Reducing Propulsion Airframe Aeroacoustic Interactions with Uniquely Tailored Chevrons: 3. Jet-Flap Interaction

Abstract

Propulsion airframe aeroacoustic (PAA) interactions, resulting from the integration of engine and airframe, lead to azimuthal asymmetries in the flow/acoustic field, e.g., due to the interaction between the exhaust jet flow and the pylon, the wing and its high-lift devices, such as, flaps and flaperons. In the first two parts of this series we have presented experimental results which show that isolated and installed nozzles with azimuthally varying chevrons (AVCs) can reduce noise more than conventional chevrons when integrated with a pylon and a wing with flaps at take-off conditions. In this paper, we present model-scale experimental results for the reduction of jet-flap interaction noise source due to these AVCs and document the PAA installation effects (difference in noise between installed and isolated nozzle configurations) at both approach and take-off conditions. It is found that the installation effects of both types of chevron nozzles, AVCs and conventional, are reversed at approach and take-off, in that there is more installed noise reduction at approach and less at take-off compared to that of the isolated nozzles. Moreover, certain AVCs give larger total installed noise benefits at both conditions compared to conventional chevrons. Phased microphone array results show that at approach conditions (large flap deflection, low jet speed and low ambient Mach number), chevrons gain more noise benefit from reducing jetflap interaction noise than they do from quieting the jet plume noise source which is already weak at these low jet speeds. In contrast, at take-off (small flap deflection, high jet speed and high ambient Mach number) chevrons reduce the dominant jet plume noise better than the reduction they create in jet-flap interaction noise source. In addition, fan AVCs with enhanced mixing near the pylon are found to reduce jet-flap interaction noise better than conventional chevrons at take-off.

Flaperon Modification Effect on Jet-Flap Interaction Noise Reduction for Chevron Nozzles

“…Thus, we see that there is a marginal beneficial effect up to 1 dB with the R-flaperon especially in the front quadrant at lower frequencies. Recall that earlier studies 10,6 have denoted this region also as the one dominated by JFI. We will corroborate this later with phased array analysis.…”

“…The two cycle conditions analyzed here are as in the authors' previous studies [8][9][10] and are tabulated below: ke measurements, say, of installed noise (with both the Narrowband SPL was measured in LSAF f *log(frequency)) ranging from band number 22 to 49. We have previously documented 8,9 that during this LSAF test the repeatability of the far field SPL is within ± 0.5 dB between band levels of 27 and 47 at all directivity angles; it is even tighter (± 0.3 dB) if we restrict to a smaller band range 8,9 around band 36.…”

“…This is the same aircraft model previously used and described by the authors 9,10 . Figure 3 shows some of the basic variations of the flaperon modifications that were tested.…”

“…(See Ref. 10 for more details on this; it shows how the line of sight from the flaperon t.e. center to the centroid of the 90º-phased array makes an angle of 81.8º, and with the inclusion of tunnel flow refraction it corresponds to far field angles near 75º which is, indeed, the region of interest for JFI.)…”

“…This paper is part of a larger study at Boeing on Propulsion Airframe Aeroacoustic (PAA) interactions, funded by NASA in 2004, and should be read in conjunction with the companion paper 10 on the effect of AVC on JFI noise without the flaperon modifications. The scope of this work is limited to modern higher bypass ratio nozzles, of the order of 7 to 8 and the focus here is on noise, not thrust.…”