Airframe Noise Study of a Bombardier CRJ-700 Aircraft Model in the NASA Ames 7-by 10-Foot Wind Tunnel

Abstract

NOMENCLATURE d fs distance from noise source to receiver full scale d ss distance from noise source to receiver small scale f frequency, Hz fs full scale f45°flap deflection angle = 45 deg i t horizontal-stabilizer incidence angle, deg Lp sound pressure level, dB relative to 20 mPa M free-stream Mach number s25°slat deflection angle = 25 deg SF scale factor = 0.07 ss small scale a or alpha model angle of attack as measured on fuselage lower surface near wing, deg DdB change in noise level relative to baseline configuration, dB l acoustic wavelength CONFIGURATION LIST ffl.25f flap fence sized to 1.25 times flap thickness, inboard flap only, fences extended forward to vane sfil slat-fence inboard tip, yawed sfi2 slat-fence inboard tip, streamwise aeroacoustics volume 3 á number 1 á 2004 Ð pages 1 Ð 42 1 sfi3 slat-fence inboard tip, streamwise, extended forward sfol slat-fence outboard tip SL3d slat inboard-tip extension, streamwise SL3e same as SL3d with fillet on outboard surface SL5 slat trailing-edge extension SL5a slat trailing-edge serrations ABSTRACT An acoustic and aerodynamic experimental study was conducted of a 7%-scale unpowered Bombardier CRJ-700 aircraft model in the NASA Ames 7-by 10-Foot Wind Tunnel for the purpose of identifying and attenuating airframe noise sources. Simulated landing, takeoff and approach configurations were evaluated at Mach 0.22 and 0.26. With a phased-microphone array mounted in the ceiling over the inverted model, various noise sources in the high-lift system, landing gear, fins, and other miscellaneous components were located and compared for sound level and frequency at one flyover location. Numerous model modifications and noise-alleviation devices were evaluated. Simultaneous with acoustic measurements, aerodynamic forces were recorded to document aircraft conditions and any performance changes caused by the geometric modifications. Such performance changes were small and are not reported here. Ten airframe noise sources were identified that might be important to approach and landing noise of the full-scale aircraft The top five noise sources were: a) slat gap, b) main gear, c) flap tips at wing crank, d) flap inboard gap, and e) slat inboard tip. Relative strengths of these sources were documented along with their dependence on aircraft configuration and operating condition. Although the data were scaled to full-scale frequencies, no extrapolation to full-scale flyover was attempted.

Quantification of Airframe Noise Using Microphone Arrays in Open and Closed wind Tunnels

“…An integration method similar to that of Brooks et al 2 was used by Soderman et al 4 to determine the relative importance of different airframe noise sources in a closed wind tunnel. However, the reliability of the absolute and relative sound levels was not investigated, probably because single-microphone levels of the wall-mounted array were contaminated by flow noise.…”

Evaluation of the noise impact of flap-tip fences installed on laminar wings

“…The conventional beamforming technique together with the diagonal removal and the spectral background noise subtraction ( [45]) provides sufficient SNR to eliminate background noise and reverberations in the closed test section ( [46,47]). More details are given in [36,37].…”

Systems and certification issues for civil transport aircraft flow control systems

“…In terms of noise, it is apparent that local flow separations are a dominant source in the landing phase. For example, an experimental study conducted for the Bombardier CRJ-700 regional jet identified the most significant sources of noise in the landing configuration (5) . Of the ten most significant sources, five were directly attributable to the high-lift system, and it was noted that both increased incidence and device deflection resulted in increased noise.…”

“…In the high lift configuration and particularly with the undercarriage deployed, significant noise sources are introduced. Some of these have been found to be attributable to the interacting vortices and wakes caused by the flap ends and slots (5) . Given the current research, it is reasonable to presume that FC could be used to eliminate some of these problems by improving simply hinged flap performance and the control of unsteadiness in separating flows.…”