1,720,982 research outputs found
Phased array transformation methods to estimate non-compact jet noise source characteristics
No full textThis paper reviews the basis of the beam former (BF) and polar correlation (PC) phased array methods and shows that these provide different information about axially distributed, non-compact noise sources, which nevertheless satisfy a simple integral relationship. The conventional BF method provides an image of the source PSD, whereas the PC method yields a ‘source strength’ which is an image of the axial wavenumber transform of the source CSD. However, the BF method can be generalised to provide an image of the source CSD. At first sight the generalised BF method is therefore more useful for diagnostic purposes but the results presented here suggest that the combined effects of resolution and source convection place serious limitations on the source CSD image information. For the same reasons, although the source PSD axial shape can be obtained with the conventional BF method, it cannot yield its absolute level for this type of source. The PC method yields a source strength axial distribution at each ‘reference’ microphone, which when integrated over the source length, yields the far-field PSD at that reference microphone. Therefore the PC source strength is arguably the more relevant quantity to measure when determining what proportion of the sound at a particular microphone position comes from each region of the jet axis, as a function of radiation angle
A weak-scattering model for tone haystacking caused by sound propagation through an axisymmetric turbulent shear layer
No full textIn aeroacoustics, spectral broadening refers to the scattering of tonal sound fields by turbulent shear layers, whereby the interaction of the sound with turbulent flow results in power lost from the tone and distributed into a broadband field around the tone frequency. Fan and turbine tone spectral broadening is known colloquially as “haystacking”. Spectral broadening causes substantial difficulties in determining the relative importance of various sources of turbofan engine noise, as it scatters sound generated by tonal sources across a wide frequency range. This obscures the importance of some key sources of tone noise by apparently reducing their prominence, the resultant broadband noise being often difficult to attribute. Recently a new analytical solution has been derived to predict weak spectral broadening of a tone radiated through a circular jet. In this paper, validation of the weak scattering model is presented. A key aspect of the modelling is the choice of the two-point turbulent velocity cross-correlation function which is used to provide a statistical description of the turbulence in the shear layer. Results obtained using three different cross-correlation functions are compared. These include cross-correlation functions which have been developed using the theory for homogeneous isotropic turbulence or using the theory for homogeneous axisymmetric turbulence. In particular, a new cross-correlation function for an axisymmetric turbulent shear layer formed by a circular jet, based on the theory for homogeneous axisymmetric turbulence, has been developed. Validation results of weak-scattering calculated using this correlation function show better agreement with measurements when compared to the results calculated using correlation functions based on the theory for three-dimensional homogeneous isotropic turbulence
Spectral broadening of tonal sound propagating through an axisymmetric turbulent shear layer
No full textIn aeroacoustics, spectral broadening refers to the scattering of tonal sound fields by turbulent shear layers, whereby the interaction of the sound with turbulent flow results in power lost from the tone and distributed into a broadband field around the tone frequency. Fan and turbine tone spectral broadening is known colloquially as “haystacking”. Recently a new analytical model has been derived to predict weak spectral broadening of a tone radiated through a circular jet. A key part of the modeling is the choice of the two-point turbulent velocity cross-correlation function, which is used to provide a statistical description of the turbulence in the shear layer. A new cross-correlation function for an axisymmetric turbulent shear layer formed by a circular jet, based on the theory for homogeneous axisymmetric turbulence, has been developed. Validation results of weak-scattering calculated using this correlation function show better agreement with measurements when compared with the results calculated using a correlation function based on the theory for homogeneous isotropic turbulence
Mach-number scaling of individual azimuthal modes of subsonic co-flowing jets
No full textThe Mach-number scaling of the individual azimuthal modes of jet mixing noise was studied for jets in flight conditions, i.e. with co-flow. The data were obtained via a series of direct numerical simulations (DNS), performed of fully turbulent jets with a target Reynolds number, based on nozzle diameter, of . The DNS included a pipe 25 diameters in length in order to ensure that the flow developed to a fully turbulent state before exiting into a laminar co-flow, and to account for all possible noise generation mechanisms. To allow for a detailed study of the jet mixing noise component of the combined pipe–jet configuration, acoustic liner boundary conditions on the inside of the pipe and a modification to the synthetic turbulent inlet boundary condition of the pipe were applied to minimize internal noise in the pipe. Despite these measures, the use of a phased-array source breakdown technique was essential in order to isolate the sources associated with jet noise mechanisms from additional noise sources that could be attributed to internal noise or unsteady flow past the nozzle lip, in particular for the axisymmetric mode. Decomposing the sound radiation from the pipe–jet configuration into its azimuthal Fourier modes, and accounting for the co-flow effects, it was found that at the individual azimuthal Fourier modes of far-field pressure for the jet mixing noise component exhibit the same scaling with the centreline jet Mach number as that experimentally documented for the overall noise field. Applying the phased-array source breakdown code to the DNS data at smaller angles to the jet axis, an increase of the velocity exponent of the jet noise source was found, approaching 10 at . At this smaller angle the higher azimuthal modes again showed the same behaviour as the axisymmetric mode
Modelling the suppression of rotor-alone fan noise with Over-Tip-Rotor liners and comparison with measurements from a high-bypass turbofan rig
No full textFan noise is one of the dominant sources of aircraft engine noise, both at approach and at take-off. Improved attenuation of fan noise with acoustic liners and the reduction of fan noise at source remain key technology challenges for 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 at NASA using a single rotor and multiple lined circumferential grooves. This paper describes an analytical OTR prediction model in which the fan noise is modelled 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 representing the ‘rotor-alone’ source with its OTR liner. OTR liner insertion loss predictions are obtained for comparison with data from the W-8 NASA experimental rig yielding peak broadband in-duct noise reductions of up to 4 dB, in line with the measurements
Aeroacoustic assessment of the performance of Over-Tip liners in reducing noise of an aerofoil over a flat surface
No full textThe reduction of fan broadband noise in the next generation of Ultra-High By-pass-Ratio (UHBR) engines remains a key technology challenge for the foreseeable future. The Over-The-Rotor (OTR) liner concept has been studied as a technology with the potential to further reduce fan noise and significant noise reductions have been measured. This paper describes a fundamental experimental evaluation that represents the Over-Tip-Rotor liner as a static airfoil with its tip located over a flat plate containing a flush-mounted lined insert and separated from the airfoil tip by a small gap. Differences in measured far-field sound spectra and in source power estimates derived from post-processed spiral array data have shown broadband gap noise reductions of 5-10 dB, even in the absence of a tip gap. Over-tip liners are found to suppress noise sources when located in the immediate vicinity, irrespective of the generation mechanism, mainly due to back-reaction effects on the source. An analytical prediction model for the over-tip liner noise reduction, based on a point source located over an infinite lined plane, is evaluated and compared with the experimental data. The model captures the back-reaction effects and gives good agreement with the measured data, including the variation of noise reduction with increasing gap size
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
Impedance modelling of acoustically treated circumferential grooves for over-tip-rotor fan noise suppression
No full textExperimental investigation of Over-Tip-Rotor circumferential groove liners has shown potential for fan noise suppression in turbofan engines whilst providing minimal penalty in fan aerodynamic performance. The validation of Over-Tip-Rotor liner analytical prediction models against published experimental data requires the modelling of an equivalent impedance for such acoustic treatments. This paper describes the formulation of two analytical groove impedance models as semi-locally reacting liners, that is locally reacting in the axial direction and non-locally reacting in the azimuthal direction. The models are cross-verified by comparison with high-order FEM simulations, and applied to a simplified Over-Tip-Rotor configuration consisting of multiple grooves excited by a monopole point source located close to the grooved surface
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