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Linne, Sweden

Boden H.,Linne Flow Center
19th AIAA/CEAS Aeroacoustics Conference | Year: 2013

This paper discusses the effect of high level multi-tone acoustic excitation on the acoustic properties of perforates. It is based on a large experimental study of the nonlinear properties of these types of samples without mean grazing or bias flow. Compared to previously published results the present investigation concentrates on the effect of multiple harmonics. It is known from previous studies that high level acoustic excitation at one frequency will change the acoustic impedance of perforates at other frequencies, thereby changing the boundary condition seen by the acoustic waves. This effect could be used to change the impedance boundary conditions and for instance increase the absorption. It could obviously also pose a problem for the correct modelling of sound transmission through ducts lined with such impedance surfaces. Experimental results are compared to a quasi-stationary model. The effect of the combination of frequency components and phase in the excitation signal is studied. Source


Boden H.,Linne Flow Center
17th AIAA/CEAS Aeroacoustics Conference 2011 (32nd AIAA Aeroacoustics Conference) | Year: 2011

This paper discusses the possibility to apply polyharmonic distortion modelling, used for nonlinear characterisation of microwave systems, to acoustic characterisation of samples with non-linear properties such as perforates and other facing sheets used in aircraft engine liners and automotive mufflers. In some previous papers multi-port techniques using sinusoidal excitation for characterization of samples with non-linear properties were developed and experimentally tested. These techniques aimed at taking non-linear energy transfer between sound field harmonics into account. Essentially linear system identification theory was however used assuming that superposition applies and that the functions studied are analytical. Polyharmonic distortion modelling does not assume that the function relating waves incident and reflected or transmitted is analytic nor does it assume application of normal superposition. This technique is tested on experimental data obtained from measurements on a perforate mounted in a duct. The similarity to the previously developed nonlinear scattering matrix techniques is demonstrated. It is shown how the results obtained can be used to analyse nonlinear energy transfer to higher harmonics. © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Source


Boden H.,Linne Flow Center
21st AIAA/CEAS Aeroacoustics Conference | Year: 2015

This paper discusses the use of nonlinear system identification techniques for determination of linear acoustic impedance and non-linear acoustic properties of perforates and other facing sheets used in aircraft engine liners. Multiple input single output nonlinear system identification techniques are revisited and applied to the problem of nonlinear acoustic characterisation of perforates. Bi-linear signal analysis techniques are also tested as well as Hilbert transform techniques applicable for non-stationary and nonlinear problems. It is shown that random excitation nonlinear system identification techniques have the potential of identifying and characterising non-linear acoustic properties of these types of samples. © 2015, American Institute of Aeronautics and Astronautics Inc, AIAA. All Rights Reserved. Source


Zhou L.,Competence Center for engine Gas Exchange | Boden H.,Linne Flow Center
21st AIAA/CEAS Aeroacoustics Conference | Year: 2016

Acoustic liners are a key part for reducing aircraft engine noise. Simulation and optimization of liner properties are critically relying on impedance measurement results, so called impedance eduction technology. Traditionally the effect of viscosity has been assumed to have negligible influence. However this paper shows that viscosity has noticeable influence even at low frequencies. The investigation is based on a comparison study using Linearized Euler equations and Linearized Navier-Stokes equations solved by finite element simulations. In the process of impedance eduction a one-dimensional straightforward method is proposed. Normal velocity and displacement have been obtained and discussed. Impedance results are further implemented into a two-dimensional wave propagation code. Finally simulation results have been compared and validated against experimental data. © 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. Source


Zhou L.,Competence Center for engine Gas Exchange | Boden H.,Linne Flow Center
Journal of Sound and Vibration | Year: 2015

The so-called impedance eduction technology is widely used for obtaining acoustic properties of liners used in aircraft engines. The measurement uncertainties for this technology are still not well understood though it is essential for data quality assessment and model validation. A systematic framework based on multivariate analysis is presented in this paper to provide 95 percent confidence interval uncertainty estimates in the process of impedance eduction. The analysis is made using a single mode straightforward method based on transmission coefficients involving the classic Ingard-Myers boundary condition. The multivariate technique makes it possible to obtain an uncertainty analysis for the possibly correlated real and imaginary parts of the complex quantities. The results show that the errors in impedance results at low frequency mainly depend on the variability of transmission coefficients, while the mean Mach number accuracy is the most important source of error at high frequencies. The effect of Mach numbers used in the wave dispersion equation and in the Ingard-Myers boundary condition has been separated for comparison of the outcome of impedance eduction. A local Mach number based on friction velocity is suggested as a way to reduce the inconsistencies found when estimating impedance using upstream and downstream acoustic excitation. © 2015. Source

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