal and heated jet
Influence of mean flow gradients on fan exhaust noise predictions
No full textAft fan noise is becoming a more dominant source as engine bypass ratio isincreased n this paper an assessment of the effect of the mean flow gradients onfan exhaust noise propagation is carried out using both analytical models forsimplified problems and numerical methods for realistic configurations. Fanexhaust noise can be significantly refracted by the mean flow gradients in the jetmixing layer, especially at high operating conditions (i.e. during take off). Therefraction effect is predicted using either Lilley’s equation or the linearizedEuler equations. For parallel base flows, an issue with these linear models is thepresence of Kelvin-Helmholtz instabilities whose unlimited exponential growthis unphysical and problematic for computational methods. This problem is lesscritical for developing mixing layer for instance where the growth of thevorticity thickness reduces the growth of the instability waves [1]. Varioustechniques have been used for suppressing the instability; these include addingnon-linear terms to saturate the growth of the instability [2], using frequencydomain analysis [3], or removing the mean flow gradient terms [4]. It is the lastapproach, termed Gradient Term Suppression (GTS), which is investigated inthe present work
Modelling of ducted noise sources in the proximity of acoustic liners
No full textThis paper focuses on modelling and predicting the acoustic field generated by ducted point sources in close proximity to an acoustic liner. Two analytical models are presented. The first model comprises a point monopole or dipole source in an infinite lined duct based on an existing Green's function. The predictions are compared with classic solutions for a source over an infinite lined plane and with a high frequency asymptotic duct approximation. The second model extends the Green's function to include a liner section of finite length connected to hard-wall extensions by using mode-matching techniques. The new model features the inclusion of point sources in the vicinity of an impedance discontinuity. The accuracy of this model is demonstrated by comparison with reference FE solutions. Both models indicate a significant impact of the source proximity to the liner surface in the source power output. These models offer insight on the source modification effects beyond the conventional approach in the design of liners solely based on acoustic absorption.</p
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/
Solving the Lilley equation with quadrupole and dipole jet noise sources
No full textThe literature contains various methods for solving the Lilley equation with different types of quadrupole and dipole sources to represent the mixing noise radiated into the far-field by isothermal and heated jets. These include two basic numerical solution methods, the ‘direct’ and the ‘adjoint’, and a number of asymptotic, analytic solutions. The direct and adjoint equations are reviewed and it is shown that their solutions are not only related through the adjoint property: the radial ODE for the adjoint displacement Green's function is the same as that governing the direct displacement Green's function because this particular Green's function obeys classical reciprocity with respect to its radial dependence. Further, by comparing the two numerical solution methods within the context of the parallel flow assumption of the Lilley equation, it is shown that the numerical effort for the two methods is equivalent. The numerical solutions are compared with analytic low frequency ‘thin shear layer’ solutions and WKB solutions, both outside and inside the cone of silence. It is concluded that the former should be used with caution at all angles, while the WKB has some limitations inside the cone of silence. Although numerical solutions can be obtained with little computational effort and are the preferred route for jet mixing noise predictions, the analytic solutions still offer important physical insights as well as verification of numeric result
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