Fidelity enhancement of a multi-rotor dynamic inflow model via system identification

Multi-rotor analytical dynamic inflow models in the literature, such as Pressure Potential Superposition Inflow Model (PPSIM) or Velocity Potential Superposition Inflow Model (VPSIM), have been shown to capture the fundamental inflow interference effects between the rotors. Some of the differences in inflow predictions seen between these analytic models and high-fidelity wake models are attributed to missing real flow effects such as wake distortion, contraction, decay, swirl, etc. As such, correction terms are needed in the analytically derived multi-rotor finite state inflow models, because of the potential flow and rigid wake assumptions they are based on, in order to capture some of the missing real flow effects in them. This paper develops a systematic methodology for arriving at the needed correction terms in the VPSIM through comparisons of its inflow predictions with those of a Viscous Vortex Particle Model (VVPM). Also, a procedure is developed to assess the relative importance of individual real flow effects and the associated corrections needed for improving the overall fidelity of the VPSIM. The developed methodology is applied to the Harrington coaxial rotor using its geometric and aerodynamic data from the literature. Because of the output-coupled nature of the VPSIM, it is shown that significantly fewer number of correction terms are needed to improve its fidelity using the developed methodology. It is shown that addition of swirl coupling correction terms to the VPSIM significantly improves its correlations with the VVPM. Further, it is shown that the required corrections are reasonably insensitive to thrust sharing ratio conditions between the rotors.