## 4 Aeroacoustic Results
### Acoustic Fields on the Ground  
The acoustic impact at ground level for the two engine cases has been evaluated. Following a standard approach [30], the time history of the Sound Pressure Level at selected microphones on the runaway has been computed (Figure 18) for a flyover condition, as long as the corresponding Sound Exposure Level (SEL) on the above-mentioned microphones and on a microphones carpet at 600 m of altitude. No atmospheric absorption and no soil reflection have been considered. The soil reflection contributes to increase the overall noise level, however the objective of the current work is to compare the two engine configuration, without considering the absolute noise value. The ACO-ENV code presented in Section 3.2.3 uses as input noise field the scattered results and direct acoustic results computed for the frequency 1333 kHz, 2000 kHz, and 2666 kHz, shown in the previous paragraph. Starting from those solutions, the code is able to evaluate the time history of SPL and OASPL values in selected points. In Figure 19a, a comparison in terms of overall SPL time history is reported at the three microphones positions. Free- (label DIRECT) and scattered field (label BEM) have been compared for the BLI case (label BLI) and the clean one (label CLEAN). Referring to the central microphone, the installation effect is more evident than on the lateral microphones. The two lateral microphones show a slightly asymmetric behavior, though their position and the acoustic field are symmetrical with respect to the median vertical plane. This is due the absence of a symmetric boundary condition in the BEM calculation and due to the use of an unstructured mesh that is asymmetric with respect to the median vertical plane considered. Figure 17: Comparison of scattered (BEM) results for three frequencies, along X axis. Figure 18: Flyover and measurement in three aligned points. SEL for a single flyover event is finally reported at the three microphones positions in Figure 20, where the solid lines represent the results including the scattering effect and the dashed lines the direct counterpart. The difference in terms of SEL levels is clearly visible showing lower levels for the BLI case. However, this beneficial effect of the BLI case is reduced at ground level by the engine-anircraft installation. As already pointed out, the beneficial of the BLI case derives from the fact that the acoustic reference design point of the fan stage was considered by including the effect of the boundary layer ingestion. Sound Exposure Levels have been also computed for a microphones carpet at an altitude of 600 m. In Figures 21 and 22, the SEL carpet is shown, respectively, for the CLEAN case and BLI one. On the left side, results without installation effects are reported, while, on the right side, the corresponding ones with the acoustic installation effect. Comparing SEL carpets, the same conclusion of Figure 20 are deduced. For both free-field and scattering propagation, SEL carpet has lower values for BLI case than the one without BLI. Note that the propulsor-airframe installation effects poorly contribute to modify the noise field for the CLEAN case. This is due to the fact that the higher BFFs, that are the frequencies mainly contributing to the scattering effects, have much lower levels than the lowest BPF, as illustrated Figure 14. This behavior reflects in a small modification of the SEL carpet for the CLEAN case. For the BLI case, as shown on the right side of Figure 14, the highest BPFs have higher SPL values comparable to the 2 nd BPF. These highest frequencies, being more affected by the scattering phenomena, contribute much more to the overall acoustic noise. As a consequence, the engine-anircraft installation effect partially degrades the benefit gained at source level by the BLI case. Figure 19: SPL time history on three locations on the runaway. Figure 21: Comparison of Sound Exposure Level carpets. Clean case without acoustic installation effects (**a**) and with installation effects (**b**).