Identification, analysis, and control of fluid mechanically generated sound from models of aircraft and automobiles in special low-noise, semi-anechoic wind tunnels are an important research endeavor. Such studies can also be done in aerodynamic wind tunnels that have hard walls if phased microphone arrays are used to focus on the noise source regions and reject unwanted reflections or background noise. Although it may be difficult to simulate the total flyover or drive-by noise in a closed wind tunnel, individual noise sources can be isolated and analyzed.

An acoustic and aerodynamic study was made of a 7-percent-scale aircraft model in a NASA Ames 7-by-10-ft (about 2-by-3-m) wind tunnel for the purpose of identifying and attenuating airframe noise sources. Simulated landing, takeoff, and approach configurations were evaluated at Mach 0.26. Using a phased microphone array mounted in the ceiling over the inverted model, various noise sources in the high lift system, landing gear, fins, and miscellaneous other components were located and compared for sound level and frequency at one flyover location. Numerous noise-alleviation devices and modifications of the model were evaluated. Simultaneously with acoustic measurements, aerodynamic forces were recorded to document aircraft conditions and any performance changes caused by geometric modifications.

Most modern microphone-array systems function in the frequency domain in the sense that spectra of the microphone outputs are computed, then operations are performed on the matrices of microphone-signal cross-spectra. The entire acoustic field at one station in such a system is acquired quickly and interrogated during post processing. Beam-forming algorithms are employed to scan a plane near the model surface and locate noise sources while rejecting most background noise and spurious reflections. In the case of the system used in this study, previous studies in the wind tunnel have identified noise sources up to 19 dB below the normal background noise of the wind tunnel. Theoretical predictions of array performance are used to minimize the width and the side lobes of the beam pattern of the microphone array for a given test arrangement.

To capture flyover noise of the inverted model, a 104-element microphone array in a 622-mm-diameter cluster was installed in a 19-mm-thick poly(methyl methacrylate) plate in the ceiling of the test section of the wind tunnel above the aircraft model (see Figure 1). The microphones were of the condenser type, and their diaphragms were mounted flush in the array plate, which was recessed 12.7 mm into the ceiling and covered by a porous aromatic polyamide cloth (not shown in the figure) to minimize boundary-layer noise. This design caused the level of flow noise to be much less than that of flush-mount designs. The drawback of this design was that the cloth attenuated sound somewhat and created acoustic resonances that could grow to several dB at a frequency of 10 kHz.

A correction methodology has been developed to account for the signal interference. The first side lobe of the beam pattern was 13.4 dB down from the peak response at 8 kHz and at an angle of 23° from the normal vector: these characteristics made it possible to obtain good acoustic signals from the model when the model was located at a distance of 1.11 m from the array. Data were acquired at 12,321 scan points in a plane encompassing the model. From these data, aerodynamic noise from sources as small as 6 mm on the model surface could be identified easily.

The microphone signals were digitized at a rate of 153,600 samples per second on 104 channels simultaneously by use of analog-to-digital converter circuits and a computer. The resulting maximum acoustic frequency was 60 kHz with a bandwidth of 300 Hz. The data for frequencies <2 kHz were found to be of marginal utility because the microphone beam pattern at those frequencies was too wide. The data for frequencies >32 kHz were found to be of marginal utility because at those frequencies, the sources were too weak and the side lobes too strong. The frequency limits of 2 and 32 kHz correspond to limits of 140 and 2,240 Hz, respectively, on the full-scale aircraft.

A sound-convection correction was included in the processing of the data so that sources appeared to come from the model rather than being swept downstream. The acoustic sources were depicted, one frequency at a time, as color contours on the scan plane with the model outline superimposed, as shown in Figure 2. Various integration schemes have been developed to compute the combined effects on a listener and to generate narrow band and third octave acoustic spectra.

Ten airframe noise sources that might be important to approach and landing noise of the full-scale aircraft were identified in the study. The relative strengths of these sources and their dependences on the configuration of the aircraft were documented. Although the data were scaled to the frequencies for the fullscale aircraft, no extrapolation to full scale flyover was performed.

This work was done by Paul T. Soderman of Ames Research Center. For further information, contact the Ames Technology Partnerships Division at (650) 604-2954. ARC-14967