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Phillip Joseph – One of the best experts on this subject based on the ideXlab platform.

  • Determining the strength of rotating broadband Sources in ducts by inverse methods
    Journal of Sound and Vibration, 2006
    Co-Authors: Christopher Lowis, Phillip Joseph

    Abstract:

    Aeroengine broadband fan noise is a major contributor to the community noise exposure from aircraft. It is currently believed that the dominant broadband noise mechanisms are due to interaction of the turbulent wake from the rotor with the stator, and interaction of the turbulent boundary layers on the rotor blades with their trailing edges. Currently there are no measurement techniques that allow the localisation and quantification of rotor-based broadband noise Sources. This paper presents an inversion technique for estimating the broadband acoustic Source strength distribution over a ducted rotor using pressure measurements made at the duct wall. It is shown that the rotation of acoustic Sources in a duct prevents the use of standard acoustic inversion techniques. A new technique is presented here for inverting the strength of rotating broadband Sources that makes use of a new Green function taking into account the effect of Source rotation. The new Green function is used together with a modal decomposition technique to remove the effect of Source rotation, thereby allowing an estimation of the rotor-based Source strengths in the rotating reference frame. It is shown that the pressure measured at the sensors after application of this technique is identical to that measured by sensors rotating at the same speed as the rotor. Results from numerical simulations are presented to investigate the resolution limits of the inversion technique. The azimuthal resolution limit, namely the ability of the measurement technique to discriminate between Sources on adjacent blades, is shown to improve as the speed of rotation increases. To improve the robustness of the inversion technique, a simplifying assumption is made whereby the Sources on different blades are assumed to be identical. It is also shown that the accuracy and robustness of the inversion procedure improve as the axial separation between the rotor and sensors decreases. Simulation results demonstrate that for a 26-bladed fan, rotating with a blade tip Mach number of Mt=0.5, the Aerodynamic Source strengths can be estimated with acceptable robustness and approximately 1 dB accuracy, when measurements are made approximately 0.1 acoustic wavelengths from the rotor.

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  • Inversion technique for determining the strength of rotating broadband Sources in ducts
    11th AIAA CEAS Aeroacoustics Conference, 2005
    Co-Authors: Christopher Lowis, Phillip Joseph

    Abstract:

    Aeroengine broadband fan noise is a major contributor to the community noise exposure from aircraft. Currently there are no measurement techniques that allow the localisation and quantification of rotor-based broadband noise Sources. This paper presents an inversion technique for estimating the broadband acoustic Source strength distribution over a ducted rotor using pressure measurements made at the duct wall. It is shown that the rotation of acoustic Sources in a duct prevents the use of standard acoustic inversion techniques. The technique presented here makes use of a new Green function that takes into account the effect of Source rotation. The new Green function is used together with a modal decomposition technique to remove the effect of Source rotation, thereby allowing an estimation of the rotor-based Source strengths in the rotating reference frame. It is shown that the pressure measured at the sensors after application of this technique is
    identical to that measured by sensors rotating at the same speed as the rotor.
    Results from numerical simulations are presented to investigate the resolution limits of the inversion technique. The azimuthal resolution limit, namely the ability of the measurement technique to discriminate between Sources on adjacent blades, is shown to improve as the speed of rotation increases. To improve the robustness of the inversion technique a simplifying assumption is made whereby the Sources on different blades are assumed to be identical. It is also shown that the accuracy and robustness of the inversion procedure improve as the axial separation between the rotor and sensors decreases. Simulations demonstrate that for a 26-bladed fan, rotating at Mt = 0.5, the Aerodynamic Source strengths can be estimated with acceptable robustness and approximately 1dB accuracy, when measurements are made 0.1 acoustic wavelengths from the rotor.

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Wim Desmet – One of the best experts on this subject based on the ideXlab platform.

  • Aerodynamic/Acoustic Splitting Technique for Computation Aeroacoustics Applications at Low-Mach Numbers
    AIAA Journal, 2008
    Co-Authors: W. De Roeck, Martine Baelmans, Wim Desmet

    Abstract:

    Hybrid computational aeroacoustics applications approaches, in which the computational domain is split into an Aerodynamic Source domain and an acoustic propagation region, are commonly used for aeroacoustic engineering applications and have proven to be of acceptable efficiency and accuracy. The different coupling techniques tend to give erroneous results for a number of applications, which are mainly encountered in confined environments. Acoustic analogies are inaccurate if the acoustic variables are of the same order of magnitude as the flow variables, and an acoustic continuation of the Source-domain simulation using the latter solution as acoustic boundary conditions is only possible if no vortical outflow is occurring. These inaccuracies can be avoided by using appropriate filtering techniques in which the Source-domain solution is split into an acoustic and an Aerodynamic fluctuating part. In this paper, such an Aerodynamic/acoustic splitting technique is developed and validated for some simple test cases. The filtering method is valid for low-Mach-number applications, assuming that all compressibility effects are caused by the irrotational acoustic field and that the incompressible Aerodynamic field is responsible for the vortical movement of the flowfield. Under these assumptions, it is shown that the Aerodynamic and acoustic fields at every time step are obtained by solving a system of Poisson equations driven by the fluctuating expansion ratio and vorticity, obtained from the Source-domain simulation. For hybrid computational aeroacoustics applications approaches, this filtering technique, generally applicable for both free-field and confined-flow applications, provides more accurate coupling information and improves the knowledge of Aerodynamic-noise-generating mechanisms.

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  • An Aerodynamic/acoustic splitting technique for hybrid CAA applications
    13th AIAA CEAS Aeroacoustics Conference (28th AIAA Aeroacoustics Conference), 2007
    Co-Authors: W. De Roeck, Martine Baelmans, Wim Desmet

    Abstract:

    Hybrid CAA-approaches, where the computational domain is split into an Aerodynamic Source domain and an acoustic propagation region, are commonly used for aeroacoustic engineering applications and have proven to be of acceptable efficiency and accuracy. The different coupling techniques tend to give erroneous results for a number of applications, which are mainly encountered in confined environments. Acoustic analogies are inaccurate, if the acoustic variables are of the same order of magnitude as the flow variables and an acoustic continuation of the Source domain simulation using the latter solution as acoustic boundary conditions is only possible if no vortical outflow is occurring. These inaccuracies can be avoided by using appropriate filtering techniques where the Source domain solution is split into an acoustic and an Aerodynamic fluctuating part. In this paper, such an Aerodynamic/acoustic splitting technique is developed and validated for some simple test cases. The filtering method is valid for low-Mach number applications, assuming that all compressibility effects are caused by the irrotational acoustic field while the incompressible Aerodynamic field is responsible for the vortical movement of the flow field. Under these assumptions, it is shown that the Aerodynamic and acoustic fields at every time step are obtained by solving a system of Poisson equations driven by the fluctuating expansion ratio and vorticity, obtained form the Source domain simulation. For hybrid CAA-approaches this filtering technique, general applicable for both free-field and confined flow applications, is able to provide more accurate coupling information and improves the knowledge of Aerodynamic noise generating mechanisms.

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Kim – One of the best experts on this subject based on the ideXlab platform.

  • Numerical Analysis and Characterization of Surface Pressure Fluctuations of High-Speed Trains Using Wavenumber–Frequency Analysis
    Applied Sciences, 2019
    Co-Authors: Lee, Cheong, Kim

    Abstract:

    The high-speed train interior noise induced by the exterior flow field is one of the critical issues for product developers to consider during design. The reliable numerical prediction of noise in a passenger cabin due to exterior flow requires the decomposition of surface pressure fluctuations into the hydrodynamic (incompressible) and the acoustic (compressible) components, as well as the accurate computation of the near aeroacoustic field, since the transmission characteristics of incompressible and compressible pressure waves through the wall panel of the cabin are quite different from each other. In this paper, a systematic numerical methodology is presented to obtain separate incompressible and compressible surface pressure fields in the wavenumber–frequency and space–time domains. First, large eddy simulation techniques were employed to predict the exterior flow field, including a highly-resolved acoustic near-field, around a high-speed train running at the speed of 300 km/h in an open field. Pressure fluctuations on the train surface were then decomposed into incompressible and compressible fluctuations using the wavenumber–frequency analysis. Finally, the separated incompressible and compressible surface pressure fields were obtained from the inverse Fourier transform of the wavenumber–frequency spectrum. The current method was illustratively applied to the high-speed train HEMU-430X running at a speed of 300 km/h in an open field. The results showed that the separate incompressible and compressible surface pressure fields in the time–space domain could be obtained together with the associated Aerodynamic Source mechanism. The power levels due to each pressure field were also estimated, and these can be directly used for interior noise prediction.

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