Noise Source Location in Turbomachinery Using Coherence Based Modal Decomposition

Abstract

Coherence based source analysis techniques can be used to identify the contribution of turbomachinery core noise sources to pressure measurements in the far-field. The usual approach is to locate a measurement sensor within the engine and to calculate the ordinary coherence function between this and the far-field pressure measurement. If the internal measurement is close to a dominant noise source, the technique will identify this sources' contribution to the overall far-field energy. Modal decomposition is an advanced technique which can provide detailed information as to the modal content of sound propagating in ducts. When applied to aero-engines, the technique can be used as a diagnostic to determine which of the many rotor-stator stages contribute most to the overall radiated sound power. The method developed in this paper discusses how the two techniques can be combined to locate the plane at which a mode is generated within an aeroengine. A proof of concept of the technique is successfully demonstrated with the use of simulated data.

Aeroacoustics of an axial ducted low Mach-number stage: numerical and experimental investigation

“…The objective of this modal decomposition is to assess the modal content of the fan module and to investigate which modes are predominant. [15][16][17][18][19][20][21][22] It is expected that modes related to the stator heterogeneity at the first two BPFs (cut-off in a homogeneous configuration) will be highlighted. Considering a hard-walled duct of circular cross-section, the acoustic pressure at any point inside the duct can be expressed as a sum of modes as follows 23,24…”

A new MEMS microphone array for the wavenumber analysis of wall-pressure fluctuations: application to the modal investigation of a ducted low-Mach number stage

“…The final application -the modal analysis of a turbofan engine -requires a small-sized microphone, whose bandwidth covers the audio frequencies up to 20 kHz. In the literature, the use of quarter inch capacitive analog microphones is quite common [3,8,12]. That type of microphone offers a very high dynamics up to 160 dB or more, together with an extended bandwidth of 80 kHz or more.…”

“…where R is the duct radius, and c 0 the speed of sound. The eigenvalue λ mn is obtained through the annulation of the derivative of the Bessel function, through [3]: J ′ m (λ mn ) = 0 In the wavenumber representation, a pressure mode propagating with an angle φ mn will produce, in the array direction, a projected wavenumber (see Fig. 4):…”

“…M analysis has become a common tool for the experimental characterization of ducted turbofan engines [3,8].…”

Source localization of turboshaft engine broadband noise using a three-sensor coherence method

“…The main objective of the European project TEENI (Turboshaft Engine Exhaust Noise Identification) was to determine its origin in order to better reduce it. Several diagnostic techniques were implemented by the various TEENI partners, such as the identification of linear and non-linear contributions, 3 a modal analysis based on spectral coherences, 4,5 or several 21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014 ICSV21, Beijing, China, 13-17 July 2014 2 coherence techniques for source identification (coherent output power, signal enhancement, conditional spectral analysis). 6 A first attempt at separating sources was made by ONERA and Turbomeca nearly three decades ago using a three-signal coherence method (TSM) to reject nonpropagating background noise.…”