ALBERT

Description: Propulsion airframe aeroacoustic (PAA) interactions arise due to the manner in which an engine is installed on the airframe and lead to an asymmetry in the flow/acoustic environment, for example, for under-the-wing installations due to the pylon, the wing and the high-lift devices. In this work we study how we can affect these PAA interactions to reduce the overall jet-related installed noise by tailoring the chevron shapes on fan and core nozzles in a unique fashion to take advantage of this asymmetry. In part 1 of this trio of papers we introduced the concept of azimuthally varying chevrons (AVC) and showed how some types of AVCs can be more beneficial than the conventional chevrons when tested on "isolated" scaled nozzles inclusive of the pylon effect. In this paper, we continue to study the effect of installing these AVC nozzles under a typical scaled modern wing with high-lift devices placed in a free jet. The noise benefits of these installed nozzles, as well as their installation effects are systematically studied for several fan/core AVC combinations at typical take-off conditions with high bypass ratio. We show, for example, that the top-enhanced mixing T-fan AVC nozzle (with enhanced mixing near the pylon and less mixing away from it) when combined with conventional chevrons on the core nozzle is quieter than conventional chevrons on both nozzles, and hardly produces any high-frequency lift, just as in the isolated case; however, its installed nozzle benefit is less than its isolated nozzle benefit. This suppression of take-off noise benefit under installed conditions, compared to its isolated nozzle benefit, is seen for all other chevron nozzles. We show how these relative noise benefits are related to the relative installation effects of AVCs and baseline nozzles.

Description: 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.

Description: The flow/acoustic environment surrounding an engine nozzle installed on an airplane, say, under the wing, is asymmetric due to the pylon, the wing and the interaction of the exhaust jet with flaps on the wing. However, the conventional chevrons, which are azimuthally uniform serrations on the nozzle lip, do not exploit the asymmetry due to these propulsion airframe aeroacoustic interactions to reduce jet noise. In this pioneering study we use this non-axisymmetry to our advantage and examine if the total jet-related noise radiated to the ground can be reduced by using different types of azimuthally varying chevrons (AVC) which vary the mixing around the nozzle periphery. Several scale models of the isolated nozzle, representative of high bypass ratio engine nozzles, were made with a pylon and azimuthally varying chevrons on both fan and core nozzles to enhance mixing at the top (near the pylon) with less mixing at the bottom (away from the pylon) or vice versa. Various combinations of fan and core AVC nozzles were systematically tested at typical take-off conditions inside a free jet wind-tunnel and, here, in Part 1 we analyze the acoustics results for the isolated nozzle with a pylon, with installation effects reported in Parts 2 and 3. Several interesting results are discovered: amongst the fan AVCs the top-enhanced mixing T-fan chevron nozzle is quieter in combination with any core AVC nozzle when compared to conventional chevrons; however, the bottom-mixing B-fan chevrons, as well as the core AVC nozzles, by themselves, are noisier. Further, the low-frequency source strengths in the jet plume, obtained via phased microphone arrays, also corroborate the far field sound, and for the T-fan chevrons such sources move further downstream than those for baseline or conventional chevron nozzles.