Report on Recent Upgrades to the Curved Duct Test Rig at NASA Langley Research Center

Abstract

The Curved Duct Test Rig (CDTR) is an experimental facility that is designed to assess the acoustic and aerodynamic performance of aircraft engine nacelle liners in close to full scale. The test section is between 25% and 100% of the scale of aft bypass ducts of aircraft engines ranging in size from business jet to large commercial passenger jet. The CDTR has been relocated and now shares space with the Grazing Flow Impedance Tube in the Liner Technology Facility at NASA Langley Research Center. As a result of the relocation, research air is supplied to the CDTR from a 50,000 cfm centrifugal fan. This new air supply enables testing of acoustic liner samples at up to Mach 0.500. This paper documents experiments and analysis on a baseline liner sample, which the authors had analyzed and reported on prior to the move to the new facility. In the present paper, the experimental results are compared to those obtained previously in order to ensure continuity of the experimental capability. Experiments that take advantage of the facility's expanded capabilities are also reported. Data analysis features that enhance understanding of the physical properties of liner performance are introduced. The liner attenuation is shown to depend on the mode that is incident on the liner test section. The relevant parameter is the mode cut-on ratio, which determines the angle at which the sound wave is incident on the liner surface. The scattering of energy from the incident mode into higher order, less attenuated modes is demonstrated. The configuration of the acoustic treatment, in this case lined on one surface and hard wall on the opposite surface, is shown to affect the mode energy redistribution.

Segmented Liner to Control Mode Scattering

“…The control system design ensures that the sound incident on the liner is predominantly composed of the selected mode and is at least 10 dB greater than any other mode in the duct. This was demonstrated previously by the authors 13 . The recorded signals from the upstream and downstream microphone arrays are analyzed to determine the mode distribution of the sound in the duct incident upon and discharged from the liner sample section, as discussed in previous papers.…”

“…The authors have established previously 12,13 that the desired source mode upstream of the liner test section is generally at least 10 dB greater than any other mode in the duct and thus, the downstream mode distribution can be considered to show the mode distribution relative to an upstream wave consisting of the source mode. The control system is programmed to generate the source mode at 130 dB.…”

“…The CDTR is an open loop wind tunnel that uses a fan to draw unconditioned atmospheric air through the test section. It has been described in previous papers, 12,13 and readers may refer to those papers for detailed description.…”

“…The attenuation is the difference between the sound power level upstream and that downstream of the liner test section. The derivation of the equations necessary to determine the sound power in the duct has been shown in a previous paper 13 .…”

Configuration Effects on Liner Performance

“…Thus, the sound incident on the liner is predominantly composed of the selected mode, which is at least 10 dB greater than any other mode in the duct. This was demonstrated previously by the authors 8 . The full array of microphones is used to determine the modal amplitudes of the sound waves entering and leaving the liner test section.…”