## 4 Aeroacoustic Results
### Direct Acoustic Fields  
Using the engine noise boundary conditions as computed in Figure 10, the direct acoustic field was evaluated on the surface of the aircraft portion and on a microphones hemisphere. The hemisphere has a radius of 150 m with 64,800 nodes, and it is centered into the engine geometrical center. The 10 m portion of the aircraft tail is discretized with about 1 million of triangular elements corresponding to a maximum number of Points Per Wavelength of 14 for the frequency 1333 kHz. In Figure 11a, the aircraft portion is visible with the engine inlet and outlet used as boundary condition. In Figure 11b, a particular of the mesh size is reported, corresponding to the junction between the vertical tail and the rear part of the fuselage (with red color the fuselage, green the vertical tail, and gray the engine nacelle).  
Figure 11: Aircraft portion geometry used in aeroacoustic simulation, with particular of the computational mesh grid.  
Figure 10: Real part of complex pressure in [Pa] for tonal RSI noise at inlet (**left**) and outlet (**right**) plane. 2 nd BPF (1333.33 Hz). The modal structure of the sound field is clearly visible.  
The engine noise boundary condition was computed for the case without boundary layer ingestion, labeled as \"Clean\", and for the case with engine boundary layer ingestion, labeled as \"BLI\". Note that the engine reference configuration was the boundary layer ingestion case, for which the fan stage is properly designed accounting for the presence of the inflow distortion. As a consequence, the Clean configuration results in an off-design case, with a flow detachment at the rotor blades. This is highlighted in Figure 9, where it is clearly visible that for the 2 nd BPF the noise of BLI case is lower than Clean case. This behavior did not emerge for the other harmonics; however, the contribution of the second harmonic is prominent on the overall sound power level and drives the global noise results.  
Sound Pressure Level (SPL) computed on the microphones hemisphere is shown in Figures 12 and 13, respectively, for the 2 nd and 5 th BPF. Note that the engine boundary layer ingestion case has a reduced noise level compared with the clean one at the 2 nd BPF whereas the opposite behavior appears at the 5 th BPF. As already mentioned, the boundary layer ingestion causes an alternation of the mean rotor wakes (in width and depth) that lead to the alternations in tonal rotor-stator-wake interaction noise leading to a noise reduction for the lowest frequency and to a noise increase for the higher ones.  
A comprehensive comparison between the simulations with and without engine boundary layer ingestion is shown in Figure 14. Looking at the direc  
Figure 12: Comparison of direct acoustic field on hemisphere, between the cases without (**a**) and with (**b**) boundary layer ingestion for the frequency 1333 Hz.  
Figure 13: Comparison of direct acoustic field on hemisphere, between the cases without (**a**) and with (**b**) boundary layer ingestion for the frequency 3333 Hz.  
(longitudinal direction), similar trends for the cases with and without BLI emerge at all frequencies. In particular, for both cases, the same shifting of the SPL levels emerge in the downstream direction, due to the presence of a uniform mean-flow. Moreover, note that the absolute values of the SPL levels decrease by increasing the frequency, even if this behavior is more pronounced for the non-BLI case than the BLI one.