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 blade resolved
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.