Predicting the aerodynamic noise of counter-rotating coaxial rotors

This work presents an aerodynamic noise prediction model for counter-rotating coaxial rotors. The aerodynamic interactions due to the flow accelerations and wake flow properties are considered in an efficient blade element and momentum theory method to compute the mean flow variables. Then, the sectional aerodynamic lifts are represented as an array of point vortexes. Using the potential flow theory, the induced flow speeds, which vary with time due to the rotational motion, can cause unsteady forces acting on the blade surfaces. The computed aerodynamic variables are then incorporated into the noise prediction model. The computed acoustic spectra and directivities for coaxial rotors with identical rotating speeds are compared with experiments, and fairly good agreements are obtained. Notably, the unsteady loading due to the interaction of potential flows surrounding the blades can lead to significant tonal noise at high-order harmonics of the blade passing frequency. The dependence of the noise radiation on rotation speed is also explored, showing different scaling laws from the isolated rotors.