Computational Investigation of Microscale Coaxial-Rotor Aerodynamics in Hover

In this work, a compressible Reynolds-averaged Navier-Stokes solver is used to investigate the aerodynamics of a microscale coaxial-rotor configuration in hover, to evaluate the predictive capability of the computational approach and to characterize the unsteadiness in the aerodynamic flowfield of the microscale coaxial systems. The overall performance is well-predicted for a range of rpm and rotor spacing. As the rotor spacing increases, the top-rotor thrust increases and the bottom-rotor thrust decreases, while the total thrust remains fairly constant. The thrusts approach a constant value at very large rotor spacing. Top rotor contributes about 55 % of the total thrust at smaller rotor spacing, which increases to about 58% at the largest rotor separation. The interaction between the rotor systems is seen to generate significant impulses in the instantaneous thrust and power. Unsteadiness is mainly caused due to blade loading and wake effect. Additional high-frequency unsteadiness was also seen due to shedding near the trailing edge. The phasing of the top vortex impingement upon the bottom rotor plays a significant role in the amount of unsteadiness for the bottom rotor. Interaction of the top-rotor tip vortex and inboard sheet with the bottom rotor results in a highly three-dimensional shedding on the upper surface of the blade in the outboard region and a two-dimensional shedding on the lower surface at the inboard portion of the blade. The wake of the top rotor contracts faster compared with that of the bottom rotor because of the vortex-vortex interaction. Further, the top-rotor wake convects vertically down at a faster rate due to increased inflow.