Numerical assessment of the tonal noise of Counter-Rotating Open Rotors at approach

Abstract

A hybrid method combining a three-dimensional unsteady Reynolds-Averaged Navier–Stokes simulation of a cropped Counter-Rotating Open Rotors at approach conditions and an acoustic analogy either based on a time formulation, an advanced-time formulation of Ffowcs-Williams and Hawkings’ analogy, or on a frequency formulation, an extension of Hanson’s model to non compact chord length, has shown three main sources, the impacts of the front-rotor wakes on the aft-rotor, of the front-rotor tip vortex on the aft-rotor tip, and of the front-rotor hub horse-shoe vortex on the aft-rotor blade foot. Consequently, this study confirms that the aft-rotor is the dominant tonal noise source and has identified another potential noise source when the Counter-Rotating Open Rotors is installed caused by the strong tip vortex of the highly loaded cropped aft-rotor. The present rotor–rotor distance should not be reduced as the potential effect from the aft rotor is already felt on the front rotor. The influence of several numerical parameters including grid refinement, data sampling, and simulation length has been evaluated on both aerodynamic and acoustic performances.