The need to minimize fan noise radiation from commercial aircraft engine nacelles continues to provide an impetus for developing new acoustic liner concepts. If the full value of such concepts is to be attained, an understanding of grazing flow effects is crucial. Because of this need for improved understanding of grazing flow effects, the NASA Langley Research Center Liner Physics Group has invested a large effort over the past decade into the development of a 2-D finite element method that characterizes wave propagation through a lined duct. The original test section in the Langley Grazing IncidenceTube was used to acquire data needed for implementation of this finite element method. This test section employed a stepper motor-driven axial-traversing bar, embedded in the wall opposite the test liner, to position a flush-mounted microphone at pre-selected locations. Complex acoustic pressure data acquired with this traversing microphone were used to educe the acoustic impedance of test liners using this 2-D finite element method and a local optimization technique. Results acquired in this facility have been extensively reported, and were compared with corresponding results from various U.S. aeroacoustics laboratories in the late 1990 s. Impedance data comparisons acquired from this multi-laboratory study suggested that it would be valuable to incorporate more realistic 3-D aeroacoustic effects into the impedance eduction methodology. This paper provides a description of modifications that have been implemented to facilitate studies of 3-D effects. The two key features of the modified test section are (1) the replacement of the traversing bar and its flush-mounted microphone with an array of 95 fixed-location microphones that are flush-mounted in all four walls of the duct, and (2) the inclusion of a suction device to modify the boundary layer upstream of the lined portion of the duct. The initial results achieved with the modified test section are provided in this report, and a comparison of these results with those achieved using the original test section is used to demonstrate that the data acquisition and analysis with the new test section can be confidently used for impedance eduction.
[1]
Willie R. Watson,et al.
Impedance Eduction In the Presence of Shear Flow
,
2001
.
[2]
Jay H. Robinson,et al.
Design and Attenuation Properties of Periodic Checkerboard Liners
,
2003
.
[3]
Willie R. Watson,et al.
Comparison of Two Waveguide Methods for Educing Liner Impedance in Grazing Flow
,
2004
.
[4]
Patricia E. Stiede,et al.
Comparison of methods for determining specific acoustic impedance
,
1997
.
[5]
G. W. Stewart,et al.
A Modification of Davidon's Minimization Method to Accept Difference Approximations of Derivatives
,
1967,
JACM.
[6]
M. K. Myers,et al.
On the acoustic boundary condition in the presence of flow
,
1980
.
[7]
Ricardo A. Burdisso,et al.
SOUND RADIATION FROM THE BOUNDARY IN A CIRCULAR LINED DUCT WITH FLOW
,
2003
.
[8]
Willie R. Watson,et al.
Comparison of Acoustic Impedance Eduction Techniques for Locally-Reacting Liners
,
2003
.
[9]
H. Saunders.
Literature Review : RANDOM DATA: ANALYSIS AND MEASUREMENT PROCEDURES J. S. Bendat and A.G. Piersol Wiley-Interscience, New York, N. Y. (1971)
,
1974
.
[10]
T. H. Melling,et al.
The acoustic impendance of perforates at medium and high sound pressure levels
,
1973
.
[11]
U. Ingard.
On the Theory and Design of Acoustic Resonators
,
1953
.
[12]
Willie R. Watson,et al.
Optimization Method for Educing Variable-Impedance Liner Properties
,
1998
.
[13]
D. L. Armstrong,et al.
Impedance measurements of acoustic duct liners with grazing flow
,
1974
.
[14]
Vincent Pagneux,et al.
Acoustic impedance measurement with grazing flow
,
2001
.