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[PhD] In-duct aeroacoustic sources identification by means of external antennae

PhD position
3 years

In the context of aircraft noise reduction, localization methods aim at identifying and quantifying the different noise sources (jet, fan, wing, etc.). Aircraft engines are the main community noise source around airports. Furthermore the current developments of aircraft engine architecture towards high dilution rates tend to make fan noise even more predominant. Its experimental characterization is based on localization methods. Two different approaches can be applied in case of in-duct propagation: (i) the localization and quantification of noise sources by beamforming, taking into account their rotation [1], or (ii) the modal characterization of the acoustic field by deconvolution methods [2,3]. Both approaches use internal wall-mounted measurements. The reduced space available on real engines induces installation difficulties when the number of microphones increases. Thus, the use of external measurements appears as a relevant alternative. At ONERA, S. Léwy [4] first estimated modes amplitudes from an external antenna, assuming that the whole acoustic power inside the duct is retrieved outside. Nevertheless, reflexions occur at the end of the duct and a good knowledge of the propagation function between modes and microphones is needed to accurately estimate the sources. For instance, cut-off modes, confined inside the duct, will obviously not be retrieved by external measurements; however their impact in term of noise pollution is limited.
The aim of this PhD project, which is part of the CleanSky2 project ADEC, is to evaluate the feasibility of in-duct aeroacoustic sources characterization by means of external measurements, taking into account the complex acoustic waves propagation from the inside to the outside of the duct. First, a simplified configuration will be considered: finite rigid cylindrical duct with constant section, without flow, in order to assess the feasibility, determine the limitations, identify the hard points and quantify the method. It will also be necessary to ascertain if the modal content of the internal acoustic field can be directly evaluated by external measurements or if it is necessary to characterize the acoustic field at the outlet of the duct, as an intermediate step. The estimation methodology has to be developed. To do so, the student will lean on bibliography, ONERA’s expertise (numerical simulation, external localization and deconvolution methods) as well as DAAA test bench facilities. Measurements using the test bench represented below will be proceed in the DAAA anechoic chamber, in order to deeply understand the propagation phenomenon, evaluate the method and assess the antennae validity. Then, configurations with increased complexity (flow, annular duct, etc.) will be studied.
[1] P. Sijtsma, Using phased array beamforming to identify broadband noise sources in a turbofan engine, International journal of Aeroacoustics, vol. 9 (3), pp 357-374 (2010).
[2] U. Tapken, B. Pardowitz, and M. Behn, Radial mode analysis of fan brodband noise, 23rd AIAA/CEAS Aeroacoustics Conference, doi : 10.2514/6.2017-3715 (2017).
[3] S. Fauqueux, and R. Davy, Modal Deconvolution Method in a Finite Circular Duct, using Flush-mounted Microphones, 24th AIAA/CEAS Aeroacoustics Conference, doi:10.2514/6.2018-3927 (2018).
[4] S. Léwy, Numerical inverse method predicting acoustic spinning modes radiated by a ducted fan from free-field test data, Journal of the Acoustical Society of America, vol. 124 (1), pp 247-256 (2008).

5/15/21 12:00 AM
Sandrine Fauqueux