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Acoustics Reflections of Full-Scale Rotor Noise Measurements in NFAC 40- by 80-Foot Wind TunnelThe objective of current research is to identify the extent of acoustic time history distortions due to wind tunnel wall reflections. Acoustic measurements from the recent full-scale Boeing-SMART rotor test (Fig. 2) will be used to illustrate the quality of noise measurement in the NFAC 40- by 80-Foot Wind Tunnel test section. Results will be compared to PSU-WOPWOP predictions obtained with and without adjustments due to sound reflections off wind tunnel walls. Present research assumes a rectangular enclosure as shown in Fig. 3a. The Method of Mirror Images7 is used to account for reflection sources and their acoustic paths by introducing mirror images of the rotor (i.e. acoustic source), at each and every wall surface, to enforce a no-flow boundary condition at the position of the physical walls (Fig. 3b). While conventional approach evaluates the "combined" noise from both the source and image rotor at a single microphone position, an alternative approach is used to simplify implementation of PSU-WOPWOP for this reflection analysis. Here, an "equivalent" microphone position is defined with respect to the source rotor for each mirror image that effectively renders the reflection analysis to be a one rotor, multiple microphones problem. This alternative approach has the advantage of allowing each individual "equivalent" microphone, representing the reflection pulse from the associated wall surface, to be adjusted by the panel absorption coefficient illustrated in Fig. 1a. Note that the presence of parallel wall surfaces requires an infinite number of mirror images (Fig. 3c) to satisfy the no-flow boundary conditions. In the present analysis, up to four mirror images (per wall surface) are accounted to achieve convergence in the predicted time histories
Document ID
20130011016
Acquisition Source
Ames Research Center
Document Type
Conference Paper
Authors
Barbely, Natasha Lydia (NASA Ames Research Center Moffett Field, CA, United States)
Kitaplioglu, Cahit (NASA Ames Research Center Moffett Field, CA, United States)
Sim, Ben W. (University Affiliated Research Center (Calif. Univ. SC) Moffett Field, CA, United States)
Date Acquired
August 27, 2013
Publication Date
January 20, 2012
Subject Category
Acoustics
Report/Patent Number
ARC-E-DAA-TN741
Meeting Information
Meeting: AHS Specialists'' Conference
Location: San Francisco, CA
Country: United States
Start Date: January 20, 2012
End Date: January 21, 2012
Sponsors: American Helicopter Society, Inc.
Distribution Limits
Public
Copyright
Public Use Permitted.

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