## 4 Results
### Aeroacoustics Measurements and Predictions
#### 4.2.3 Dual Rotor Interactions  
The interactional aerodynamics of the dual rotors predicted by Helios suggests an increase in noise from the single rotor case. The computed and measured sound at mic 3 of the linear array for the dual rotor configuration is plotted in Figure 17. The Helios/PSU-WOPWOP results show a significant increase in noise from the isolated rotor case, with greater than 15 dB increase at the BPF. This is greater than the effect of a pressure doubling, which means the computational methodology is able to capture the effect of the interaction. The experiment is also able to capture this effect, although it is less severe.  
An even greater interactional effect lies in the higher harmonics. Although the single rotor simulations do show some weak harmonics, the dual rotor simulations are rich with strong BPF tones into the 2 kHz range. [PERSON] et al. were able to identify increases in higher harmonic noise with the installation of different obstructions in the wake that represented a typical airframe [3]. These results indicate that the presence of an additional rotor is able to induce periodic unsteady loading, due to the impact of the blade-crossover events. The experiments show stronger higher harmonics for the dual rotor case as compared to the single rotor and they are much higher than predicted. This is, again, attributed to the effect of recirculation in the experiment, which is not modeled computationally.  
It is interesting to note that the high frequency broadband of the Helios/PSU-WOPWOP results shows a negligible change between the isolated rotor and dual rotor configurations. This is attributed to the fact that the influence of the vortices of the adjacent rotor is due largely to leading edge interactions, whereas the high frequency broadband is sourced at the trailing edge. [PERSON] et al.'s experiments of a confined rotor also found that flow recirculation did not increase the trailing edge noise in the high frequency by much, and localized the increase in low-frequency noise at the leading edge [21]. This result is reflected in the present study, where the simulated low frequency broadband of the dual rotor is much greater than that of the single rotor.  
Direct comparison between the computed results and experimental data at Mic 3 of the linear array for the co and counter rotating rotors at \(t=0.04D\) is provided in Figure 18 There is negligible difference in the broadband between the two configurations for the computations. The Helios/PSU-WOPWOP method resolves the acoustics in the low frequency, due to the leading edge interactions, while the UCD-Quietfly models the high frequency trailing edge noise. Therefore, the shape of the broadband shows excellent agreement with the experiments, although the amplitude is slightly low. It is possible that neither Helios/PSU-WOPWOP nor UCD-Quietfly is able to capture the effect of the blade  
Figure 17: Computations using Helios/PSU-WOPWOP and UCD-Quietfly (**a**) and experimental measurements (**b**) of the dual rotor configurations with a separation distance of \(t=0.04d\) at mic 3 of the linear array. The results of the isolated rotor are also included for reference.  
wake or blade vortex interactions on the trailing edge noise, which would be greater for the dual-rotor case.