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Dual Rotating Rake Measurements of Higher-Order Duct Modes: Validation Using Experimental and Numerical DataA rotating rake mode measurement system was designed to measure acoustic duct modes generated by a fan stage. After analysis of the measured data, the mode coefficient amplitudes and phases were quantified. Early studies using this system found that mode power levels computed from rotating rake measured data would agree with the far-field power levels. However, this agreement required that the sound from the noise sources within the duct propagated outward from the duct exit without reflection and previous studies suggested conditions could exist where significant reflections could occur. This paper shows that mounting a second rake to the rotating system, with an offset in both the axial and the azimuthal directions, measures the data necessary to determine the modes propagating in both directions within a duct. The rotating rake data analysis technique was extended to include the data measured by the second rake. The analysis resulted in a set of circumferential mode coefficients at each of the two rake microphone locations. Radial basis functions were then least-squares fit to this data to obtain the radial mode coefficients for the modes propagating in both directions within the duct while accounting for the presence of evanescent modes. The validation of the dual-rotating-rake measurements was conducted using data from a combination of experiments and numerical calculations to compute reflection coefficients and other mode coefficient ratios. Compared to results from analytical and numerical computations, the results from dual-rotating-rake measured data followed the expected trends when frequency, mode number, and duct termination geometry were changed.
Document ID
20180002981
Acquisition Source
Glenn Research Center
Document Type
Technical Memorandum (TM)
Authors
Dahl, Milo D. (NASA Glenn Research Center Cleveland, OH, United States)
Hixon, Duane R. (Toledo Univ. Toledo, OH, United States)
Sutliff, Daniel L. (NASA Glenn Research Center Cleveland, OH, United States)
Date Acquired
May 21, 2018
Publication Date
August 1, 2018
Subject Category
Acoustics
Report/Patent Number
GRC-E-DAA-TN50872
E-19515
NASA/TM-2018-219898/REV1
Funding Number(s)
WBS: WBS 081876.02.03.50.03.06
Distribution Limits
Public
Copyright
Public Use Permitted.
Keywords
Validation
Measuring Technique
Duct modes

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