Computational Investigation of Micro-Scale Coaxial Rotor Aerodynamics in Hover

Abstract

In this work, a compressible Reynolds-Averaged Navier Stokes (RANS) solver is extended to investigate the aerodynamics of a micro-scale coaxial rotor configuration in hover. This required the following modifications to the solver: implementation of a time-accurate low Mach preconditioner, implementation of a sliding mesh interface boundary condition, improvements in the grid connectivity and parallelization of the code.

First, an extensive validation study on the prediction capability of the solver is performed on a hovering micro-scale single rotor, for which performance data and wake characteristics have been measured experimentally. The thrust and power are reasonably well predicted for different leading and trailing geometries. Blunt leading edge geometries show poorer performance compared to the sharp leading edge geometries; the simulations show that this is mainly because of the large pressure drag acting at the blunt front. The tip vortex trajectory and velocity profiles are also well captured. The predicted swirl velocities in the wake for the micro-rotor are found to be significantly larger as compared to those for a full-scale rotor, which could be one of the reasons for additional power loss in the smaller scale rotors. The use of twist and taper is studied computationally and is seen to improve the performance of micro-rotor blades.

Next, the solver is applied to simulate the aerodynamics of a full-scale coaxial rotor configuration in hover, for which performance data is available from experiments. The global quantities such as thrust and power are predicted reasonably well. In the torque trimmed situation, the top rotor shares significant percentage of the total thrust at lower thrust levels, which decreases to about 55% of the total thrust at higher thrust values. The simulations reveal that the interaction between the rotor systems is seen to generate significant impulses in the instantaneous thrust and power. The characteristic signature of this impulse is explained in terms of the blade thickness effect and loading effect, as well as blade-vortex interactions for the bottom rotor (wake effect).

Finally, the RANS solver is applied to investigate the aerodynamics of a micro-scale coaxial rotor configuration in hover. The overall performance is well predicted. The interaction between the rotor systems is again seen to generate 3­8% fluctuation in the instantaneous thrust and power. The wake effect in the simulation is seen to be very prominent and the phasing of the impingement of the tip vortex from the top rotor upon the bottom rotor plays a significant role in the amount of unsteadiness on the bottom rotor. Interaction of the top rotor vortex and inboard sheet with the bottom rotor results in significant shedding on the bottom rotor blade, and this is believed to be caused by the of sharp leading edge geometry. Significant blade-vortex and vortex-vortex interactions are observed for coaxial systems.