Optimisation of Sensor Arrays for Radial Mode Analysis in Flow Ducts

For the evaluation and improvement of fan noise reduction notions and the verification of broadband sound power measurement in flow ducts, special interest was given to the analysis of the broadband noise fields in duct. Two axial sensor arrays were wall-flush mounted upstream of a single-stage axial fan in the circular duct section of the fan inlet, staggered by 180° in the circumferential direction. During the tests, the rotating drum was rotated by step of 6 degrees to give a total 840 measurement points. There are two static reference microphones mounted upstream of rotating measurement section. Time-series of 12 seconds duration were recorded at each of the 30 azimuthal positions of the rotating duct in order to allow for a statistically accurate cross-correlation data analysis. This method is able to discriminate the sound waves propagating in upstream and downstream directions. Special attention was given to the blade passing frequencies of the axial fan. Rotor-stator interaction mode dominates the incident sound field, while modes with low azimuthal order play an important role in the reflected sound field. The reflected broadband sound power is almost 10dB lower than the transmitted sound field in a broad-frequency range. On the whole, this method behaves robustly in decomposition of broadband noise in flow duct and delivers physically meaningful broadband mode amplitudes.

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Impact of Swirl on the Sensitivity of the Radial Mode Analysis in Turbomachinery

Volume 6C: Turbomachinery ◽

10.1115/gt2013-95460 ◽

2013 ◽

Cited By ~ 1

Author(s):

Juan D. Laguna ◽

Michael Bartelt ◽

Joerg R. Seume

Keyword(s):

Sensitivity Analysis ◽

Measurement Errors ◽

Sound Propagation ◽

Swirling Flow ◽

Signal To Noise Ratio ◽

Sound Field ◽

Radial Mode ◽

Mode Analysis ◽

Mean Flow ◽

Sound Structure

Sound measurements in turbomachinery are a prerequisite for the study and consequent understanding of sound propagation mechanisms. For analyzing these measurements, the Radial Mode Analysis (RMA) is applied. This method decomposes the transmitted sound field in dominant acoustical modes at specific frequencies. Before an experimental campaign is carried out, measurement parameters are selected such that the uncertainty in the results from the application of the RMA is minimized. In order to minimize uncertainties, a sensitivity analysis of the parameters which influence the overall error of the RMA is performed. This analysis focuses mainly on the output of a measurable quantity, namely on the propagating mode amplitudes. Using a numerical simulation, modal structures are generated based upon real turbine operating data with swirling flow and a characteristic operating temperature. The swirling flow is generated by adding an axial vortex to a constant flow-velocity profile. The results show that the sound field varies under consideration of swirling mean flow compared to uniform flow conditions. In the present case, higher-order modes dominate the propagating sound structure. The parameters studied for assessing the sensitivity are the signal-to-noise ratio of the measurement sensors, the number of triggered revolutions, the azimuthal spacing of the sensors, and a triggering delay. The sensitivity analysis gives a detailed insight into the measurement parameters influencing the output of the RMA, e.g. that small triggering delays cause appreciable measurement errors. This knowledge is used to define the requirements for high fidelity measurements.

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An Acoustic Excitation System for the Generation of Turbomachinery Specific Sound Fields: Part I — Design and Methodology

Volume 2A: Turbomachinery ◽

10.1115/gt2016-56020 ◽

2016 ◽

Cited By ~ 2

Author(s):

Akif Mumcu ◽

Christian Keller ◽

C. Mandanna Hurfar ◽

Joerg R. Seume

Keyword(s):

Sound Propagation ◽

Microphone Array ◽

Sound Field ◽

Radial Mode ◽

Mode Analysis ◽

Acoustic Excitation ◽

Axial Distribution ◽

Excitation System ◽

Sound Fields ◽

Duct Wall

A strong focus in the development of modern aircraft engines is the reduction of the engine tonal core noise. For the development of efficient noise reduction techniques, a detailed understanding of the sound transmission throughout all turbomachinery components of the engine is mandatory. In this paper an excitation system is developed to generate turbomachinery-specific sound fields by controlling their circumferential and radial mode order. The excitation system consists of two rows of eight loudspeakers distributed circumferentially around the outer duct wall. This paper gives a detailed description of the analytically- and numerically-supported design methodology of an optimized excitation system, as well as an optimized microphone array mounted flush with the outer duct wall. A sensitivity analysis of the loudspeaker array and of the microphone array with respect to distance and frequency is then carried out numerically. To analyze the microphone signals and to deconstruct the propagating sound field into its modal components, a Radial Mode Analysis (RMA) is carried out. To ensure high-quality RMA results, the axial distribution of the microphones is optimized with respect to the condition number of the array’s transfer matrix. The procedure explained in this paper shall help guide the development of acoustic excitation and microphone array systems for experiments to better understand sound propagation in turbomachinery and flow ducts.

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