A Comparison of High-Fidelity Simulation Approaches for Interactional Aerodynamics of Multirotor Systems in Forward Flight

Recent advancements in distributed electric propulsion for urban air mobility applications have made interactional aerodynamics more common on modern rotorcraft designs. Rotor-to-rotor interactions are associated with complex flow features, and require high-fidelity numerical tools for adequate analysis and prediction. The computational cost associated with resolving these aerodynamic interactions can become prohibitively expensive, particularly when simulations over a multitude of operating conditions/configurations are desired. As an alternative to the typical bladeresolved DDES (BR-DDES) approach, an actuator line model with LES (ALM-LES) is considered for its high fidelity aerodynamic prediction capabilities at a reduced computational cost. In this study, flow field and rotor performance predictions using ALM-LES are compared to BR-DDES in order to evaluate the merits of ALM-LES for interactional aerodynamic analysis. Overall, the wake structure of the two-rotor system shows good agreement between the two methods. Integrated thrust is also predicted similarly, with a difference of 2.4% for the front rotor and 4.3% for the aft rotor. While integrated thrust is predicted well by ALM-LES, some discrepancies in sectional thrust are observed in areas with blade-vortex interaction (BVI). The vortex position compares well between the two methods, so the sectional thrust difference is tied to differences in vortex strength and how well ALM is able to represent a BVI. Sectional thrust differences are also observed on the aft rotor and are associated with secondary vortices convecting into the rotor plane. Despite differences in parts of the rotor disk with BVI, ALM-LES is shown to be capable of predicting the interactional aerodynamics of a two-rotor system in forward flight at about 1% the computational cost of BR-DDES.