# Jet Noise in Airframe Integration and Shielding
## 5 Concluding Remarks  
This paper examined the available experimental investigations on the effect of propulsion airframe integration on jet noise. The reviewed data generally indicates that the perceived noise increases as a result of the jet plume interaction with the pylon, wing, and flaps, mainly due to the reflection of acoustic waves from aerodynamic surfaces. These interactions have led to an exploration of revolutionary designs, such as the hybrid wing body, aft-deck nozzle extensions, and engine-top design. Such design approaches, while promising in noise reduction, seem to be sensitive to geometrical or operating parameters. Optimizing these new design configurations toward achieving higher noise reduction would require accurate and efficient numerical simulations. Thus, the available numerical options to achieve this task were reviewed in this paper. Over the past two decades, computational aeroacoustics has significantly advanced by developing high-order schemes, proper boundary treatments, and the increase in computational power, aiming for accurate noise predictions of isolated jets. However, this study suggests that there are currently three options available to predict the generated noise in various complex scenarios of jet exhaust interaction with the airframe. The first option avoids solving the full, unsteady Navier-Stokes equations, and relies mainly on time-averaged data from the Reynolds-averaged Navier-Stokes equations or some empirical formula. This technique, though, is the fastest, and would be unreliable in testing new proposed geometries due to the built-in models involved. In the second option, large-eddy simulation was used to resolve the noise sources over the relevant range of frequencies. The presence of a solid boundary would make full large-eddy simulations impractical because of the resolution requirement near the boundary. The third option is the hybrid LES--URANS, which is often referred to as very large-eddy simulations or detached eddy simulations. In this approach, large-eddy simulation is used everywhere in the domain except when it switches to unsteady Reynolds-averaged simulations near the walls to avoid the prohibitive requirements for resolving the boundary layer. Once the near field is resolved via Hybrid LES--URANS, Fowcs Williams-Hawkings approach can be used for extending the near field to the far field. Thus, this option seems to be the viable one for jet-airframe interactions. Figure 31: Evolution of the fluctuating pressure R.M.S along jet axis. (**a**) Baseline (BSL) case, (b) Shielded case. To provide measures of existing noise mitigation approaches, such as chevrons, ejector-mixture, and high-bypass engines, open-loop noise control of jet was reviewed. For instance, excitation of the jet and consequently changing the spreading rate have shown promising results. Such alterations in the flow field of the jet are desired to achieve effective noise reductions in designs that incorporate propulsion-airframe integration. The effectiveness of the hybrid LES URANS approach was demonstrated by considering the shielding effect of solid surfaces on supersonic jets. In this case, an ideally expanded heated jet exhausting from a rectangular nozzle was considered while shielded by a flat plate parallel to the flow direction. Transient flow fluctuations were resolved using this hybrid approach. Here, a density-based compressible finite volume solver was employed coupled with the Fowcs Williams-Hawkings surface integral approach to predict the far-field acoustics. The numerical results were shown to be consistent with experimental observations. Both simulations and experimental data showed that the shielding plate effectively reduces the noise in the shielded direction. The acoustic spectra from the numerical simulations strongly agree with the experimental measurements, and confirmed the shielding effect of the flat plate, as well as the increase of noise in the reflected side when the flat plate is touching the jet exit.