Development of an Experimental Rig for Investigation of Higher Order Modes in Ducts

14th AIAA/CEAS Aeroacoustics Conference (29th AIAA Aeroacoustics Conference)

et al. 2008

Self Cite

Continued success in aircraft engine noise reduction necessitates ever more complete understanding of the effect that flow path geometry has on sound propagation in the engine. The Curved Duct Test Rig (CDTR) has been developed at NASA Langley Research Center to investigate sound propagation through a duct of comparable size (approximately ½ the gap of GE90) and physical characteristics to the aft bypass duct of typical aircraft engines. The liner test section is designed to mimic the outer/inner walls of an engine exhaust bypass duct that has been unrolled circumferentially. Experiments to investigate the effect of curvature along the flow path on the acoustic performance of a test liner are performed in the CDTR and reported in this paper. Flow paths investigated include both straight and curved with offsets from the inlet to the discharge plane of ½ and 1 duct width, respectively. The test liners are installed on the side walls of the liner test section. The liner samples are perforate over honeycomb core, which design is typical of liners installed in aircraft nacelles. In addition to fully treated side walls, combinations of treated and acoustically rigid walls are investigated. While curvature in the hard wall duct is found not to reduce the incident sound significantly, it does cause mode scattering. It is found that asymmetry of liner treatment causes scattering of the incident mode into less attenuated modes, which degrades the overall liner attenuation. It is also found that symmetry of liner treatment enhances liner performance by eliminating scattering into less attenuated modes. Comparisons of measured liner attenuation with numerical results predicted by an analytic model based on the parabolic approximation (CDUCT-LaRC) have also been made and are reported in this paper. The effect of curvature in the rigid wall configuration estimated by CDUCT-LaRC is similar to the observed results, and the mode scattering seen in the measurements also occurs in the analytic model results. The analytic model and experiment show similar differences of overall attenuation between one wall treated and both walls treated.

Section: Analysis Methodologymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Configuration Effects on Acoustic Performance of a Duct Liner

Gerhold

Brown

14th AIAA/CEAS Aeroacoustics Conference (29th AIAA Aeroacoustics Conference)

et al. 2008

Self Cite

“…The most notable of these are the Grazing Incidence Tube 16 (GIT) and the Curved Duct Test Rig (CDTR). 17 In 2009, LaRC brought online a replacement flow duct for the GIT that is now referred to as the Grazing Flow Impedance Tube (GFIT). Lessons learned from the GIT were applied to the design of the GFIT.…”

Section: Introductionmentioning

confidence: 99%

Explanation of Anomalous Behavior Observed in Impedance Eduction Techniques Using Measured Data

Watson

16th AIAA/CEAS Aeroacoustics Conference

2010

Several enhancements that improve the accuracy and robustness of an impedance eduction technique that use an automatic optimizer are presented. These enhancements are then used to launch an intensive investigation into the cause of anomalous behavior that occurs for a small number of test conditions. This anomalous behavior is investigated for both a hardwall insert and a conventional liner. The primary conclusions of the study are that: (1) for the hard wall insert, the anomalies are due to narrow peaks in the objective function, (2) For the conventional liner, the anomalies are due to the presence of an extremely flat objective function, and (3) the anomalies appear to be triggered by inconsistencies between the duct propagation model and the measured data. At high frequencies, the duct propagation model may need to include the effects of higher-order duct modes, whereas at low frequencies, the effects of the mean boundary layer may have to be included. Meas, s,t = a measured quantity, a source plane quantity, an exit plane quantity min, max = minimum value of a quantity, maximum value of a quantity

“…The most notable of these are the Grazing Incidence Tube (GIT) 16 and the Curved Duct Test Rig (CDTR). 17 Impedances are educed for test liners in the GIT using the following four-step procedure: 4 1. Measure the plane wave acoustic pressures at a microphone array distributed along the lower wall in the hardwall section upstream of the test liner.…”

Section: Introductionmentioning

confidence: 99%

Impedance Eduction in Ducts with Higher Order Modes and Flow

Watson

15th AIAA/CEAS Aeroacoustics Conference (30th AIAA Aeroacoustics Conference)

2009

An impedance eduction technique, previously validated for ducts with plane waves at the source and duct termination planes, has been extended to support higher-order modes at these locations. Inputs for this method are the acoustic pressures along the source and duct termination planes, and along a microphone array located in a wall either adjacent or opposite to the test liner. A second impedance eduction technique is then presented that eliminates the need for the microphone array. The integrity of both methods is tested using three sound sources, six Mach numbers, and six selected frequencies. Results are presented for both a hardwall and a test liner (with known impedance) consisting of a perforated plate bonded to a honeycomb core. The primary conclusion of the study is that the second method performs well in the presence of higher-order modes and flow. However, the first method performs poorly when most of the microphones are located near acoustic pressure nulls. The negative effects of the acoustic pressure nulls can be mitigated by a judicious choice of the mode structure in the sound source. The paper closes by using the first impedance eduction method to design a rectangular array of 32 microphones for accurate impedance eduction in the NASA LaRC Curved Duct Test Rig in the presence of expected measurement uncertainties, higher order modes, and mean flow.