Farfield radiation of aft turbofan noise

Abstract

The farfield radiation of turbofan noise from the inlet of an engine has recently received considerable study. Relatively simple approximate expressions have been developed for single mode radiation as a function of cutoff ratio and for multimodal radiation. Convective effects upon radiation which are essential for interpreting static engine tests have been developed. This information has been essential for the design of inlet suppressors. This paper represents an initial effort to provide the same type of radiation expressions for the noise radiation from the aft duct. The results of approximate aft radiation equations are compared to more exact Wiener-Hopf radiation results. Refraction as well as convective effects in the multiple flow streams must be considered. The possibility that the acoustic power distribution is altered during transmission is also discussed. This has an impact on the design and testing of aft duct suppressors.

Wind tunnel measurements of blade/vane ratio and spacing effects on fan noise

“…The directivities for both stator configurations show a decrease beyond 130 deg which is predicted by Ref. 22 as a "zone of silence" due to the shear layer refraction associated with the boundary of the fan exhaust stream.…”

“…The geometric acoustics analysis of Ref. 22 predicts the aft peak to be slightly forward of this angle for the 11-vane stator case. Although an aft peak was observed, it was not expected for the 25-vane stator case since residual tone noise is likely to tend toward a smooth equal energy distribution as occurred in the inlet quadrant.…”

Flight effects on fan noise with static and wind tunnel comparisons

“…This is especially true for aft-radiated spinning modes well above cut-on which tend to radiate in the forward quadrant. (28) aft radiated fan noise, tests were conducted with and without inlet liners and with and without bypass duct noise suppressors. bypass liners utilized bulk absorber material for improved suppression bandwidth.…”

Aircraft Turbofan Noise

“…The ray acoustics, mode cutoff ratio approach to an approximate solution can also be applied with more confidence to the aft slip layer. Starting from the zero-flow flanged duct radiation equation, a coordinate transformation was applied to account for the duct flow, and ray acoustics arguments were applied across the slip layer (74). Single mode aft directivity from the approximate expression is compared to the full Wiener-Hopf solution (75) in Fig.…”

“…The location of the principal lobe in the far field, ',' f is found from the approximate theory (74) to be: p __ 2 M cos 1,7f = _MD + 1 Ducts between the turbomachinery components and the observer can be lined with sound absorbing materials to greatly reduce the radiated noise. Early workers (77) on suppressors considered the use of splitter rings in the duct to increase treated area and decrease the distance between treated surfaces.…”