Computational Investigation of the Effects of Rotor-on-Rotor Interactions on Thrust and Noise

Recent advancements in electric propulsion systems have made electric vertical takeoff and landing aircraft a reality, and one that is seen as a partial solution to the growing issue of urban traffic congestion. Designing an aircraft with multiple smaller motors and rotors spread across the wings–referred to as distributed electric propulsion (DEP)–has shown great potential in help- ing improve electric aircraft performance by offering increased propulsive efficiency, augmented lift, and structural load distribution. For these reasons, DEP is one configuration that is currently being implemented into multiple prototype designs (e.g. NASA’s Maxwell X-57, Airbus Vahana, Opener BlackFly, and Joby S2). However, while a DEP configuration has many potential benefits, it complicates the aerodynamics by introducing complex rotor-on-rotor interactions which can significantly affect noise generation. In this study we use unsteady Reynolds-averaged Navier–Stokes (RANS) simulations (STAR-CCM+) with an aeroacoustic solver (PSU-WOPWOP) to quantify thrust fluctuations and noise generation for two distinct rotor-rotor configurations. The configurations investigated in this study are: 1) coplanar rotors with a varying tip separation distance and 2) one rotor downstream of the other at varying distances for a fixed tip separation distance. Both configurations are investigated using an APC 10x7E and DJI-based 0.24 m rotor. It was found that tip-to-tip separation distance has a stronger influence on noise generation than the downstream separation distance does. A one diameter change in tip separation distance resulted in a ∼15 dBA change in noise while a three diameter change in downstream separation distance only resulted in a ∼9 dBA change in noise for the same rotor. Changes in thrust fluctuations were found to predict trends in noise generation well for multi-rotor configurations. Additionally, it was shown that when rotors are located less than 10% of the diameter apart from each other, noise can be decreased by up to 9 dBA by moving one rotor ∼0.5 diameter downstream of the other.