Installation Effects on the Flow and Noise of Wing Mounted Jets

T IS known experimentally, since the late 1970s, that a jet installed under a wing of an aircraft radiates more noise than the same jet in a standalone condition. The excess noise is the propulsion-airframe integration noise or commonly referred to as installation noise. When a jet is placed near a wing, there is an increase in noise in the flyover directions because of the reflection of sound by the wing. Here, installation noise includes not merely the noise increase due to the reflection of sound by the wing. The major part of this noise is generated aerodynamicly by the nonlinear interaction between the flow around the wing flap and the jet. In this work,ourprimaryinterestistomodelandtopredictinstallationnoise of aerodynamic origin. Installation noise increases not only the total aircraftnoiseinthe flyoverplanebutalsointhesidelinedirections.It is especially important during landings and takeoffs when the flaps are down. Duringthe1980s,anumberofexperimentswerecarriedouttrying to quantify the characteristics and intensity of installation noise [1– 4]. Most of these experiments involved the measurements of the jet alone noise and the noise when the jet was placed near a model of an aircraft wing inside an anechoic chamber. The experimental measurements by Wang [2] were the most systematic. In his experiment,ascaledmodelofthewingofaDC-10aircraftwasused. Large noise increase was observed in the flyover plane in the lowfrequencypartofthespectrum.Theincreaseinhigh-frequencynoise was less. In directions at small exhaust angles, the installation noise intensity was quite low. In the sideline, the radiated noise characteristics, on the other hand, were quite different. Overall, the measured data indicated that installation noise had a unique spectral shape and a directional pattern of its own. Recently, there is a renewed interest in propulsion-airframe integration noise. Mead and Strange [5] investigated the under-thewing installation effects on jet noise with special emphasis on the sidelinedirections.Theirinterestinthesidelinewasmotivatedbythe experience that it was generally more difficult to meet legislative limit on sideline noise level requirements. They reported the measurement of high installation noise level in the low-frequency range. One drawback of the Mead and Strange experiment [5], as well as most of the previous works, is that the experiments were carried out in static conditions. Upon realizing that the effect of forward flight is extremely important in the interaction between the flow around the wing flapandthejet,aseriesofnewexperimentsoninstallationnoise was conducted by engineers of The Boeing Company (Shivashankara and Blackner [6], Blackner and Bhat [7], and Bhat and Blackner [8]). They employed an open wind tunnel at M � 0:28 tosimulatetheforwardmotionoftheaircraft.Byusingellipticmirror microphones and the newly developed phase-array microphones, they were able to obtain noise source location maps as well as farfield noise data.