## 4 Results
### Aeroacoustics Measurements and Predictions
#### 4.2.1 Isolated Rotor Noise Sources  
The sound pressure level measured at Mic 3 of the azimuthal array, representing an in-plane observer, and Mic 3 of the linear array, representing an above-plane observer, are plotted in Figure 14 against predictions using combined Helios/PSU-WOPWOP with UCD-Quietfly. The in-plane mic shows an overprediction at the BPF and reasonable agreement at the second BPF harmonic. Although the azimuthal directivity of the isolated rotor should be uniform due to the symmetric loading, there were experimental installation effects which impacted the results. These effects showed a greater impact on tonal noise, and can explain the discrepancy in results when comparing a single mic location. The details of this discovery are explained further in Appendix A. Although the relative magnitude of the tones varied slightly, the higher harmonic tones present in the experiment were still noticeable around the entire azimuth. These tones are not present in the predictions,  
Figure 13: Instantaneous planar slice of downward vertical velocity along the centerline of the mesh system at the final time step for the dual rotor configuration with \(t=0.04D\).  
as there are no higher harmonic amplitudes that stand out from the broadband signal. On the other hand, the computations at the above-plane mic demonstrate some weak harmonics into the 5 kHz range. The thickness and loading acoustic pressure signals of the isolated rotor at the two mic locations are plotted in Figure 15. We see that the thickness contribution is fairly smooth and a low source of noise mainly at the BPF at both mics. The loading noise contribution is shown to dominate and contains slightly higher harmonic content, which is more prevalent at the out-of-plane mic than the in-plane mic. As an aside, BEMT predictions of the thrust are uniform around the azimuth, due to the steady leading of the blade and as such, would predict only noise at the BPF. This was demonstrated previously [24].  
In previous papers that discuss the presence of higher harmonics, they are attributed to recirculation [4], random motor fluctuations [22], or self-generated turbulent inflow. Since the effects of recirculation and motor fluctuations are not modeled here, it must be the self-generated turbulent inflow that is responsible for the higher harmonics in the CFD simulations. However, the amplitude of the harmonics predicted by the computational methodology is low compared to the experimental measurements, which is most likely because the recirculation effect is not modeled. The low higher harmonic amplitudes match the findings of LBM predictions by [PERSON] et al. [25].  
Figure 14: Sound pressure level of the isolated rotor in hover at 6000 RPM measured at two different mic locations.  
Figure 15: Thickness and loading acoustic pressure signals computed using Helios/PSU-WOPWOP of the isolated rotor in hover over the final rotor revolution. Mic 3 of the azimuthal array is plotted in blue, while mic 3 of the linear array is plotted in red.  
Turbulent inflow will also contribute to the broadband noise as will blade-wake interactions. These mechanisms are considered leading edge interactions as opposed to the trailing edge mechanisms modeled by UCD-Quietfly and have been shown to mainly affect the lower frequency [21]. The underprediction of the broadband by the present methodology in the frequency range under 1 kHz may indicate that the leading edge interaction is not fully resolved by the computation. The trailing edge contribution to the broadband is fairly well predicted, except that it has a more uniform directivity with a similar broadband hump occurring at high frequencies for both observers. The overprediction of the in-plane high frequency broadband by UCD-Quietfly as compared to the present experimental data is not understood at this time.