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1. Correction: Sub-Doppler infrared spectroscopy of resonance-stabilized hydrocarbon intermediates: ν ₃ / ν ₄ CH stretch modes and CH ₂ internal rotor dynamics of benzyl radical

   https://doi.org/10.1039/c9cp90148e  

   Kortyna, A.; Samin, A. J.; Miller, T. A.; Nesbitt, D. J. (June 2019, Physical Chemistry Chemical Physics)

   Correction for ‘Sub-Doppler infrared spectroscopy of resonance-stabilized hydrocarbon intermediates: ν 3 / ν 4 CH stretch modes and CH 2 internal rotor dynamics of benzyl radical’ by A. Kortyna et al. , Phys. Chem. Chem. Phys. , 2017, 19 , 29812–29821. 

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2. Rotor-on-Rotor Aeroacoustic Interactions of Multirotor in Hover

   Alvarez, E. J.; Schenk, A.; Critchfield, T.; and Ning, A. (July 2020, Journal of the American Helicopter Society)

   Multirotor configurations introduce complicated aerodynamic and aeroacoustic interactions that must be considered during aircraft design. In this paper we explore two numerical methods to model the acoustic noise caused by aerodynamic rotor-on-rotor interactions of rotors in hover. The first method uses a conventional mesh-based unsteady Reynolds-average Navier-Stokes (URANS) solver, while the second consists of a meshless Lagrangian solver based on the viscous vortex particle method (VPM). Both methods are coupled with an aeroacoustics solver for tonal and broadband noise predictions. Noise predictions are validated for single and multi-rotor configurations, obtaining with the VPM a similar accuracy than URANS while being two orders of magnitude faster. We characterize the interactions of two side-by-side rotors in hover as the tip-to-tip distance and downstream spacing are varied. At an observer located six diameters away, multirotor noise is the strongest above and below the rotors, increasing by about 10 dBA directly underneath as the rotors are brought closer together. The interactions show no sensitivity to blade loading distribution, indicating that multirotor interactions are not alleviated with a lighter tip loading. We found that noise can be mitigated by spacing the rotors in the downstream direction—with the optimal spacing being about half a diameter—achieving a noise decrease of about 4 dBA without any aerodynamic penalties. 

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3. Rotor-on-Rotor Aeroacoustic Interactions of Multirotor in Hover

   Alvarez, E. J.; Schenk, A.; Critchfield, T.; and Ning, A. (July 2020, Journal of the American Helicopter Society)

   Multirotor configurations introduce complicated aerodynamic and aeroacoustic interactions that must be considered during aircraft design. In this paper we explore two numerical methods to model the acoustic noise caused by aerodynamic rotor-on-rotor interactions of rotors in hover. The first method uses a conventional mesh-based unsteady Reynolds-average Navier-Stokes (URANS) solver, while the second consists of a meshless Lagrangian solver based on the viscous vortex particle method (VPM). Both methods are coupled with an aeroacoustics solver for tonal and broadband noise predictions. Noise predictions are validated for single and multi-rotor configurations, obtaining with the VPM a similar accuracy than URANS while being two orders of magnitude faster. We characterize the interactions of two side-by-side rotors in hover as the tip-to-tip distance and downstream spacing are varied. At an observer located six diameters away, multirotor noise is the strongest above and below the rotors, increasing by about 10 dBA directly underneath as the rotors are brought closer together. The interactions show no sensitivity to blade loading distribution, indicating that multirotor interactions are not alleviated with a lighter tip loading. We found that noise can be mitigated by spacing the rotors in the downstream direction—with the optimal spacing being about half a diameter—achieving a noise decrease of about 4 dBA without any aerodynamic penalties. 

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4. Helicopter Rotor Optimization via Operator Overloading-Based Discrete Adjoint Approach

   https://doi.org/10.2514/6.2021-0170  

   Djeddi, Reza; Ekici, Kivanc (January 2021, AIAA Scitech 2021 Forum)

   null (Ed.)

   A memory efficient framework is developed for the aerodynamic design optimization of helicopter rotor blades in hover. This framework is based on a fully-automated discrete-adjoint toolbox called FDOT. The in-house toolbox is capable of computing sensitivity or gradient information very accurately, and uses an operator-overloading technique that takes advantage of a unique expression-template-based approach for memory and computational efficiency while still being fully-automated with minimal user interventions. The main goal of the present work is to "design" helicopter rotor blades with increased figure-of-merit. Therefore, the flow around the Caradonna-Tung rotor in non-lifting and lifting hover conditions is studied in order to validate the primal and adjoint solvers based on a rotating frame of reference formulation. The efficacy of the optimization framework is first demonstrated for drag minimization of a rotating NACA 0012 airfoil, which resembles a Vertical-Axis Wind Turbine (VAWT) configuration. Finally, the single- and multi-point design optimization results for the Caradonna-Tung rotor are presented. It is important to note that the current approach (FDOT) can be directly coupled -- in a "black-box" manner -- to other existing codes in the Helios computational platform, which is part of CREATE-AV. 

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5. Rotor Fault Detection and Isolation in Aerial Vehicles with Dozens of Rotors

   https://doi.org/10.1109/LARS64411.2024.10786448  

   Gandhi, Neeraj; Xu, Jiawei; Saldaña, David; Phan, Linh_Thi Xuan (November 2024, IEEE)

   Aerial vehicles with dozens of rotors are becoming increasingly common in important applications such as transportation and construction. One challenge with building such a system is to ensure that the system is robust against faults: as the number of rotors increases, the likelihood of a rotor failing during operation also increases; despite the spare thrust capacity provided by the redundant rotors, a rotor fault can significantly impact the motion and safety of the system. This paper presents an efficient fault detection and isolation (FDI) method for aerial vehicles with a large number of rotors. Our approach relies on two key insights: First, the effect of a faulty rotor directly affects the tracking error in roll and in pitch. This property can be used to order our faulty rotor search space. Second, the error in either roll or pitch is related to both the distance from the (relevant) axis and the severity of a fault. With these observations, we can use probe faults to isolate faulty rotors. Evaluation results show that our technique can efficiently detect and isolate faults in multi-rotor aerial vehicles with up to 64 rotors (8 more rotors than in existing FDI work), and that it can help improve robustness. To the best of our knowledge, our FDI method is the first that scales to several dozens of rotors. 

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