Reynolds number effects on the interference factors for a counter-rotating coaxial rotor

Counter-rotating coaxial rotors (CCR) have experienced a resurgence in interest as their performance features are beneficial for military and space applications. This experimental investigation provides a baseline experiment for quantifying the inter-rotor spacing and Reynolds number effect on the induced power of both an isolated single rotor and a CCR system. The facility consisted of two mechanically decoupled, two-bladed rotors with a NACA0015 airfoil profile, and the rotational speed of the rotors were varied to achieve different tip Reynolds numbers. In the isolated single rotor, it was found that a change of Reynolds number by 120,000 causes the power to increase by 15%, and the CCR results showed that the rotor-rotor interference was invariant to Reynolds number. However, the influence effect on the individual rotors in CCR was shown to have a dependence on the Reynolds number, as the upper rotor performed worse as Reynolds number decreased, but the lower rotor performed better. It is hypothesized that the flow structures and inflow characteristics of rotors at low Reynolds numbers are primarily responsible for the change in individual rotor influences; however, future experiments are required to fully understand the phenomena driving this trend.