## 5 Conclusions
It also shows the challenge for computations for application to realistic vehicle settings in which the rotors are not perfectly phased or at the exact same RPM, the flow near the rotors are disturbed by the rest of the vehicle's structure, and the flow field may not be quiescent. It was demonstrated that high-fidelity CFD modeling captures a Reynolds number performance effect that inviscid models do not capture. It was also shown that the high-fidelity CFD model can capture the impact of interactional effects on the creation of higher harmonics. The high-fidelity CFD together with an impermeable surface FWH, however, does not capture all of the noise sources; their effect was added via the use of UCD-Quietfly. The single rotor permeable surface FWH calculation presented in the paper suggests that more of the noise sources are captured using the permeable surface. However, an inconsistency in the BPF prediction with the permeable surface called into question the accuracy of this implementation. It is surmised that the issue is due to imperfect placement or topology of the surface, which requires further investigation. The Helios/PSU-WOPWOP simulation framework requires computational resources for the dual rotor cases on the order of \(10^{5}\) CPU hours. Such computations are not helpful for design, but will continue to help identify important contributors to the performance and acoustics of different multi-rotor configurations. **Author Contributions:** Conceptualization, A.D.T., S.M.G. ; methodology, A.D.T., S.M.G. ; software, A.D.T. ; validation, A.D.T., S.M.G. ; formal analysis, A.D.T., S.M.G. ; investigation, A.D.T., E.D.P., L.G.S., A.D.T., S.M.G. ; resources, A.D.T., R.C., S.M.G. ; data curation, A.D.T., E.D.P., L.S., A.D.M., S.M.G. ; writing--original draft preparation, A.D.T., E.D.P., S.M.G. ; writing--review and editing, A.D.T., E.D.P., S.M.G. ; visualization, A.D.T. ; supervision, A.D.M., R.C., R.T., S.M.G. ; project administration, R.T., S.M.G. ; funding acquisition, R.T., S.M.G. All authors have read and agreed to the published version of the manuscript. **Funding:** The computational aspect of this research was partially funded by NSF Grant Number 1728277 and the Boston University Mechanical Engineering Department. The U.S. Army provided the computational resources, funding support, and software for the first author through the Department of Defense High Performance Computing Modernization Program. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** The data can be obtained upon request from the corresponding author. **Acknowledgments:** APC, the manufacturer of the rotor used in this study, provided the CAD model of the rotor geometry. [PERSON] and [PERSON] provided the computing hours and administrated the internships of the first author. [PERSON] provided the software for the coupling between Helios and PSU-WOPWOP. [PERSON] provided the PSU-WOPWOP software and aided in Helios/PSU-WOPWOP coupling. [PERSON] and [PERSON] provided the UCD-Quietfly code. [PERSON], [PERSON], and [PERSON] provided insight into the higher harmonic noise. The experimental test campaign was carried out in support of the EU funded CLEAN SKY 2 Joint Undertaking Project ERACLE (Grant Agreement No. 687015-8). **Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. **Abbreviations**  
The following abbreviations are used in this manuscript: