Computational Investigation of Coaxial Rotor Aerodynamics in Hover

In this work, a compressible Reynolds Averaged Navier Stokes (RANS) solver is used to investigate the aerodynamics of a coaxial rotor configuration under hovering conditions. The objective of this work is to evaluate the predictive capability of the computational approach and to characterize the unsteadiness in the aerodynamic flow-field of a coaxial system. Additionally, the effect of rotor spacing on the unsteadiness is studied. To assess the accuracy of the baseline methodology, single rotor simulations are compared with experiments. On gaining sufficient confidence in the approach, extensive validation studies are performed on a two-bladed coaxial rotor system. Although detailed yaw-trimming is not achieved, global quantities such as thrust and power are predicted to within 5% accuracy. 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 (a venturi effect) and loading (an upwash-downwash effect). As expected, increased rotor spacing is seen to reduce both the thickness and loading effects. Further, interaction of the top-rotor wake with the blades of the bottom rotor results in low-harmonic unsteadiness.