1,720,971 research outputs found
Assessment of a frequency-domain linearized Euler solver for turbofan aft radiation predictions with comparison measurements
Full text linkThis paper presents a frequency-domain computational aeroacoustics tool for predicting aft noise radiation through turbofan ducts and jets and its application to two realistic engine exhaust configurations which have been experimentally tested. The tool is based on the discretised axisymmetric form of the linearised Euler equations in conjunction with perfectly matched layer equations at the inlet and far-field boundaries using high-order finite differences. The resultant linear system of equations is inverted by the state-of-the-art parallel sparse solver MUMPS. The far-field prediction is carried out by integrating Kirchhoff’s formula in frequency domain. The code has already been verified extensively for idealized semi-infinite duct cases with comparisons to available analytical solutions with very good agreement. Therefore, we concentrate in this paper on numerical solutions to the experimental cases tested in the EC FP6 Project TURNEX (TUrbomachinery noise Radiated through the engine EXhaust) to assess and partially validate the present solver further. Comparisons of the computed results with the measured data reveal that the solver predicts the general noise radiation patterns and sound levels reasonably well, so long as the target in-duct azimuthal mode remains dominant as it radiates to the far-field. The agreement strongly suggests that, at least for the range of mean flows and acoustic conditions considered, the physical aeroacoustic radiation processes are fully captured through the frequency-domain solutions to the linearised Euler equations.<br/
Broadband noise prediction from wake-rotor interaction in contra-rotating propfans
No full textA semi-analytical model for the prediction of the broadband noise due to the interactionbetween turbulent rotor wakes and a rotor in contra-rotating propfans (CRPs) is presented.The unsteady loading of the rear rotor is modelled using classical isolated flat-plate theory.Strip theory is used to treat the spanwise variations of aerodynamic quantities and bladegeometry. The turbulent wake is assumed to be homogeneous and isotropic that is modulatedby a wake train of wake profiles and is modelled using an analytical 3D turbulentvelocity spectrum (Von Karman’s model). The model is presented in detail and insight intoits modal behaviour is provided. Predictions and measurements differ by no better than14dB. This may indicate the presence of other broadband noise sources or measurementerror due to the facilit
Application of frequency-domain linearized Euler solutions to the prediction of aft fan tones and comparison with experimental measurements on model scale turbofan exhaust nozzles
No full textAlthough it is widely accepted that aircraft noise needs to be further reduced, there is an equally important, on-going requirement to accurately predict the strengths of all the different aircraft noise sources, not only to ensure that a new aircraft is certifiable and can meet the ever more stringent local airport noise rules but also to prioritize and apply appropriate noise source reduction technologies at the design stage. As the bypass ratio of aircraft engines is increased – in order to reduce fuel consumption, emissions and jet mixing noise – the fan noise that radiates from the bypass exhaust nozzle is becoming one of the loudest engine sources, despite the large areas of acoustically absorptive treatment in the bypass duct. This paper addresses this ‘aft fan’ noise source, in particular the prediction of the propagation of fan noise through the bypass exhaust nozzle/jet exhaust flow and radiation out to the far-field observer. The proposed prediction method is equally applicable to fan tone and fan broadband noise (and also turbine and core noise) but here the method is validated with measured test data using simulated fan tones. The measured data had been previously acquired on two model scale turbofan engine exhausts with bypass and heated core flows typical of those found in a modern high bypass engine, but under static conditions (i.e. no flight simulation). The prediction method is based on frequency-domain solutions of the linearized Euler equations in conjunction with perfectly matched layer equations at the inlet and far-field boundaries using high-order finite differences. The discrete system of equations is inverted by the parallel sparse solver MUMPS. Far-field predictions are carried out by integrating Kirchhoff's formula in frequency domain. In addition to the acoustic modes excited and radiated, some non-acoustic waves within the cold stream-ambient shear layer are also captured by the computations at some flow and excitation frequencies. By extracting phase speed information from the near-field pressure solution, these non-acoustic waves are shown to be convective Kelvin–Helmholtz instability waves. Strouhal numbers computed along the shear layer, based on the local momentum thickness also confirm this in accordance with Michalke's instability criterion for incompressible round jets with a similar shear layer profile. Comparisons of the computed far-field results with the measured acoustic data reveal that, in general, the solver predicts the peak sound levels well when the farfield is dominated by the in-duct target mode (the target mode being the one specified to the in-duct mode generator). Calculations also show that the agreement can be considerably improved when the non-target modes are also included, despite their low in-duct levels. This is due to the fact that each duct mode has its own distinct directionality and a non-target low level mode may become dominant at angles where the higher-level target mode is directionally weak. The overall agreement between the computations and experiment strongly suggests that, at least for the range of mean flows and acoustic conditions considered, the physical aeroacoustic radiation processes are fully captured through the frequency-domain solutions to the linearized Euler equations and hence this could form the basis of a reliable aircraft noise prediction metho
Direct numerical simulation of turbulent flow with an impedance condition
No full textDNS solutions for a pipe/jet configuration are re-computed with the pipe alone to investigate suppression of previously identified internal noise source(s) with an acoustic liner, using a time domain acoustic liner model developed by Tam and Auriault (AIAA Journal, 34 (1996) 913–917). Liner design parameters are chosen to achieve up to 30 dB attenuation of the broadband pressure field over the pipe length without affecting the velocity field statistics. To understand the effect of the liner on the acoustic and turbulent components of the unsteady wall pressure, an azimuthal/axial Fourier transform is applied and the acoustic and turbulent wavenumber regimes clearly identified. It is found that the spectral component occupying the turbulent wavenumber range is unaffected by the liner whereas the acoustic wavenumber components are strongly attenuated, with individual radial modes being evident as each cuts on with increasing Strouhal number
Modeling of over-tip-rotor liners for the suppression of fan noise
No full textFan noise is one of the dominant sources of aircraft engine noise, both at approach and at takeoff. Improved attenuation of fan noise with acoustic liners and the reduction of fan noise at the source remain key technology challenges for civil aviation in the foreseeable future. Over-tip-rotor (OTR) acoustic treatments have been investigated experimentally during the last decade, and significant fan noise reductions have been measured, most recently using a single rotor and multiple lined circumferential grooves. This paper describes an analytical OTR prediction model in which the fan noise is modeled with point or distributed, static or rotating monopole and dipole sources based on Green’s functions for infinite hard or lined cylindrical ducts containing uniform mean flow; these are combined with an anechoic or unflanged inlet termination and an embedded finite length lined section in which the rotor-alone source is located with its OTR liner. OTR liner insertion loss predictions are obtained for comparison with data from the W-8 NASA experimental rig. These yield peak broadband in-duct noise reductions of up to 4 dB, in line with the measurements.</p
An analytic Green's function for a lined circular duct containing uniform mean flow
Full text linkAn analytic Green’s function is derived for a lined circular duct, both hollow and annular, containing uniform mean flow, from first principles by Fourier transformation. The derived result takes the form of a common mode series. All modes are assumed to decay in their respective direction of propagation. A more comprehensive causality analysis suggests the possibility of upstream modes being really downstream instabilities. As
their growth rates are usually exceptionally large, this possibility is not considered in the present study.
We show that the analytic Green’s function for a lined hollow circular duct, containing uniform mean flow, is essentially identical to that used by Tester e.a. in the Cargill splice scattering model. The Green’s function for the annular duct is new. Comparisons between the numerically obtained modal amplitudes of Alonso e.a. and the present analytic results for a lined, hollow circular duct show good agreement without flow, irrespective of how many modes are included in the matrix inversion for the numerical mode amplitudes. With flow, the mode amplitudes do not agree but as the number of modes included in the matrix inversion is increased the numerically obtained modal amplitudes of Alonso e.a. appear to be converging to the present analytical result.
In practical applications our closed form analytic Green’s function will be computationally more efficient, especially at high frequencies of practical interest to aero-engine applications, and the analytic form for the mode amplitudes could permit future modelling advances not possible from the numerical equivalent
Development of an improved core noise prediction method for long-cowl engines
No full textThis paper describes the process of developing an improved method to predict the core noise radiated by long-cowl engines under static and flight conditions. The proposed method follows the same approach as the industry standard SAE ARP 876 method in terms of structure and source model, but uses: (1) a new, analytical frequency-dependent directivity accounting for radiation, convection and refraction effects through the jet and (2) a new empirical source spectrum determined using a source breakdown code applied to external phased array data acquired on a Rolls-Royce BR700-type engine.</p
Shear layer modelling with a finite element model - variants of the Munt problems
No full textThis paper describes an unusual application of a finite element code Actran, which was
originally designed to simulate acoustic propagation in potential mean flows. The code is
applied to the problem of duct mode radiation through an exhaust nozzle to the far-field;
the effects of refraction across the jet shear layer are accounted for, this shear layer being
represented as a vortex sheet. The first section presents details of a membrane model
of the vortex sheet and a preliminary study of a simple problem which has been used to
investigate the membrane parameters and to assess their effect on solutions in the near and
far-field. The second section presents some comparisons with Munt-type analytic solutions
to demonstrate the validity of this approach. In the third section, a methodology is outlined
that will, in future, enable users of the Actran code to use this membrane representation of
vortex sheets for more complex problems. Some preliminary results are given to illustrate
its application
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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