Anechoic Noise Facility
NASA Langley Research Center
Curved Duct Experiment
The ANRF currently houses an experimental facility to evaluate sound propagation in a rectangular, acoustically treated duct with a curved flow path. The purpose of the experiment is to evaluate duct liner properties in full scale with higher order mode sound generation and to validate analytic models of sound propagation. The duct cross-section is 6”x15” and the maximum flow rate is Mach 0.3. Air to the duct is supplied from a 600 psia bottle field, and passes through a muffler under the chamber floor. Air is turned through an acoustically lined 90 deg elbow and up into the chamber from a 3-foot diameter nozzle whose mounting flange is at floor level. The duct transitions to a 30” square cross-section and then flow is turned in a 90 deg elbow to horizontal through the anechoic chamber. Flow control including screens and boundary layer control are utilized to reduce the turbulence of the flow through the test section. Sound is generated in the duct by an array of 12 loudspeakers, and is measured by two arrays of 35 surface mounted microphones, one upstream and one downstream of the wall treatment test section. The duct layout is shown in figure 1. A feedback control system is utilized to generate a user-specified mode, and to suppress all other modes in the duct. The sound system can generate tones in the duct up to 2500 Hz in frequency and modes up to 6 on the long dimension and 2 on the short dimension. The microphone arrays are used to measure the absorption and the mode scattering of sound in the duct. An acoustically treated diffuser provides an anechoic termination to the flow duct. The facility can accommodate single- or multiple-degree of freedom passive liners with offsets up to one cross dimension (6 inch). Active liner concepts as well as advanced passive concepts including Herschel-Quincke tubes can be evaluated in the facility. The Curved Duct Experiment will be fully operational by October 2005.
Jet Nozzle
When the Curved Duct Experiment is not operational in the chamber, the ANRF is a low-speed anechoic wind tunnel. Figure 2 is a cross-sectional view of the anechoic chamber, showing both the jet nozzle and the 12-inch ADP demonstrator (discussed below) set-ups. The jet nozzle configuration of the ANRF, which is referred to as Mode I operation, is a vertical flow path through the anechoic chamber. Air to this system is supplied from a 600 psia bottle field, and passes through a muffler under the chamber floor. Air is turned through an acoustically lined 90 deg elbow and up into the chamber from a 3-foot diameter nozzle whose mounting flange is at floor level. A transition to 8-inch size is available to accommodate nozzles in horizontal and vertical flow orientations. The air is collected in an eductor that is mounted on the ceiling of the chamber. The air goes through an acoustically lined torturous path before exhausting to the outside through acoustically lined hoods. The model air system has the capacity to supply dry air at pressures up to 255 psig and at a flow rate of 20 lb/sec to research nozzles mounted on the mating flange. Mode I operation is used for acoustic evaluation of nozzles and the study of fluid/solid interaction noise.
12-inch ADP Demonstrator
The facility can also be configured in Mode II operation, in which the 600 psi supply air is used to drive the 1000 HP, four-stage air turbine of the high power 12-inch Advanced Ducted Propeller (ADP) engine Demonstrator as shown in figure 3. The 12-inch ADP Demonstrator is a research tool that is used to investigate the fan noise generation mechanisms of high bypass ratio aircraft engines. It is also used for validation of noise control concepts. The 12-inch ADP Demonstrator has 16 wide-chord rotor blades with a diameter of 12-inches and hub-to-tip ratio of 0.445. The stator vanes can be located 1.0-, 1.5-, or 2.0-blade chords downstream of the rotors. Two stator vane sets have been fabricated. One consists of 20 vanes; this number of stator vanes is used to generate a tone at blade passage frequency due to impingement of the blade wakes on the stator vanes. The other stator vane set consists of 40 vanes; this number of vanes is used in order that the lowest mode of rotor/stator interaction noise is evanescent, and no tone at the blade passage frequency due to rotor/stator interaction is expected to radiate. The nacelle is supported by the stator vanes, so that there are no other obstructions in the flow path to produce interaction noise. The maximum rotational speed of the rotor is approximately 17,500 rpm, and rotor tip speed at 100% is subsonic at 905 feet/second. The blade angle setting is fixed relative to the hub at the takeoff condition such that the fan pressure ratio is 1.27 at 100% speed. The inlet of the 12-inch ADP demonstrator can be removed from the nacelle, so that the effects of inlet shape can be investigated or inlet noise control concepts can be evaluated. Similarly, the bypass duct nozzle is detachable in order to allow investigation of different nozzle shape effects. The 12-inch ADP Demonstrator is mounted on a sting that also serves as a muffler to reduce the noise from the drive turbine from influencing the fan-radiated noise. Air for the fan enters the chamber via the eductor/inductor and, after passing through the fan, enters a collector that redirects it up and discharges it at roof level. The 12-inch ADP Demonstrator is used for critical fan noise measurements where forward flight is not required. Since this is a static facility, the model is equipped with an inflow control device. The inflow control device, which can be seen in figure 3, is a hollow sphere that fits over the fan inlet. It is made of honeycomb material configured such that the axis of the honeycomb is oriented along the lines of potential flow into the inlet. The inflow control device breaks up the large-scale turbulence that is ingested by the fan and, thereby, simulates forward flight.
The facility has a machine shop for model preparation and facility mechanical repair work. A second floor open area of 600 square feet can be used for model preparation, auxiliary experiments, or meeting space. The control room on the second floor features a permanent 24-channel high-speed data acquisition system, data links to remote computers, all the controls for the air system for the Curved Duct Experiment, or Mode I and Mode II operation, and an office area. The portable 64-channel high speed data acquisition system can be used for noise data acquisition on full scale and model aircraft engines at remote facilities.
Figure 1. Sketch of the Curved Duct experiment in the ANRF. Hardware consists of: 1- anechoic termination, 2-downstream microphones, 3-test section, 4-upstream microphones, 5-sound source, 6-transition, 7-flow control, 8-elbow, 9-square-to-round transition.
The chamber is sealed during testing in which air is supplied either to the jet nozzle in Mode I operation or to the 12-inch ADP Demonstrator in Mode II operation. Observation of the test article in the chamber is made possible by a closed-circuit video camera with pan, tilt, and zoom capability.
The ANRF uses a linear hoop array of microphones to acquire acoustic data from the 12-inch ADP Demonstrator, as shown in figure 3, or other test article mounted on the test stand. The hoop holds up to 20 Bruel-Kjaer 1/4-inch microphones on a 6-foot diameter. It can be rotated up to 20 degrees. The hoop is mounted on a linear track and can be translated as much as 15 feet. Operation of the hoop array, in both rotation and translation, is achieved by computer control from the control room. Additional microphones can be mounted on fixed stands throughout the chamber. The acoustic signals are high-pass filtered and amplified using a 24-channel Precision filter. Acoustic data are acquired on a 24-channel Neff 495 data acquisition system, which can simultaneously sample and store data from all channels at rates up to 1 M-samples/second per channel. Buffer size on each channel allows acquisition of 1 M-samples before download to the computer. Control of the data acquisition process and the data analysis are performed on a DEC Alpha computer. Typical analysis includes Digital Fast Fourier Analysis, providing auto- and cross-spectral analysis. Programming to provide data analysis specific to a particular test is done using FORTRAN language. Graphics such as 3-dimensional sound contour maps are generated using TECPLOT. All raw data are archived on data tape and optical disc.A portable 64-channel data acquisition system has recently been acquired for use in remote facilities or field tests. This data acquisition system acquires data on each channel simultaneously at rates up to 200 k-samples/second. The system writes data directly to the hard disc, with a capacity of 17 G-bytes. On-line analysis includes maximum and minimum alarms and Digital Fast Fourier Analysis. Data are archived on DVD and can be transferred via FTP to another site.
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